Introduction to LAN Protocols

What Is a LAN?

A LAN is a high-speed data network that covers a relatively small geographic area. It typically connects workstations, personal computers, printers, servers, and other devices. LANs offer computer users many advantages, including shared access to devices and applications, file exchange between connected users, and communication between users via electronic mail and other applications.

LAN Protocols and the OSI Reference Model

LAN protocols function at the lowest two layers of the OSI reference model, as discussed in Chapter 1, "Internetworking Basics," between the physical layer and the data link layer. The following figure illustrates how several popular LAN protocols map to the OSI reference model.

Popular LAN Protocols Mapped to the OSI Reference Model

LAN Media-Access Methods

Media contention occurs when two or more network devices have data to send at the same time. Because multiple devices cannot talk on the network simultaneously, some type of method must be used to allow one device access to the network media at a time. This is done in two main ways: carrier sense multiple access collision detects (CSMA/CD) and token passing.

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In networks using CSMA/CD technology such as Ethernet, network devices contend for the network media. When a device has data to send, it first listens to see if any other device is currently using the network. If not, it starts sending its data. After finishing its transmission, it listens again to see if a collision occurred. A collision occurs when two devices send data simultaneously. When a collision happens, each device waits a random length of time before resending its data. In most cases, a collision will not occur again between the two devices. Because of this type of network contention, the busier a network becomes, the more collisions occur. This is why performance of Ethernet degrades rapidly as the number of devices on a single network increases.

In token-passing networks such as Token Ring and FDDI, a special network packet called a token is passed around the network from device to device. When a device has data to send, it must wait until it has the token and then sends its data. When the data transmission is complete, the token is released so that other devices may use the network media. The main advantage of token-passing networks is that they are deterministic. In other words, it is easy to calculate the maximum time that will pass before a device has the opportunity to send data. This explains the popularity of token-passing networks in some real-time environments such as factories, where machinery must be capable of communicating at a determinable interval.

For CSMA/CD networks, switches segment the network into multiple collision domains. This reduces the number of devices per network segment that must contend for the media. By creating smaller collision domains, the performance of a network can be increased significantly without requiring addressing changes.

Normally CSMA/CD networks are half-duplex, meaning that while a device sends information, it cannot receive at the time. While that device is talking, it is incapable of also listening for other traffic. This is much like a walkie-talkie. When one person wants to talk, he presses the transmit button and begins speaking. While he is talking, no one else on the same frequency can talk. When the sending person is finished, he releases the transmit button and the frequency is available to others.

When switches are introduced, full-duplex operation is possible. Full-duplex works much like a telephone—you can listen as well as talk at the same time. When a network device is attached directly to the port of a network switch, the two devices may be capable of operating in full-duplex mode. In full-duplex mode, performance can be increased, but not quite as much as some like to claim. A 100-Mbps Ethernet segment is capable of transmitting 200 Mbps of data, but only 100 Mbps can travel in one direction at a time. Because most data connections are asymmetric (with more data travelling in one direction than the other), the gain is not as great as many claim. However, full-duplex operation does increase the throughput of most applications because the network media is no longer shared. Two devices on a full-duplex connection can send data as soon as it is ready.

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Token-passing networks such as Token Ring can also benefit from network switches. In large networks, the delay between turns to transmit may be significant because the token is passed around the network.

L A N T r a n s m i s s i o n M e t h o d s

LAN data transmissions fall into three classifications: unicast, multicast, and broadcast. In each type of transmission, a single packet is sent to one or more nodes.

In a unicast transmission, a single packet is sent from the source to a destination on a network. First, the source node addresses the packet by using the address of the destination node. The package is then sent onto the network, and finally, the network passes the packet to its destination.

A multicast transmission consists of a single data packet that is copied and sent to a specific subset of nodes on the network. First, the source node addresses the packet by using a multicast address. The packet is then sent into the network, which makes copies of the packet and sends a copy to each node that is part of the multicast address.

A broadcast transmission consists of a single data packet that is copied and sent to all nodes on the network. In these types of transmissions, the source node addresses the packet by using the broadcast address. The packet is then sent on to the network, which makes copies of the packet and sends a copy to every node on the network

N e t w o r k

A network is a collection of computers and other devices that allow computer users to send and receive information to and from each other.

