basic & advance hardware

8/8/2019 Basic & Advance Hardware

1/38

Networking Concepts

What is a NIC?

A Network Interface Card (NIC), also called a Network Adapter, is used to connect acomputer to the cabling used in a local area network (LAN). Typically, the NIC

attaches to the computers expansion bus via an ISA slot (8-bit or 16-bit) or a PCI

slot (32-bit). The NIC has one or more external ports with which to attach thenetwork cable.

The primary function of a NIC is to allow the computer to communicate on the

network. It does this by transmitting/receiving and controlling traffic with othercomputers or devices on the network. When transmitting, the NIC converts data

from parallel to serial, encodes and compresses it, and then places it on the wire in

the form of an electrical or optical signal. The process is reversed on the receivingend. The NIC translates the electrical signal it receives off the wire into bits that can

be read by the computer.

Each NIC has a unique identifying MAC address hard-coded onto the card. In

addition, a NIC must have a network adapter driver that enables it to communicatewith the network protocols. (The network adapter driver is discussed under thesection 'Network Driver Standards: NDIS and ODI.') A NIC is specific to a particular

type of LAN architecture (e.g., Ethernet, Token Ring, or Fiber-Optic). It is possible toinstall more than one NIC in the same computer.

Most NICs connect directly to a computers system bus via a 32-bit PCI slot or an 8-

bit or 16-bit ISA slot for older computers.

A NIC operates at the Physical layer, the lowest layer of the OSI model.

What is the MAC address?

Each network interface card (NIC) has a unique MAC address assigned to it. The

MAC address, also referred to as the physical address, is a 6-byte (48-bit),

hexadecimal serial number hard-coded onto the card by the manufacturer.

A NIC manufacturer (e.g., Intel, 3Com, Cisco, etc.) must apply to the IEEE (institute

of Electrical and Electronics Engineers) for a block of addresses to assign to its cards.

The leftmost 3 octets of the address identify the manufacturer and the rightmost 3

octets represent the unique serial number for the card.

If your system is running TCP/IP, you can learn your MAC address by going to a

command prompt and typing IPCONFIG /ALL (Windows NT) or WINIPCFG (Windows95). Shown below is a portion of the information returned by the IPCONFIG /ALL

command showing the MAC (physical) address of a NIC:


2/38

Note that the name MAC stands for Media Access Control, a division of the DataLink layer under the IEEE 802 model. Thus, MAC address is something of a

misnomer. The MAC address is referenced at this layer but actually resides at the

physical layer. Although physical address is the more accurate description, MAC

address is the term most often used in practice.

What does the MAC address do?

The MAC Address provides a way to distinguish one NIC from any other NIC.

Operations at the lower layers of the OSI model (the physical and data-link layers)

do not directly recognize computer names or protocol addresses (e.g., the IP

address). The MAC address allows a card to recognize the data that is sent to it. A

NIC checks each frame or packet on the network traffic to see if it contains its MAC

address. Once the NIC identifies a frame with its MAC address, it decodes the frame

and processes it.

A bridge, which operates at the data-link layer, looks at MAC addresses. A router,

which operates at the network layer, looks at protocol addresses. Thus, a router is

able to perform more sophisticated traffic management functions than a bridge.

How does a NIC connect to the cable?

An interface connector is the physical connection between the Network Interface

Card (NIC) and the cable. There are three common types of interface connectors

used with LANs:

BNC (British Naval Connector). Used with coax cable

RJ-45. An 8-wire modular connector that looks similar to a telephone jack

connector (RJ-11). Used with twisted-pair cable. (An RJ-11 has only 6 pins.

An RJ-45 has 8 pins.)

AUI (Attachment Unit Interface). Normally a 15-pin interface used with

Thicknet coax. Also called a DIX (Digital Intel Xerox) interface. Not verycommon anymore.

Some NICs have more than one interface connector (e.g., an RJ-45 and a BNC)


3/38

What is the difference between a local and an expansion bus?

Originally there were two types of buses: local and expansion.

The local bus (also called a processor or system bus) is the path directly

connected to the CPU. It is actually four buses (data, address, control, and

power). Components on the local bus are synchronized with the CPU. This

means that they can operate on a 32-bit path at full clock speed (386

processors and above). Generally only the processors main memory and

controller logic are connected to the local bus.

The expansion bus (also called an AT Bus) allows the CPU to communicate

with controllers and peripheral devices. It connects various peripherals such

as the disk and video controllers, serial and parallel cards, and a network

interface cards (NIC) Typically these components communicate through 16-bit

ISA expansion cards at 8 MHz. In other words, they perform tasks at a much

slower speed than the processor. Because of its speed and data path are

inferior to the local bus, the expansion bus can become the "bottleneck" insystem performance.

The PCI (Peripheral Connection Interface) bus design provides the capability to tap

directly into the local bus (32-bit) for faster transfer of data. This allows peripherals

like a NIC to bypass the expansion bus. A VLB (VESA Local bus) also attaches

directly to the local bus.

What are the common bus architectures and their characteristics?

The most popular expansion bus designs are distinguished as follows:

PCI - Peripheral Connection Interface. Currently the most popular and fastest

bus design. PCI was developed by Intel and has essentially become the

industry standard for Pentium systems. It can exchange data with the CPU at

32- or 64-bits per second and supports bus mastering (allows intelligent

cards to do processing independent of the CPU). It also supports multiplexing

(the sharing or mixing of signals on the bus). PCI slots only accept PCI cards

and PCI cards only fit in PCI slots.


4/38

VLB - VESA (Video Electronics Standards Association) Local Bus. VLB was

originally designed to remove the display adapter as the system bottleneck.

Supports 3 slots for high speed peripherals on what is essentially the 32-bit

local bus (CPU). It also supports bus mastering and offers backward

compatibility (i.e., a VLB slot will accept an ISA card but the ISA card still

functions as an ISA card; it does not access the local bus.).

MCA - Micro Channel Architecture. Developed by IBM for its PS/2 computers,this design never really caught on. Data path is 16- or 32-bits wide. An MCA

slot accepts only a MCA card.

EISA - Extended Industry Standard Architecture. A counter to IBMs MCA

design that transfers data at 32-bits and offers backward compatibility (i.e.,

EISA slot will accept ISA card). EISA never really caught on except in some

specialized servers.

ISA - Industry Standard Architecture. The original bus design. Also known as

the PC/AT bus. Data is transferred at either 8-bits or 16-bits depending on

the card. A 16-bit ISA slot will accept an 8-bit ISA card or a VLB card.

What is an IRQ conflict?

When a device such as a keyboard or printer needs the attention of the CPU, it sends

a signal via its assigned IRQ (interrupt request) line. When the CPU detects the

interrupt, it knows which device is signaling and allocates its time based on the

requesters priority. Some IRQs are permanently assigned (e.g., the keyboard is

always IRQ 1). However, other IRQs can be assigned by the user.

Generally only one device can be assigned to a specific IRQ line. If two devices are

inadvertently assigned to the same IRQ, the CPU cannot distinguish which device is

signaling and the result is an IRQ Conflict.

Two devices can share the same IRQ; they just cant use it at the same time. For

example, if both the mouse and a modem are assigned to IRQ 3 (COM2), a conflict


5/38

could develop. Typically one or both of the devices will lock up or start behavingerratically. It is possible for an IRQ conflict to render the entire system inoperable.

What are the common assignments for IRQs 0 to 15?