A network allows you to:

share information without having to carry or mail disks or paper ensure that your staff has the same software release communicate with a colleague on another campus access shared information share printers or other devices

I n t r o d u c t i o n t o N e t w o r k s

Individual workstations are sometimes connected by cable to a shared computer known as a server. The server is usually located relatively close to the individual workstations. There is either an Ethernet card or token ring board in each computer that allows it to be

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connected to the network. Both workstation and server use software that allows the computers to speak the same language.

Local Area Network (LAN) - A network (often in a department or office) intended to serve a small area. The network allows computers to work together and people to share resources.

Wide Area Network (WAN) - A network that covers a large geographic area. Individual computers can be connected directly to a Wide Area Network through a data line from the office or a modem from home without first going through a Local Area Network.

T h e T h r e e C o m p o n e n t s o f a N e t w o r k

In order to have full access to a network (local or wide) from your workstation, three components are required.

1. The first component is hardware.

Your workstation must have an Ethernet card or token ring board installed and a cable running from this card to the data jack in your office.

The data jack must be wired from your office through the building to the campus broadband. Once this hardware wiring connection is made, you have the infrastructure in place to access the network.

2. The second component is network software that recognizes the hardware and will use it. Different software is required depending on the network access you want.

o For a Local Area Network (LAN), you will need network operating system software (i.e., Novell or Windows NT). If you want to access the Wide Area Network and the Local Area Network, you will need both kinds of software. Contact your local support person to find out what kind of software you have.  

3. The third component is application software running on the Local Area Network. Examples of these would be any network version of word processors (i.e., Microsoft Word, WordPerfect), databases (Paradox, Dbase), spreadsheets (Lotus, Excel), etc. These packages are designed to provide multiple access to files and records and to lock files and records so that a particular document can be edited by only one person at a time.

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N e t w o r k P r o s a n d C o n s

P r o s

A network gives you the ability to:

share and transfer files between people on the Local Area Network (platform dependent DOS/Windows or Macintosh)

share common printers connect your computers to other sites on the WAN and at other locations access a variety of network search facilities such as Gopher, Mosaic, and World

Wide Web transfer files between systems using FTP save disk space on your hard drive by putting software on the server use E-Mail

C o n s  

A network:

needs someone on-site to provide network support needs someone to back up the server may require a longer time to start up applications

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OSI ModelOpen Standards Interconnection Model

A Layered Approach to Networking

OSI

MODEL - Basic Operation

I. Network -capable Applications produce DATA. II. Each layer in the OSI Model adds its own information to the front of the data it

receives from the layer above it. This information in front of the data is called a header and contains information specific to the protocol operating at that layer. The process of adding the header is called encapsulation. Encapsulated data is transmitted in Protocol Data Units (PDUs). There are Presentation PDU's, Session PDU's, Transport PDU's etc. Thus, PDU's from an upper layer are encapsulated inside the PDU of the layer below it.

III. PDU's are passed down through the stack of layers (called 'the stack' for short) optionally repeating the encapsulation process until they can be transmitted over the Physical layer. The physical layer is the wire connecting all the computers on the network.

IV. The OSI standards specify that a layer on host #1 speaks the same language as the same layer on host #2 or any other host on the network. Thus, all hosts can communicate via the Physical layer. This communication between layers is represented by the symbols in the diagram above. For example, the Transport layer on Host 1 should speak the same language as the Transport layer on Host 2.

V. DATA passed upwards is unencapsulated before being passed farther up (represented by the colored brackets [[[[[[ ).

VI. All information is passed down through all layers until it reaches the Physical layer (represented by the vertical red arrows).

Host #1  Host #2

DATAApplicatio

nApplicatio

nDATA 

[DATAPresentati

on Presentati

on [DATA 

[[DATA Session Session [[DATA [[[DATA Transport Transport [[[DATA [[[[DAT

ANetwork Network

[[[[DATA 

[[[[[DATA

Data Link Data Link[[[[[DAT

A [[[[[[DA

TAPhysical Physical

[[[[[[DATA 

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VII. The Physical layer chops up the PDU's and transmits the PDU's over the physical connection (copper wire, fiber optic cable, radio link etc.). The Physical layer provides the real physical connectivity between hosts over which all communication occurs (represented by ).

LAYER EXAMPLE FUNCTION/ACTIVITY

APPLICATION  Web Browser

A web browser such as Internet Explorer or Netscape provides the means for your computer to contact a web server and download several files that go together to produce a single web page. You can request a web page by typing in a web address or by clicking a link in an open web page. The web browser is an APPLICATION. The web browser application gives you the means to select a web server, contact the server and request a web page. The web browser handles the process of finding the web server , requesting the desired file and displaying all the files contained in the web page.