For an 80286 processor and above, IRQ settings are typically assigned as shown

below.

system timer

keyboard

cascade to IRQ 9/programmable interrupt controller

COM2/COM4 (communication ports)

COM1/COM3 (communication ports)

LPT2 parallel port; typically used by the sound card

Floppy Disk Controller

parallel port; LPT 1

system or real-time clock/CMOS

cascade from IRQ 2

open

sometimes OS/2 mouse or trackball

math co-processor

hard disk controller

second hard disk controller

Exam Tip: For the exam you will need to know the IRQ associated with common

devices. For example, you may be given a situation where you have a modem on

COM2 and a NIC on IRQ 3. Although not given, you need to know that COM2 uses

IRQ 3 and the NIC and modem are in conflict.

Which IRQs are commonly used for a NIC? How can I tell which IRQs are

available?

Typically, IRQs 10, 11 and 15 can usually support a Network Interface Card (NIC). If

no sound card is present, IRQ 5 is often assigned to a NIC. Or IRQ 3 can be used for

a NIC if the second serial port (COM2/COM4) is not being used by another device

(e.g., a modem).

The status of IRQs on a Windows NT system can be checked using Administrative

Tools | Windows NT Diagnostics | Resources | IRQ. On a windows 95 computer, the

same information is available under Control Panel | System | Device Manager |Computer | Interrupt Request.


6/38

What is the DMA Channel?

DMA (Direct Memory Access) Channels are used to transfer data directly between

memory and an 'intelligent' I/O device like a network interface card (NIC). A DMA

channel actually consists of 2 signal lines; a receive line and an acknowledgement

line.

A DMA channel allows a device on an expansion board to transfer data to or from the

system's RAM without going through the CPU. When the device needs to access

memory, it signals the CPU via its assigned DMA channel. The CPU then gets out of

the way and the data transfer takes place directly between the device and memory.

The downside to this method is that CPU operations are put on hold until the transfer

is complete.

Not all devices use DMA channels. A tape backup device is an example of a device

that normally does. Most NICs generally do not use DMA channels because of the

interruption they cause with the CPU. However, if a device does require a DMA

channel, the assigned channel must be unique or conflicts can develop.

The number of DMA channels available depends on the system bus. There are only 4

channels available to an older 8-bit ISA bus. For other buses, there are 8 channels

(numbered 0-7) of which only 7 are available to the expansion slots. Typically the

DRAM refresh, floppy disk controller, the DMA controller itself, and LPT1 use DMA

channels 0, 2, 4, and 7, respectively. Others channels are usually free to be

assigned.


7/38

What is attenuation?

Attenuation refers to the weakening of the signal as it travels over cable. It is

sometimes referred to as roll off.

As a signal travels across wire, its square wave becomes deformed in proportion to

the distance travelled. Thus, attenuation is primarily a function of cable length. If the

signal travels too far, it can degrade so much that the receiving station may not be

able to interpret it and communication will fail.

A repeater is can be used to boost signal strength on a baseband network so that it

can travel greater distances. An amplifier performs the same function on a

broadband network.

2. What is a repeater?

A repeater is a device that regenerates the signal transmitted on a cable. Repeatersallow signals to travel beyond the normal cable length limitations. A repeater does

not translate or filter packets.

Repeaters have the following characteristics:

Used to regenerate an existing baseband signal

Used primarily in a coaxial bus (linear) topology

Segments connected by a repeater must use the same media access control

(MAC) method (e.g., a repeater does can not pass traffic between Ethernet

and Token Ring)

A repeater can pass traffic between different types of media (i.e., coax tofiber optic) if the appropriate interfaces exist

Segments connected by a repeater must have the same network address

A repeater does not accelerate or change the signal; it simply regenerates it

A repeater does not filter packets or limit congestion

A repeater will pass a broadcast

A repeater operates at the physical layer of the OSI model

The basic functionality of a repeater (signal regeneration) can be incorporated

into other connectivity devices (e.g., a hub or bridge)

Because a repeater does nothing to filter or limit traffic on the network, it should be

viewed primarily as a means of connecting distant workstations, not as a means of

adding additional workstations. In other words, repeaters should be used to extend

the length of a network but not its density.


8/38

3. Is there a limit on the number of repeaters that can be employed?

Yes. The number of repeaters is limited by the "5-4-3 rule." Under this rule there can

be no more than 5 segments with no more than 4 repeaters between any two

stations. Only 3 of the 5 segments can be populated (i.e., contain nodes or hosts). In

this case a "station" is a bridge, router, or gateway. Thus, adding a bridge can allow

additional repeaters to be used on the LAN.

What is a hub?

A hub is a generic term used to describe a central connection point for computers on

a network. The basic function performed by a hub is to receive signals from one

computer and transmit them on to other computers.

A hub can be active or passive. An active hub acts as a repeater; it regenerates andsends out a stronger signal. A passive hub is basically just a junction box; it splits

incoming signals for transmission across the network.

The hub is the center of a star topology and allow computers to be added or moved

on the network with relatively ease.

How does a hub work?

A hub is basically a signal splitter; it takes the incoming bits from one port and sends

them out to all of the other ports. A eight port hub would receive a packet on one of

its ports and send copies out to each of the other seven ports. Each host connect tothe hub will view the packet but only the host to which it is addressed will process it.

This can cause network traffic problems because a packet addressed to one host is

actually sent to all hosts (even though it is only processed by the one it is addressed

to).

What is switch?

A switch is an 'intelligent hub' that has the ability to determine the destination MAC

address of a packet. Instead of passing the packet to all ports, the switch forwards it

only to the port to which it is addressed. Thus, a switch can drastically reduce

network traffic

A switch maintains a list of MAC address associated with its various ports that it uses

to determine where to send the packet. Because it operates on the MAC address

rather than an IP address, a switch is generally faster than a router.


9/38

What is a MSAU?

A MSAU is a Multistation Access Unit that functions like a hub in a Token Ring

network. A MSAU has the ability to bypass a cable break or station failure in order to

maintain the integrity of the ring.

What is a concentrator?

A concentrator or wiring concentrator is similar to a hub and the two terms are often

used interchangeably. It serves a central connection point where a number of

incoming cables converge. A hub or switch performs the function of a concentrator.

In its most simple form, a concentrator is a multi-port repeater. It merges a number

of incoming lines with a number of outgoing lines. Some concentrators can control

errors, determine the quality of cable, and perform other administrative functions.

Like a MSAU the concentrator in a ring network keeps the ring intact even if

workstations are removed.

What are "interconnectivity" devices and what functions do they perform?

A concentrator or wiring concentrator is similar to a hub and the two terms are often

used interchangeably. It serves a central connection point where a number of

incoming cables converge. A hub or switch performs the function of a concentrator.

Bridges, routers, and gateways are interconnectivity devices. Their primary function

is to connect segments or multiple networks into a single, heterogeneous system

across which computers can communicate. These devices are sometimes referred to

collectively as "relays."

What is a bridge and what can it do?

A bridge is a relay or interconnecting device that is use to join to LANs.

Bridges, routers, and gateways are interconnectivity devices. Their primary function

is to connect segments or multiple networks into a single, heterogeneous system

across which computers can communicate. These devices are sometimes referred to

collectively as "relays."

A bridge can:

Link segments of a network together

Filter traffic by reading packet addresses

Link dissimilar networks (Ethernet and Token Ring) if higher level protocols

are the same or can be translated


10/38

A bridge cannot:

Determine the most efficient path to transmit data

Provide traffic management functions (congestion bypassing)

The functions of a bridge may be performed by a server or an independent bridge

device. A general rule of thumb is that there should be no more than four bridges ona LAN.

How does a bridge pass traffic between LAN segments?

A bridge checks the MAC address of each packet it receives. The bridge examines the

packet's destination address and compares it to the routing table it maintains. Based

on this comparison, the bridge performs one of three tasks:

If the destination address is in the routing table and is located on the same

segment as the sender, the packet is dropped. (There is no need to pass it

across the bridge if it is addressed to a host on the near side.) If the destination address is in the routing table and is located on a different

segment, the packet is forwarded to that segment.

If the destination address is not in the routing table, the packet is forwarded

to all segments.

Note that when a bridge receives a packet from one host that is addressed to a host

on the other side, the bridges passes the data frame across the connection. When

the bridge detects traffic that is addressed to a host on the originators segment, it

does not allow it to pass. In this manner a bridge performs a filtering function that

reduces the overall traffic on the network.

Although a bridge can learn the MAC addresses of the stations on the network, it

cannot determine the most efficient data path to send the data. This job requires a

router.

How does a bridge create a routing table?

The routing table for a bridge is different than a routing table used by a router. The

routing table used by a bridge is based on MAC addresses rather than IP addresses.

Unlike routers, bridges don't 'talk' to each other to update their routing tables.

However, most bridges can monitor and "learn" the addresses of each station on the

network. They use this information to construct a routing table.

When the bridge receives a packet, it exams the packet's source MAC address and

uses this information to add to or update its routing table. A bridge only needs to

know which MAC addresses are on which segments so it can properly forward the

packets. When it examines a packet, it may not know on which segment the

destination address resides, but it is does know from which segment the packet


11/38

originated. So the source MAC address and segment are added to the routing table.

Over time the routing table will be updated with all MAC addresses and segments.

What is network partitioning?

If network performance is being degraded by excessive traffic, one possible solution

is to partition the network into two or more segments. A network can be partitionedby using a bridge. The bridge is inserted between two segments of the LAN and as

traffic flows across the LAN, the bridge filters and forwards it according to the

address. Only traffic destined for another segment is allowed to "cross" the bridge.

Partitioning is also referred to as segmenting a network.

How does partitioning (segmenting) benefit a network?

When excess traffic starts to degrade performance, one solution is to partition or

segment the network.

For example, assume that a company network connects the accounting andmarketing departments which generate the following number of messages during the

course of a typical day: Marketing to Marketing: 200; Marketing to Accounting: 100;

Accounting to Marketing 100; and Accounting to Accounting: 200.

Without partitioning, all traffic will flow across both departments. Thus, 600

messages will be directed to each host. However, if the two departments are

partitioned by separating them with a a bridge, all of marketings interdepartmental

messages will stay on their side of the network. Only messages addressed to

accounting will cross the bridge to the accounting segment. The reverse is also true;

the marketing segment will not be cluttered by internal messages from accounting.

The result is a 50% decrease in overall network traffic; each segment now onlyprocesses 300 messages.

As a general rule, if all you want to do is partition a network, a bridge is a better

choice than a router because it operates with lower overhead.

What are some of the potential problems with connecting Ethernet and

Token Ring LANs with a bridge.

Bridges are capable of connecting LANs that use different physical and MAC-layer

protocols, such as Ethernet and Token Ring. However, there are several potential

problems with these connections:

Token ring has a mechanism for setting transmission priority. Ethernet does

not.*

Ethernet packets are smaller and structured differently than Token Ring

packets

Ethernet packet information is encoded differently than Token Ring packets


12/38

Ethernet uses transparent bridging to identify network addresses and token

ring uses source routing

*100VG-AnyLAN Ethernet can establish transmission priorities but by an different

method than that used by a Token Ring network.

A translational bridge can be used to eliminate may of the problems associate withlinking Ethernet and Token Ring LANs (e.g., packet format differences, address

detection methods, etc.) Among other things, this type of bridge is able to remove

data from one frame type (e.g., Ethernet) and repackage it into another frame (e.g.,

Token Ring).

What are the benefits provided by a bridge?

A bridge is a relay or interconnecting device that can be used to provide the following

capabilities:

Expand the length of an existing network Increase the number of workstations on the network

Reduce traffic congestion (network partitioning)

Provide a connection to a dissimilar network (e.g., Ethernet to Token Ring)

Move data across a intermediate network with a dissimilar protocol

What is transparent bridging and how does it compare to source routing?

Transparent bridging and source routing are two learning techniques used by

bridges to build a table of addresses for all of the stations on a network. Without

these techniques, system administrators would have to load the addresses into the

bridge manually.

Transparent Bridging is used in Ethernet networks. Bridges learn station

addresses by examining data frames as they pass. The address table

associates the source address of the data frame with a network address. The

table is reviewed continually and updated as the network changes.

Source Routing is used in Token Ring networks. Bridges learn station

addresses by having a source node send out explorer packets. Multiple

explorer packets are sent out over the network to their destination. They then

report back to the source node which determines the most efficient path. The

path information is stored in the bridge and all subsequent transmission use

this path.

Keep in mind that the objective of both transparent bridging and source routing is to

prevent bridges from creating loop in a network. The distinction is that transparent

bridging is an Ethernet method that blocks loops and source routing is a Token Ring

method that avoids loops.


13/38

What is a spanning tree algorithm?

The spanning tree algorithm (SPA) is a process used to detect and close circular

traffic patterns (loops). It operates by closing down certain ports on bridges that

would provide multiple data paths through the network.

SPA identifies a "root" bridge and then determines the pathway through the network

to the root bridge based on the "least cost." Bridge ports that provide redundant

paths are disabled to prevent loops. However, these ports may be assigned as

backup in the event the primary port fails.

A major distinction between a bridge and a router is that a router will support

multiple paths among networks. A bridge can only recognize one path.

What is a router?

A router is a relay device that is used to connect two or more networks, either locallyor through a WAN. The key feature of a router is its to determine the shortest path

to a destination. In addition, a router offers "fault tolerance" capability in that it can

route traffic across an alternate link if the primary link is broken or busy.

Routers can transfer data between two networks with dissimilar lower layer protocols

(Physical and Data Link). Thus, routers can connect Ethernet with Token Ring.

However, in order for a router to function, protocols at the Network and higher layers

must be identical. In addition, the protocol must be routable. Some protocols like

NetBEUI are not routable.

Routers operate at the Network Layer of the OSI model. Unlike bridges, routers lookat IP addresses, not MAC addresses. Specifically, they look only at the network ID of

the address, not the host ID.

Routers do not pass broadcasts. A broadcast 'storm' can occur when many hosts are

broadcasting at the same time.

What is a routing table?

A routing table contains entries with the IP addresses of router interfaces for other

networks. It does not contain addresses for the hosts on each network. It only stores

the address of the router that provides an interface to that network.

Typically a routing table consists of four pieces of information: a network address

(the destination net ID); a net mask (to identify subnetting); the address of the

router for that network ID (near-side address); and a metric (number of hops to that

router).


14/38

For example, the following command adds a static entry in the routing table that

allows packets to be forwarded to network 192.168.222.0 from network

192.168.211.0. This entry would be added to the router interfacing with network

192.168.211.0. Note the no subnetting is used and the near-side address of the

router providing access to 192.168.222.0 is 192.168.211.100.

ROUTE ADD 192.168.222.0 MASK 255.255.255.0 192.168.211.100

You can view static entries in a routing table by issuing the ROUTE PRINT command.

How does a router work?

A router uses the routing table stored in it's memory to make decisions about where

and how to send packets.