PRESENTATION 

HTTPYour web browser supports varous image file formats, audio files and HTML. The web browser handles PRESENTATION of the web page to the user by converting the files stored at the web server into formats used to display them on your computer. Conversion of data from one format to another is the job of the PRESENTATION layer. A web browser can convert these file formats into the local formats used on the local computer for displaying images, playing sounds and displaying text; if it cannot, it often can launch an application which does understand the format. Much of the PRESENTATION layer conversions are handled in the program you're running.

SESSIONWhen you request a web page, a the web browser opens a TCP connection to the web server. The web server sends back the web page and closes the connection. Your web browser then parses the HTML of the web page. Within the web page are instructions written in HTML tags which tell the browser where to find additional files to be displayed within the web page such as style sheets, sound files, images, movies, Flash files and applets. Your web browser automatically opens additional TCP

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Open Standards Interconnect Model - an Operational Example

connections to the web server. Each TCP connection is a SESSION.

TRANSPORT TCP

To communicate with a web server your computer must open a TCP connection to the web server and request a web page. The TCP connection breaks up theweb page into managable chunks, lables them with numbers so they can be reassembled in the correct order and TRANSPORTS the pieces across the correct SESSION.

NETWORKIP

ARP

Internet Protocol (IP) is a NETWORK layer protocol that uses unique addresses for the web server and for your computer. IP provides the means for your computer to determine whether the web server is a local computer or a computer located somewhere on the Internet. To reach a web server on the Internet, IP protocol also allows your computer to figure out how to reach the Internet web server via your default gateway. Your computer creates a message addressed to the web server with your computer's return IP address. Your computer uses ARP to figure out the physical MAC address of the default gateway and then passes the data to the NETWORK layer.

DATA LINK

ETHERNET

LLCOnce the request from your web browser has been created it is sent to the network card. Once it reaches your network card it must be converted into a message that is sent from your computer to the default gateway which will forward the message to the Internet. At the DATA LINK layer, the web request is inserted inside a network request to the default gateway.

MAC

PHYSICALCSMA/

CD The physical layer provides the means to transmit the web page request to the default gateway.

 

Transmission Media

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Data is transmitted over copper wires, fiber optic cable, radio and microwaves. The term 'media' is used to generically refer to the physical connectors, wires or devices used to plug things together.

Basic Communications Media Types

Copper o Unshielded Twisted Pair (3,5,5e,6,7) o Shielded Twisted Pair o Coaxial Cable (Thinnet, Thicknet) o Heliax

Fiber Optic o Single-mode o Multi-mode

Infrared Radio & Microwave

COPPER

Coaxial Cabling

Coaxial cabling is used in bus-style Ethernet networks. Coaxial cable consists of a copper wire core surrounded by a plastic cladding sheathed in a wire mesh. Coaxial cable comes in two sizes which are called thinnet and thicknet.

Unshielded Twisted Pair (UTP)

If you use two pairs of wires to enable two communications circuits, one for transmit, and one for receive. If you twist the wires of each pair, you can place them much closer together. There are several grades of coaxial cable with category ratings. There are Category 3 (<10 Mbps), Category 5 (10 Mbps), Category 5e (10/100 Mbps) and Category 6 (100/1000 Mbps) versions of unshielded twisted pair.

Fiber Optic

Single Mode

Single mode fiber refers to the fact that only a single wavelength (one color of light) is transmitted over the physical medium. Typically, single mode fiber is true doped fiberglass fibers wrapped in a plastic cladding. Single mode typically has much longer reach, but a larger bend radius than multi-mode.

o Dispersion Shifted o Non-Dispersion Shifted o Non-Zero Dispersion-Shifted

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Multi-Mode

Multi-mode fiber can carry multiple wavelengths, is made of special clear plastic materials and has a much smaller bend radius than single mode fiber. Multi-mode does not have as long a reach as single mode fiber.

o Step Index o Graded-Index

Infra-Red

There are many systems today using infra-red communications. This is usually a directional infrared light signal transmitted into the air and received by nearby devices. Such systems came into use in the early 90's for use with laptops, printers and later in the 90's with cameras and handhelds.

Radio and Microwave

Last of all, but certainly not least are radio and microwave signals. These are the signals we think of as being radio, television and satellite, but they are now being put to use in wireless Ethernet and Bluetooth communications technologies.