The router looks at the information in the packets it receives to examine the network

ID of the IP address. It checks the network ID against the routing table to determine

the destination. If the router can forward the packet directly to the destination, itdoes so. If it cannot, it finds the address of the router than serves as an interface for

that network. It then send the packet to that router for forwarding. If no entry exists

in the routing table, the router sends the packet to its default gateway.

A hop is a passage through a router. If a packet must travel through three routers to

get to its final destination, it is said to go through 3 hops.

When passing packets through a router, the original source and destination MAC

addresses are removed and recreated. (But the original source and destination IP

addresses are not altered.) The source MAC address of the sending host is replaced

with that of the router and the new destination MAC address is that the next routeror, if the packet can be forwarded directly, that of the host. The process of removing

and recreating MAC addresses requires processing resources.

Can a PC with Windows NT Server function as a router?

Yes. A PC running Windows NT Server 4.0 can function as a 'poor man's' router if it is

properly configured.

The server must have two NICs, one configured with the IP address on one network

and the other configured with an IP address on the other network. A server so

configured is referred to as a multi-homed server. Each workstation's default

gateway is configured with the IP address of the multi-homed server's local card. The

only other step required is to select the 'Enable IP-Forwarding' under the 'Routing'

tab of the TCP/IP property sheet on the multi-homed server. The multi-homed server

will now pass traffic between the two networks.

Although not a very sophisticated router, an NT Server can perform the basic

function of moving packets from one network to another.


15/38

How do routers exchange information?

Dynamic routers use what are called 'interior gateway protocols' to exchange routing

information. Two of the most common protocols are RIP (Routing Information

Protocol) and OSPF (Open Shortest Path First). RIP uses a distance-vector (DVA)

algorithm to calculate routing paths. DVA bases routing decisions on the least

number of hops and exchanges routing tables among routers about every 30

seconds. OSPF uses a link-state algorithm that requires more processing power than

DVA but offers more control. Routing tables are updated as necessary.

Basic Cablling

Describe the three most common types of cabling media used in LANs.

The three most common types of cabling media used in LANs are:

Coaxial Cable. Coaxial cable comes in two versions: Thinnet and Thicknet. Thinnet

looks like regular TV cable.* It is about 1/4 inch in diameter and is very flexible and

easy to work with. In contrast, Thicknet is about 1/2 inch in diameter and not very

flexible. Thicknet is older and not very common anymore except as a backbone

within and between buildings. Coax transmits at 10 Mbps..

Twisted Pair. Twisted pair looks like telephone wire and consists of insulated strands

of copper wire twisted together. There are two versions of twisted pair cable:

Shielded Twisted Pair (STP) and Unshielded Twisted Pair (UTP). STP is commonly

used in Token Ring networks and UTP in Ethernet networks where it is referred to as

"10baseT." Transmission rates vary between 10-100 Mbps..

Fiber-Optic Cable. Fiber-optic cable consists of a thin cylinder of glass surrounded by

glass cladding, encased in protective outer sheath. Fiber-optic cable is very fast (100

Mbps). It can transmit over long distances (2 km +) but is expensive.

*Dont confuse Thinnet cable (RG 58) with cable TV cable (RG 59). They look alike

but they are not interchangeable.

What are the recommended maximum segment lengths for each type of

cable?

The type of cable plays a role in how fast a signal will degrade as it is transmitted.

The following are the recommended maximum cable lengths:


16/38

Thinnet- maximum length of segment (terminator to terminator) is 185

meters (607 feet)

Thicknet -maximum length of segment (single run) is 500 meters (1,640 feet)

Unshielded Twisted Pair (UTP) - maximum length of cable between hub and

computer is 100 meters (328 feet)

Fiber-Optic - maximum length of cable is 2 kilometers. (6,562 feet)

What is a backbone?

A backbone is a generic term used to describe media that interconnects a number of

computers, segments or subnets.

In its most common form, a backbone is used to connect hubs. Each hub represents

a segment on which individual workstations are connected via UTP cable. The hubs

from different segments are then connected to each other with thinnet cable. In this

case, the thinnet cable functions as the backbone that links the hubs.

In another example, a backbone may be a length of cable that serves as a trunk.

Drop cables are attached from the backbone to connect individual workstations.

A backbone is often used to connect networks in separate buildings. Organizations

typically use fiber-optic cable for this type of backbone. Thicknet is also used as a

backbone.

What are the characteristics of coaxial cable?

Coaxial cable (coax) has the following characteristics:

Transmission rate of about 10 Mbps

Maximum cable length of 185 meters for Thinnet, 500 meters for Thicknet Good resistance to electrical interference

Less expensive than fiber-optics but more expensive than twisted pair.

Flexible and easy to work with (Thinnet)

Wire type is 20 AWG for Thinnet (R-58) and 12 AWG for Thicknet.

Ethernet designation is 10base2 (Thinnet) or 10base5 (Thicknet, also referred

to as 'standard Ethernet').


17/38

What are the different types of coaxial cable?

The following designations are used to distinguish the different types of coaxial

cable:

RG-58 A/U Thinnet, stranded wire core, 50 ohms

RG-58 /U Thinnet, solid wire core, 50 ohms RG-59 Thicknet, cable television, broadband

RG-62 ArcNet, 75 ohms

A cable's designation is typically printed on the its outer sheathing. As a general rule,

you cannot mix coax cable types on the network.

What connection hardware is used with thinnet coaxial cable?

Thinnet utilizes the following connection hardware referred to as BNC components:

Terminator a resister used to absorb the signal once it reaches the end ofthe bus; connects to a T- or barrel connector; RG-58 requires a 50-ohm

terminator; RG-62 requires a 75-ohm terminator.

Cable Connector the interface at the end of the cable that is used to connect

to a barrel or T-connector

T-Connector used to connect to a NIC and another cable connector or a

terminator (you cannot connect directly to a NIC with a cable connector; you

must use a T-connector)

Barrel Connector used to splice to segments of cable together or attach a

terminator at the end of the cable


18/38

What connection hardware is used with thicknet coaxial cable?


Thicknet utilizes an AUI (Attachment Unit Interface) connector to attach to a

Network Interface Card (NIC). The AUI connector has 15 pins and is alternatively

referred to as a DB-15 or DIX connector.

One of the unique characteristics of Thicknet is the use of an external transceiver.

(The transceiver is used to convert signals from parallel to serial for transmission on

the network.) The Thicknet cable attaches to the transceiver via a clamp or vampire

tap. The NIC connects to the transceiver using a drop cable with AUI connectors.

What is Plenum cable?


Plenum refers to the space in buildings between the ceiling and the next floor above

it. Because of the potential fire hazard, building codes are very specific about what

type of wiring can be placed in this area. refers to the space in buildings between the

ceiling and the next floor above it. Because of the potential fire hazard, building

codes are very specific about what type of wiring can be placed in this area.

Plenum cable refers to coaxial cabling that meets the minimum standards to allow it

to be strung in the plenum area without having to use special conduit. The insulation

and jacket on plenum cabling must be fire resistant and not give off toxic fumes

when burned. refers to coaxial cabling that meets the minimum standards to allow it

to be strung in the plenum area without having to use special conduit. The insulationand jacket on plenum cabling must be fire resistant and not give off toxic fumes

when burned.

Not all coaxial cable is plenum cable. The most common type of coaxial cable is PVC

(polyvinyl chloride) which is more flexible and easier to work with than plenum but

does not have the same fire resistance features. PVC cable can give off toxic fumes

when burned.

What are the key characteristics of UTP?

Unshielded Twisted Pair (UTP) has the following key characteristics:

Transmission rate of 10-100 Mbps

Maximum cable segment of 100 meters

Most susceptible to electrical interference or crosstalk (although shielding

may lessen the impact)

Less expensive than coax or fiber-optic. In some cases, preinstalled telephone

wire may be used in the network (if it is of sufficient grade).


19/38

Very flexible and easy to work with

Wire type is 22-26AWG

Uses an RJ-45 connector

Ethernet designation is 10baseT

What are the characteristics of shielded twisted pair (STP)?

Shielded twisted pair (STP) is similar to UTP except it contains a copper braid jacket

to shield the wires from electrical interference. It can support transmissions over

greater distances than UTP.

What are the 5 categories of UTP and what is the minimum acceptable

category for 10baseT?

The following categories of unshielded twisted pair (UTP) cable were established by

the EIA/TIA* to support the networks indicated:

Category 1 - Traditional telephone cable; supports voice only, not data Category 2 - Data transmissions up to 4 Mbps (but not token ring)

Category 3 - 10 Mbps Ethernet

Category 4 - 16 Mbps token-ring

Category 5 - 100 Mbps; supports ATM

The minimum acceptable cable for 10baseT Ethernet is Category 3.

*EIA/TIA 568 is the standard developed by the Electronic Industries

Association/Telecommunications Industry Association applies to all UTP that works

with networks.

Are there other cable specifications?

Yes. There are several different specifications used to classify cable. One of the

oldest is the AWG (American Wire Gauge) rating. This rating measures the thickness

or gauge of the wire with the size being inverse to the rating. For example, a 22

AWG cable is thicker than a 24 AWG cable. 22 AWG wire is typically used in

telephone wire and UTP.

IBM uses its own system of cable classification whereby cables are categorized as

types. For example, Type 3 wire is basically equivalent to the Category 3 wire

discussed earlier. However, not all of the types used by IBM coincide exactly with a

particular category. In the non-IBM world, UTP cable is typically referred to by its

category classification and coax by its RG designation.

What connectors are used with UTP?


20/38

UTP uses a connection called the RJ-45 connector. It looks similar to a common

telephone connector (RJ-11) except it is slightly larger. The RJ-45 has 8 pins while

the RJ-11 has only 4.

UTP cable typically runs from a computers NIC and plugs directly into a wall plate

much like a regular telephone. It is not uncommon to find wall plates in newer

buildings labeled as Voice (for telephones) or Data (for UTP). The cable runningfrom the wall jack is rarely attached directly into a hub. Instead the individual cables

are collected and organized with patch panels located in a wiring closet.

Do I have to have a hub to use UTP cable?

No. If you have two computers and their NICs have RJ-45 connections, then you can

simply connect them with UTP cable. However, you will need a special type of UTP

cable called a crossover cable. This cable is a normal UTP cable with the pin

connections switched on one end so that the NICs can talk to each other. You can

make a crossover cable by removing the RJ-45 connector at one end and switching

(or crossing) the wire pairs. The downside to using a crossover cable is that it onlyallows you to connect two computers. If you need to connect more than two

computers, you have to buy a hub or use coax cable.

What is fiber-optic cable and how does it work?

Fiber-optic cable uses optical rather than electrical pulses to transmit signals.

Fiber-optic cable consists of pure silicon glass cylinders or strands surrounded by

cladding. Each strand can pass a signal in only one direction so fiber-optic cable on a

network typically consists of at least two strands: one for sending and one for

receiving. Electronic signals generated by the computer are converted to opticalsignals in the form of photons which are transmitted (flashed) down the cable by a

laser or light-emitting diode. A photo-detector on the other end collects the optical

signals and they are converted back to electrical signals.

Unlike copper cable, the signals on fiber-optic cable are not subject to the problems

of attenuation, capacitance, or crosstalk. This greatly increases the potential

transmission distance. In addition fiber-optic cable is more secure than copper cable.

It does not generate electromagnetic signals and any external tap is easily detected

by a reduction in signal strength. Fiber-optic cable is generally more expense than

copper cable.


21/38

LAN Architecture

What are the 3 most common LAN architectures?

The 3 most common types of LAN architectures* are:

Ethernet

Token Ring

ArcNet

* These architectures are sometimes referred to as "lower-level protocols" because

they represent the specifications for the IEE802 model which encompasses the

physical (1st) and data link (2nd) layers of the OSI model.

What is Ethernet? What IEEE standard does it conform to?

Ethernet is a popular, relatively inexpensive, easy-to-install LAN architecture withthe following characteristics:

Uses the CSMA/CD media access control.

Data transmission normally occurs at 10Mbps.

Typically implemented in a bus or star topology.

Ethernet LANs are normally distinguished by the type of cable they use

(Thinnet, Thicknet, or Twisted Pair).

The Ethernet architecture conforms to most but not all of the IEEE 802.3

specification (the physical layers are identical but the MAC layers are somewhat

different).

How does the nomenclature "10base2" describe Ethernet cable?

An Ethernet LAN is often described in terms of three parameters: transmission rate,

transmission type, and segment distance.

"10base2" means:

10 - transmission rate or through put of 10Mbps

base - transmission type is baseband rather than broadband network (i.e.,

the signal is placed directly on the cable, one signal at a time)

2 - the maximum segment distance in meters times 100; in this case 200

meters (actually only 185 meters)

(In "10baseT," the T stands for twisted pair. Segment distance is 100 meters.)


22/38

What are the key characteristics of 10Base2 Ethernet.

A 10Base2 Ethernet LAN conforms generally to the IEEE 802.3 standard. Also known

as Thinnet Ethernet, it has the following key characteristics:

Transmits at 10 Mbps

Uses Thinnet coaxial cable Supports a maximum of 30 nodes per segment

Uses local bus topology

Minimum distance between computers is 0.5m (not including drop cables)

Maximum length of segment is 185m

Up to 5 segments can be connected (but only 3 can accommodate nodes)

Connected with BNC connectors (T-connectors)

Used primarily for smaller workgroups or departments

What is 100VG-AnyLAN?

100VG-AnyLAN. The VG stands for "Voice Grade" in recognition of the fact that ittransmits at 100 Mbps over voice grade twisted-pair cable (Category 3).* Supports

cable segments of only 100m. Uses a new media access control method called

demand priority rather than the older CSMA/CD. IEEE standard 802.12.

*100VG-AnyLAN can transmit on Category 3 cable because it uses all 4-wire pairs

(quartet signaling). 10baseT uses only 2 of the wire pairs in Category 3 UTP.

What are the key characteristics of 10baseT Ethernet.

A 10baseT Ethernet LAN conforms generally to the IEEE 802.3 standard. Also known

as Twisted Pair Ethernet, it has the following key characteristics:

Transmits at 10 Mbps

Normally uses UTP cable

Minimum grade of UTP cable is Category 3

Typically supports about 12 nodes per hub*

Uses star or bus topology

Minimum distance between nodes is 2.5m

Maximum length of segment is 100m

Most common type of Ethernet LAN; used in both small and large networks

*There is no hard and fast rule here. Hubs can be connected together so in total a

10BaseT LAN can support up to 1024 nodes without a bridge.


23/38

What is Token Ring? What IEEE standard does it conform to?

Token ring is a relatively expensive LAN architecture that is strongly influenced by

IBM. It is very stable and can be expanded without a significant degradation in

network performance.

Token ring uses the token passing media access control. Data transmission normally

occurs at 4 or 16 Mbps depending on the cable.