 

Circuits

Circuits

Circuit is another word for circle or loop. Since most computer technology uses electricity and electricity won't function unless the circuit is open, the concept of a circuit is important. For most discussions, the term circuit simply refers to an electrical loop. You see, electricity is a funny animal. It won't flow outwards unless there is a return path back to the source. This means that electricity really does flow in a circle, or 'circuit'. This concept is also used to describe other things like radio communication and fiber optics as well because they too are energy and behave much as electricity does.

There are several terms used in describing the behavior, conditon or state of a circuit.

OPEN vs. SHORT CIRCUITS

An OPEN circuit is a circuit where there is power all the way around and everything is working correctly.

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A SHORT is when the electricity doesn't make it all the way around. This could be because the connection is cut, disconnected or because something is cross-connecting things where it shouldn't. This is why we talk about burnt wiring having shorted out.

INTEGRATED CIRCUITS

This is the term often used to describe a computer chip. These 'gates' too are part of an electrical circuit, except they have the ability to 'swing' open or closed depending on the ammount of power applied. Modern computer chips use compounds that are called semi-conductors for these microscopic 'gates'. When a small voltage is applied, these gates block the flow of electricity, but when a larger ammount of voltage is applied, they conduct electricity. Thus, they can be 'open' (conducting electricity) or 'short' (blocking electricity) depending upon how much current they are handling. This characteristic allows an integrated circuit to store the value of zero or one, and to change state based on input.

 

WHAT YOU'RE TALKING ABOUT

Really, which term you are using, and what the word 'circuit' means is a bit dependant upon what your topic of discussion is, but they are all interrelated concepts. Besides, we're only worried about the fundamentals right now.

Frames, Packets and PDU's

Frames, Packets, Datagrams and PDU's

A large part of the use of the terms frame, packet and PDU is semantics. These terms are not completely interchangeable though. This tutorial tries to highlight the differences between them.

Frames Packets Segments Datagrams Protocol Data Units

WHAT IS A FRAME?

The term frame is most frequently used to describe a chunk of data created by network communication hardware such as a network interface cards (NIC cards) and router interfaces. Switch ports primarilly forward existing frames and don't usually create

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frames of their own (unless they are participating in Spanning Tree or dynamic VLANs etc.).

Types of Frames

There are ethernet frames, token ring frames, FDDI frames etc. A frame is simply a chunk of data, frequently with some soft of pattern of bits at the start (called a header) and bits at the end (called a trailer). Frames are created by hardware protocols that do not have separate control circuits in the physical media to which they are attached.

Contents of Frames

Frames contain hardware addresses, such as a MAC address, frame delimiters and data.

WHAT IS A PACKET?

The Request For Comments (RFC) documents frequently use the term packet to mean a stream of binary octets of data of some arbitrary length. It is typically used to describe chunks of data created by software, not by hardware. Internet Protocol (IP) creates packets. This term is NOT synonymous with the term frame even though many people make that mistake. Information that has been broken into packets is sometimes described as packetized.

Types of Packets

Internet Protocol is often described as transmitting packets.

Contents of Packets

Packets contain logical addressing information, such as an IP address, and data.

What is a Segment?

The term segment is most often used to refer to a chunk of data that has been prepared for transmission by Transmission Control Protocol (TCP). The term segment is used most often in the Request For Comments (RFC) documents that describe the TCP protocol because TCP is said to chop a 'data stream' into segments.

Types of Segments

Transmission Control Protocol is described as transmitting segments.

Contents of Segments

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Segments contain logical addressing information, such as an IP address, logical connection identifiers, such as port numbers, and data that came from a computer application. TCP guarantees delivery of the segments.

WHAT IS A DATAGRAM?

This is a more generic term that is often used in describing protocols that function at higher levels of the OSI model, ususally the network layer and up. Datagram is a less specific term than PDU.

Types of Datagrams

User Datagram Protocol is described as transmitting datagrams.

Contents of Datagrams

Datagrams contain logical addressing information, such as an IP address, logical connection identifiers, such as port numbers, and data that came from a computer application. The UDP protocol does not guarantee delivery of the datagrams.

WHAT IS A PROTOCOL DATA UNIT (PDU)?