Token ring is normally implemented in a logical ring/physical star topology with a

MAU (Multistation Access Unit) as the hub. The maximum number of stations on one

ring is 260 for shielded twisted pair and 72 for unshielded twisted pair (UTP). There

can be up to 33 MAUs per ring.

Token Ring LANs normally use shielded twisted pair (STP) but may also use

unshielded twisted pair (UTP) or fiber-optic cable. The maximum distance to the MAU

from the workstation depends on the cable and varies from 45 meters for UTP to 100meters for STP.

The Token Ring architecture conforms generally to the IEEEs 802.5 specification

What is ArcNet? What IEEE standard does it conform to?

ArcNet (Attached Resource Computing Network) is a relatively inexpensive, reliable,

and easy-to-install LAN architecture with the following characteristics:.

Additional workstations are easily added.

ArcNet is a baseband, token passing media access control architecture ArcNet is relatively slow. Data transmission occurs at 2.5 Mbps. (20 Mbps for

ArcNet Plus)

ArcNet can be implemented in a bus or star topology.

ArcNet LANs normally use coaxial cable but can also use twisted pair or fiber-

optic cable. Maximum cable segment depends on the type of cable and hub

connection (120m -600m)

The ArcNet architecture conforms very loosely to the IEEEs 802.4 specification.

ArcNet is a baseband star or bus and 802.4 defines a broadband bus.

What factors should be considered when selecting a LAN architecture?

The following factors should be considered when selecting a LAN architecture:

Cost - How much is available in the budget and what quantifiable benefits will

the LAN provide?

Reliability - How dependent is the organizations operations on the LAN?

Speed - How many concurrent users and how critical is response time?


24/38

Size - How many computers will be connected and what is the overall

distance between them?

Security - How important is secure communications to the network?

Growth - What are the organizations plans for growth?

Administration - How will the LAN be administered? Who will assume what

responsibilities?

What is the most common bus topology normally used with the following

architectures: 10base2, 10base5, 10baseT, Token Ring, and ArcNet?

Common LAN architectures and their topologies include:

Ethernet 10base2 - local bus

Ethernet 10base5 - regular bus

Ethernet 10baseT - bus or star

Token Ring - ring (physical star)

ArcNet - star or distributed star (interconnected)

Media Access Methods

What is a Media Access Method ?

A media access method refers to the manner in which a computer gains and controls

access to the networks physical medium (e.g., cable). Common media access

methods include the following:

CSMA/CD

CSMA/CA

Token Passing

Demand Priority

One of the primary concerns with media access is how to prevent packets from

colliding when two or more computers try to transmit simultaneously. Each of the

methods listed above takes a different approach to this problem.

What is CSMA/CD?

CSMA/CD stands for Carrier-Sense Multiple Access with Collision Detection. It is a

media access method which means it defines how the network places data on the

cable and how it takes it off. CSMA/CD specifies how bus topologies such as Ethernet

handle transmission collisions. A collision occurs when two or more computers

transmit signals at the same time.


25/38

"Carrier Sense" means that each station on the LAN continually listens to (tests) the

cable for the presence of a signal prior to transmitting. "Multiple Access" means that

there are many computers attempting to transmit and compete for the opportunity

to send data (i.e., they are in contention). "Collision Detection" means that when a

collision is detected, the stations will stop transmitting and wait a random length of

time before retransmitting.

CSMA/CD works best in an environment where relatively fewer, longer data frames

are transmitted. This is in contrast to token passing which works best with a

relatively large amount of short data frames. Because CSMA/CD works to control or

manage collisions rather than prevent them, network performance can be degraded

with heavy traffic. The greater the traffic, the greater the number of collisions and

retransmissions.

CSMA/CD is used on Ethernet networks.

Describe how CSMA/CD operates

CSMA/CD is a media access control (MAC) method designed to control the number of

collisions that occur on a network. In its most simple form it operates as follows:

1. A station that wishes to transmit on the network checks to see if the cable

is free. 2. If the cable is free, the station starts transmitting. 3. However,

another station may have detected a free cable at the same instant and also

start transmitting. The result is a "collision." 4. Once the collision is detected,

all stations immediately stop transmitting. 5. Stations then wait a random

length of time before checking the cable and then retransmit

What is CSMA/CA?

CSMA/CA stands for Carrier-Sense Multiple Access with Collision Avoidance and is a

media access method very similar to CSMA/CD. The difference is that the CD

(collision detection) is changed to CA (collision avoidance). Instead of detecting and

reacting to collisions, CSMA/CA tries to avoid them by having each computer signal

its intention to transmit before actually transmitting. In effect, the transmitting

computer gives a 'heads up' prior to transmitting.

Although CSMA/CA can prevent collisions, it comes with a cost in the form of the

additional overhead incurred by having each workstation broadcast it's intention prior

to transmitting. Thus, CSMA/CA is slower than CSMA./CD.

CSMA/CA is used on Apple networks.


26/38

What is a token passing?

Token passing is a media access method by which collisions are prevented. A

collision results when two computers transmit a signal at the same time.

Collisions are eliminated under token passing because only a computer that

possesses a free token (a small data frame) is allowed to transmit. The token

passing method also allows different priorities to be assigned to different stations on

the ring. Transmissions from a stations with higher priority take precedence over

stations with lower priority.

Token passing works best in an environment where a relatively large number of

shorter data frames are being transmitted. (As opposed to CSMA/CD which works

best in an environment where relatively fewer, longer data frames are being

transmitted.)

Token passing is used on Token Ring and ArcNET networks.

Describe how token passing works.

Token passing is a media access method designed to prevent collisions. In its most

simple form it operates as follows:

1. A station that wishes to transmit on the network waits until it the token is

free. 2. The sending station transmits its data with the token. 3. The token

travels to the recipient without stopping at other stations. 4. The receiving

station receives the token with the data and returns the token to the sender

as an acknowledgment. 5. The sender receives acknowledgment and releasesthe token to next station. 6. The token continues being passed along the ring

until it is "grabbed" by the next station that wants to transmit.

What is the demand priority media access method?

Demand priority is the new Ethernet media access method that will probably replace

the popular but older CSMA/CD method. Demand priority access has the following

characteristics:

Used with 100 Mbps Ethernet

Requires a 'smart' hub that controls access to the network

Stations must request permission from the hub before they can transmit

Transmissions are sent through hub and are not broadcast to all other

stations

Stations can transmit and receive at the same time

Transmits on all four wire-pairs of twisted pair wire (i.e., can use Category 3

cable)


27/38

Transmissions can be prioritized (e.g., time sensitive traffic such as video is

given priority)

Describe how demand priority works.

Demand priority utilizes a 'hub-centric approach to media access. A 'smart hub'

controls access to the network. When a workstation needs to transmit, it sends arequest to the hub. The hub grants permission to transmit based on network

conditions and requester priority. Because they are under the control of the hub,

workstations do not 'compete' for access to the network.

Unlike 'regular' Ethernet in which a transmission is transmitted to all stations,

demand priority utilizes a directed transmission. The hub directs the transmission

from sender to intended recipient rather than sending it to all stations.

With demand priority, workstations can transmit and receive at the same time. This

is because demand priority employs 'quartet signaling' on four pairs of wires. Regular

10BaseT Ethernet transmits over two wire pairs.

Compare token passing with CSMA/CD

CSMA/CD is used primarily by Ethernet LANs and token passing is used

primarily by Token Ring LANs.

CSMA/CD works best in larger networks with relatively fewer, longer data

frames. Token passing works best in small to medium size networks with

many short data frames.

Token passing allows different priorities to be assigned to stations. CSMA/CD

does not.

Token passing is normally more expensive than CSMA/CD.

Describe two types of error conditions on a token-ring LAN.

There are two common error conditions that can occur on a token passing LAN:

Persistent Frame Error. A token cannot be acknowledged and continues to be

passed around the ring.

Lost Token Error. A token is inadvertently "hung up" or removed from the

ring.

Most token passing schemes can detect these errors and provide a mechanism for

clearing the ring or initializing a new token.


28/38

OSI Model

What is the OSI model?

The OSI (Open Systems Interconnection) model is a standard developed by the ISO

(International Standards Organization) to describe the flow of data on a computernetwork. The model takes into account the flow of data from the physical

connections up to the end user application.

The OSI model defines a "layered" architecture in the form of a protocol stack. There

are specific, discrete functions that take place at each layer of the protocol stack with

lower level layers providing services to upper layers. When two systems

communicate on the network, information is sent down through the protocol stack of

one system, over the cable and then up through the protocol stack to the

appropriate layer on the other system.

Data flow under the OSI model is organized into the following seven layers: Physical,Data Link, Network, Transport, Session, Presentation, and Application.

What is the purpose of the OSI model?

The OSI model (as well as IEEE 802 and NDIS) were established to standardize the

design and construction of computer networks for developers and hardware

manufacturers. Standards allow hardware and software components from a variety

of different vendors to operate together. Without standards, everything would be

vendor-specific and interoperability would suffer.

Almost all vendors implement standards in slightly different ways. This is normally

done to take advantage of unique functionality in their product that just won't fit

neatly into the model. However, if a vendor strays too far from the standards, they

risk creating a 'closed' or proprietary product that is not supported by the rest of the

industry.

IBM mainframes use Systems Network Architecture (SNA) that is a set of layered

protocols like the OSI model. However, the SNA layers are not directly comparable to

the OSI model layers. This makes interoperability between PC-based networks and

IBM mainframes more difficult.

Exam Tip. Certification candidates need to understand the OSI and IEEE models

because they provide a framework from which many of the technical concepts are

taught. This understanding can be a big help when troubleshooting network

problems in the real world.


29/38

What are the seven layers of the OSI model?

Application Layer - user interface to network services such as file

transfer/database access, terminal emulation, etc. (e.g., RPCs)

Presentation Layer - translates data format; data compression; redirectoroperations; network security (e.g., SMBs & NCPs)

Session Layer - establish/maintain/terminate a connection; synchronization

between user tasks with checkpoints, two way communication; name

recognition (e.g., NetBIOS & WinSock)

Transport Layer - Flow control; ensures reliable delivery of packets in

sequence with no losses or duplications (e.g., TCP & UDP)

Network Layer - translation of logical and physical addresses; determines

route from source to destination; traffic control (IP & IPX)

Data Link Layer - converts frames into bits (send side) and bits into frames

(receive side); performs error checking; resends if no acknowledgement (e.g,

the NIC driver) Physical Layer - transportation of raw (i.e., binary) data; defines cable &

signaling specifications (e.g., the NIC & cable)

In real life things are never quite as precise as they are in models. For practical

purposes you are generally only concerned with three 'composite' layers: 1) the

'lower layers' (physical and data link layers) that define the network architecture

(e.g., Ethernet); 2) the 'network' or 'internetwork' layer (network and transport

layers) where most of the addressing and logical transport functions are performed

(e.g., TCP/IP); and the upper or 'application' layer (session, presentation, and

application) where high level networking functions take place (e.g., redirector and

RPCs). The boundaries between these 'composite' layers are separated by well-

defined interfaces which on a Microsoft network are called NDIS and TDI.

Exam Tip. For purposes of certification testing, the following pneumonic can be

helpful in memorizing the names and order of the OSI layers:

All People Seem To Need Data Processing


30/38

What is the function of the physical layer of the OSI model?

The Physical Layer of the OSI model establishes the physical characteristics of the

network (e.g., the type of cable, connectors, length of cable, etc.) It also defines the

electrical characteristics of the signals used to transmit the data (e.g. signal voltageswing, duration of voltages, etc.). The Physical Layer transmits the binary data (bits)

as electrical or optical signals depending on the medium.

A repeater functions at the Physical Layer.

What is the function of the Data Link Layer of the OSI model?

The Data Link Layer defines how the signal will be placed on or taken off the NIC. It

is here that the data frames are broken down into individual bits that can betranslated into electronic signals and sent over the network. On the receiving side,

the bits are reassembled into frames for processing by upper levels.

Error detection and correction is also performed at the data link layer. If an

acknowledgement is expected and not received, the frame will be resent. Corrupt

data is also identified at the data link layer.

Because the Data-Link Layer is very complex, it is sometimes divided into sublayers

(as defined by the IEEE 802 model). The lower sublayer, called the MAC sublayer,

provides network access. The upper sublayer, called the LLC sublayer, is concerned

with sending and receiving packets and error checking.

A bridge functions at the Data Link Layer.

What is the function of the Network Layer of the OSI model?

The Network Layer is primarily concerned addressing and routing. Logical addresses

(e.g., an IP address) are translated into physical addresses (i.e., the MAC address)

for transmission at the network layer. On the receiving side, the translation process

is reversed.

It is at the network layer where the route from the source to destination computer is

determined. Routes are determined based on packet addresses and network

conditions. Traffic control measures are also implemented at the network layer.

Common protocols that operate at this level include IP, IPX, and X.25 (frame relay).


31/38

A router functions at the Network layer.

What is the function of the Transport Layer of the OSI model?

The Transport Layer is responsible for flow control and ensuring messages are

delivered error free.

On the sending side, messages are packaged for efficient transmission at the

Transport Layer and assigned a tracking number so they can be reassembled in

proper order. On the receiving side, the packets are reassembled, checked for errors

and acknowledged. The Transport Layer performs error handling in that it ensures all

data is received in the proper sequence and without errors. If there are errors, the

data is retransmitted.

Common protocols that operate at the transport layer include TCP, UDP, SPX, and

NetBEUI.

What is the function of the Session Layer of the OSI model?

The Session Layer is responsible for establishing, maintaining, and terminating a

connection called a 'session'. A session is an exchange of messages between

computers (a dialog). Managing the session involves synchronization of user tasks

and dialog control (e.g., who transmits and for how long). Synchronization involves

the use of checkpoints in the data stream. In the event of a failure, only the data

from the last checkpoint has to be resent.

Logon, name recognition and security functions take place at the Session Layer.Winsock and NetBIOS are usually shown as functioning at the session layer.

What is the function of the Presentation Layer of the OSI model?

The Presentation Layer is responsible for data translation (formatting), compression,

and encryption.

The Presentation Layer is primarily concerned with translation; interpreting and

converting the data from various formats. For example, EBCIDIC characters might be

converted into ASCII. It is also where data is compressed for transmission and

uncompressed on receipt. Encryption techniques are implemented at the

Presentation Layer.

The redirector operates at the presentation layer by redirecting I/O operations across

the network. Common protocols that operate at the presentation layer include SMB,

NCP, & NFS.


32/38

What is the function of the Application Layer of the OSI model?

The Application Layer provides the operating system with direct access to network

services. It serves as the interface between the user and the network by providing

services that directly support user applications. Note that a user-based application

like Excel does not operate at the application layer. Instead, the application layer

provides an interface so that processes such as Excel or Word that are running onthe local machine can get access to network services (e.g., retrieving a file from a

network server).

IPCs such as RPCs and named pipes are usually shown as functioning at the session

layer.

Describe how is information sent through the OSI model.