A protocol data unit is a term used in much of the documentation and educational literature for networking technologies. It simply means a chunk of data created and/or labled by a particular protocol. TCP, UDP, IP, OSPF and RIP (and other protocols) could be said to create "protocol data units". The term is somewhat synonymous with packet or frame, especially when used in the process of discussing routing protocols or spanning tree.

Duplex vs. Simplex

SIMPLEX

Simplex communication is permanent unidirectional communication. Some of the very first serial connections between computers were simplex connections. For example, mainframes sent data to a printer and never checked to see if the printer was available or if the document printed properly since that was a human job. Simplex links are built so that the transmitter (the one talking) sends a signal and it's up to the receiving device (the listener) to figure out what was sent and to correctly do what it was told. No traffic is possible in the other direction across the same connection.

You must use connectionless protocols with simplex circuits as no acknowledgement or return traffic is possible over a simplex circuit. Satellite communication is also simplex communication. A radio signal is transmitted and it is up to the receiver to correctly

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determine what message has been sent and whether it arrived intact. Since televisions don't talk back to the satellites (yet), simplex communication works great in broadcast media such as radio, television and public announcement systems.

HALF DUPLEX

A half duplex link can communicate in only one direction, at a time. Two way communication is possible, but not simultaneously. Walkie-talkies and CB radios sort of mimic this behavior in that you cannot hear the other person if you are talking. Half-duplex connections are more common over electrical links. Since electricity won't flow unless you have a complete loop of wire, you need two pieces of wire between the two systems to form the loop. The first wire is used to transmit, the second wire is referred to as a common ground. Thus, the flow of electricity can be reversed over the transmitting wire, thereby reversing the path of communication. Electricity cannot flow in both directions simultaneously, so the link is half-duplex.

FULL DUPLEX

Full duplex communication is two-way communication achieved over a physical link that has the ability to communicate in both directions simultaneously. With most electrical, fiber optic, two-way radio and satellite links, this is usually achieved with more than one physical connection. Your telephone line contains two wires, one for transmit, the other for receive. This means you and your friend can both talk and listen at the same time.

Half or Full-Duplex is required for connection-oriented protocols such as TCP. A duplex circuit can be created by using two separate physical connections running in half duplex mode or simplex mode. Two way satellite communication is achived using two simplex connections.

 

Connection Oriented vs. Connectionless

The terms connection oriented and connectionless are descriptive words used to describe different kinds of communication.

Connection Oriented

Connection-Oriented means that when devices communicate, they perform handshaking to set up an end-to-end connection. The handshaking process may be as simple as syncrhonization such as in the transport layer protocol TCP, or as complex as negotiating communications parameters as with a modem.

Connection-Oriented systems can only work in bi-directional communications environments. To negotiate a connection, both sides must be able to communicate with each other. This will not work in a unidirectional environment.

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Connectionless

Connectionless means that no effort is made to set up a dedicated end-to-end connection.

Connectionless communication is usually achieved by transmitting information in one direction, from source to destination without checking to see if the destination is still there, or if it is prepared to receive the information. When there is little interferance, and plenty of speed available, these systems work fine. In environments where there is difficulty transmitting to the destination, information may have to be re-transmitted several times before the complete message is received.

Walkie-talkies, or Citizens Band radios are a good examples of connectionless communication. You speak into the mike, and the radio transmitter sends out your signal. If the person receiving you doesn't understand you, there's nothing his radio can do to correct things, the receiver must send you a message back to repeat your last message.

IP, UDP , ICMP, DNS, TFTP and SNMP are examples of connectionless protocols in use on the Internet.

Reliable vs. Unreliable

The terms reliable and unreliable don't refer to whether it works or not. It refers to whether something is done to guarantee

RELIABLE

End stations running reliable protocols will work together to verify the transmission of data to ensure accuracy and integrity of the data. A reliable system will set up a connection and verify that: all data transmitted is controlled in an orderly fashion, is received in the correct order and is intact. Reliable protocols work best over physical medium that loses data, and is prone to errors. The error correction, ordering and verification mechanisms require overhead in the data packets and increase the total ammount of bandwidth required to transmit data. Transmission Control Protocol (TCP) is a typical reliable protocol. TCP often usually adds an average of 42-63 bytes of overhead to datagrams. For a Telnet connection which transmits each keystroke individually, this is horribly inefficient because up to 64 bytes of data are transmitted to communicate just 1 byte of useful information.