The OSI model defines a modular approach to networking with each module, called a

layer, responsible for performing a discrete networking function.

Normally a communication request originates at the highest layer (Application

Layer). The request is passed down through the lower layers in the form of a packet

called a protocol data unit (PDU). Layers in the protocol stack communicate with

their adjacent layers via one or more Service Access Points (SAP). Each succeeding

layer in the stack adds its own information to the PDU that will be read by its

counterpart (peer) layer on the receiving system. Once the data arrives at the lower

layers, the PDU is encoded into data frames and placed onto the cable for

transmission. The data frames make their way to the receiving system and the entireprocess is reversed as the PDU makes its way up the protocol stack. As it moves up

the stack, each layer "unwraps" the PDU and receives the information from its peer

layer on the sending system.

Note that there is no by-passing of layers. Also note that it is not possible for one

layer to communicate directly with its peer layer on the other system. Peer-to-peer

layer communication takes place by adding messages to the PDU and sending it


33/38

down through the lower layers, across the cable, and then up the stack to the peer

layer on the receiving system.

What is a data frame?

A data frame, also called a frame, is an organized structure into which data is placed

for transmission across a network. The data frame defines how the data is organized(i.e., the relative position and length of fields and the type and placement of control

information)

In serial communications, data is sent in streams one bit at a time. The data frame

breaks the stream up into discrete chunks so that it can be better managed. If there

is an error in transmission, only the affected frames need to be resent, not the entire

transmission.

The structure of a data frame depends on the type of network. The original Ethernet

data frame is shown below:

Note that the original Ethernet frame is slightly different than the IEEE 802.3

standard. In fact there are at least four different types of "Ethernet" frames in use

today.

What is a packet?

A packet is chunk of information that is exchanged between computers on a network.

A packet can contain data, control codes, messages, or commands.

Packets originate at the Application Layer where the information is organized into

Protocol Data Units (PDUs). As the PDU moves down through the protocol stack,


34/38

each layer attaches its own information to the PDU until it arrives at the Data Link

Layer where it is encoded into data frames for transmission. Once it arrives at its

destination, the PDU moves up the protocol stack, each layer "unwrapping" the

information attached by its peer layer on the sending side. This is how peer protocol

layers communicate.

Although the terms are often used interchangeably, a packet is not necessarily thesame as a data frame. A packet can be segmented into several frames. Data is

transmitted in packets (PDUs) through the protocol stack and then as data frames

over the physical connection.

Topologies

What is a topology?

A topology refers to the manner in which the cable is run to individual workstations

on the network. The dictionary defines topology as: the configurations formed by the

connections between devices on a local area network (LAN) or between two or moreLANs

There are three basic network topologies (not counting variations thereon): the bus,

the star, and the ring.

It is important to make a distinction between a topology and an architecture. A

topology is concerned with the physical arrangement of the network components. In

contrast, an architecture addresses the components themselves and how a system is

structured (cable access methods, lower level protocols, topology, etc.). An example

of an architecture is 10baseT Ethernet which typically uses the start topology.

What is a bus topology?

A bus topology connects each computer (node) to a single segment trunk. A trunk

is a communication line, typically coax cable, that is referred to as the bus. The

signal travels from one end of the bus to the other. A terminator is required at each

end to absorb the signal so it does not reflect back across the bus.

In a bus topology, signals are broadcast to all stations. Each computer checks the

address on the signal (data frame) as it passes along the bus. If the signals address


35/38

matches that of the computer, the computer processes the signal. If the address

doesnt match, the computer takes no action and the signal travels on down the bus.

Only one computer can talk on a network at a time. A media access method called

CSMA/CD is used to handle the collisions that occur when two signals are placed on

the wire at the same time.

The bus topology is passive. In other words, the computers on the bus simply listen

for a signal; they are not responsible for moving the signal along.

A bus topology is normally implemented with coaxial cable

What are the advantages and disadvantages of the bus topology?

Advantages of bus topology:

Easy to implement and extend

Well suited for temporary networks that must be set up in a hurry Typically the least cheapest topology to implement

Failure of one station does not affect others

Disadvantages of bus topology:

Difficult to administer/troubleshoot

Limited cable length and number of stations

A cable break can disable the entire network; no redundancy

Maintenance costs may be higher in the long run

Performance degrades as additional computers are added

What are the key features of a star topology?

All of the stations in a star topology are connected to a central unit called a hub.

The hub offers a common connection for all stations on the network. Each station has

its own direct cable connection to the hub. In most cases, this means more cable is


36/38

required than for a bus topology. However, this makes adding or moving computers

a relatively easy task; simply plug them into a cable outlet on the wall.

If a cable is cut, it only affects the computer that was attached to it. This eliminates

the single point of failure problem associated with the bus topology. (Unless, of

course, the hub itself goes down.)

Star topologies are normally implemented using twisted pair cable, specifically

unshielded twisted pair (UTP). The star topology is probably the most common form

of network topology currently in use.

What are the advantages and disadvantages of a star topology?

Advantages of star topology:

Easy to add new stations

Easy to monitor and troubleshoot

Can accommodate different wiring

Disadvantages of ring topology:

Failure of hub cripples attached stations

More cable required

What are the key features of a ring topology?

A ring topology consists of a set of stations connected serially by cable. In other

words, its a circle or ring of computers. There are no terminated ends to the cable;

the signal travels around the circle in a clockwise direction.


37/38

Note that while this topology functions logically as ring, it is physically wired as a

star (see FAQ 9). The central connector is not called a hub but a Multistation Access

Unit or MAU. (Dont confuse a Token Ring MAU with a Media Adapter Unit which is

actually a transceiver.)

Under the ring concept, a signal is transferred sequentially via a "token" from one

station to the next. When a station wants to transmit, it "grabs" the token, attachesdata and an address to it, and then sends it around the ring. The token travels along

the ring until it reaches the destination address. The receiving computer

acknowledges receipt with a return message to the sender. The sender then releases

the token for use by another computer.

Each station on the ring has equal access but only one station can talk at a time.

In contrast to the passive topology of the bus, the ring employs an active

topology. Each station repeats or boosts the signal before passing it on to the next

station.

Rings are normally implemented using twisted pair or fiber-optic cable.

What are the advantages and disadvantages of a ring topology?

Advantages of ring topology:

Growth of system has minimal impact on performance

All stations have equal access

Disadvantages of ring topology:

Most expensive topology

Failure of one computer may impact others

Complex

Why is a ring topology wired as a star?

A ring topology has the same outward appearance as a star; all the stations are

individually connected to a central location. In the star topology the device at the

center is called a hub. In a ring topology, the center is called a MAU.

While they look the same, a closer examination reveals that the ring actually consists

of a continuous circuit. Signals are passed along the circuit and accessed by stations

in sequence. In a star topology the signal is split and sent out simultaneously to all

stations.


38/38

What is a counterrotating ring?

A counterrotating ring is a ring topology that consists of two rings transmitting in

opposite directions. The intent is to provide fault tolerance in the form of redundancy

in the event of a cable failure. If one ring goes, the data can flow across to the other

path, thereby preserving the ring.

Can you mix topologies?

Yes, you can mix various topologies on the same network.

One very common example is a large Ethernet network with multiple hubs. Usually the

hubs are located on different floors in a building or perhaps outside in another building.

Each hub is wired in the typical star configuration. However, the hubs are connected

together along a bus, typically referred to as a backbone. The backbone between hubs

might consist of fiber optic cable while the workstations are wired to each individual hub

with UTP (unshielded twisted pair) cable.

basic & advance hardware

Documents