UNRELIABLE

Unreliable protocols make no effort to set up a connection, they don't check to see if the data was received and usually don't make any provisions for recovering from errors or lost data. Unreliable protocols work best over physical medium with low loss and low error rates. User Datagram Protocol (UDP) is an example of an unreliable protocol. UDP makes no provisions for verifying whether data arrived or is intact. However, UDP adds a minimum of overhead when compared to TCP and is thus much faster for data transfers

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over high quality physical links that are high speed and exhibit little or no errors in communication.

 

Asynchronous vs. Synchronous

Most communications circuits perform functions described in the physical and data link layer of the OSI Model. There are two general strategies for communicating over a physical link: Asynchronous and Synchronous. Each has it's advantages and disadvantages.

ASYNCHRONOUS

Sending data encoded into your signal requires that the sender and receiver are both using the same enconding/decoding method, and know where to look in the signal to find data. Asynchronous systems do not send separate information to indicate the encoding or clocking information. The receiver must decide the clocking of the signal on it's own. This means that the receiver must decide where to look in the signal stream to find ones and zeroes, and decide for itself where each individual bit stops and starts. This information is not in the data in the signal sent from transmitting unit.

When the receiver of a signal carrying information has to derive how that signal is organized without consulting the transmitting device, it is called asynchronous communication. In short, the two ends do not synchronize the connection before communicating. Asynchronous communication is more efficient when there is low loss and low error rates over the transmission medium because no data is not retransmitted. In addition, there is no time spent at the beginning of setting up the connection. One side simply transmits, and the other does it's best to receive.

EXAMPLES: Asynchronous communication is used on RS-232 based serial devices such as on an IBM-compatible computer's COM 1, 2, 3, 4 ports. Asynchronous Transfer Mode (ATM) also uses this means of communication. Your PS2 ports on your computer also use this method. This is the method is also used to communicate with an external modem. Asynchronous communication is also used for things like your computer's keyboard and mouse.

Think of asynchronous as a faster means of connecting, but less reliable.

SYNCHRONOUS

Synchronous systems negotiate the connection at the data-link level before communication begins. Basic synchronous systems will synchronize two clocks before

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transmission, and reset their numeric counters for errors etc. More advanced systems may negotiate things like error correction and compression.

It is possible to have both sides try to synchronize the connection at the same time. Usually, there is a process to decide which end should be in control. Both sides can go through a lengthy negotiation cycle where they exchange communications parameters and status information. Once a connection is established, the transmitter sends out a signal, and the receiver sends back data regarding that transmission, and what it received. This takes longer on low error-rate lines, but is highly efficient in systems where the transmission medium itself (an electric wire, radio signal or laser beam) is not particularly reliable.

Serial vs. Parallel

The two most basic types of communication are serial and parallel. They are so common that even the cabling bears the name serial cable and parallel cable. Since electricity behaves according to the laws of physics, it is impossible to get the electrical signal to go any faster. There are two ways to get the data from one place to the other faster. The first is to squish the data bits tighter together (leave less distance between them when they travel down the wire). The second way is to transmit more bits simultaneously.

Keep in mind that the information below is very general and not exactly correct from an engineering standpoint. We're just focusing on getting you to understand concepts here.

SERIAL

When information is sent across one wire, one data bit at a time, its called serial. Every computer on the face of the earth has some form of serial communications connector on it, whether internally or externally. Most people are familliar with the 'D' shaped 9-pin connector on the back of thier computer. This is a serial connector. The typical 9-pin 'D' shaped connector on the back of your computer uses 2 loops of wire (1 in each direction) for data communication, plus additional wires to control the flow of information. However, in any given direction, data is still flowing over a single wire.

PARALLEL

Instead of squishing bits together, bits are sent over more wires simultaneously. In the case of an 25-pin parallel port, you have eight data-carrying wires so that eight bits can be sent simultaneously. Because there are 8 wires to carry the data, the data finishes being transferred eight times faster than a serial connection.

 

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Local Area Network Topologies

Local Area Networks (LANs) use one of the following physical layout designs. These designs are referred to as 'topologies'.

Topology Types

Bus(Logical Ethernet)

Hub and Spoke (Star)(Physical Ethernet)

Hybrid (Bus & Star) Ethernet

Point To Point / Daisy Chaining

Serial

Point to Multipoint Frame Relay

RingFDDI, Token Ring

BUS TOPOLOGY

Networks employing a bus topology use a common physical connection for communication. That means the physical media is shared between stations. When one station transmits on the bus, all devices hear the transmission. If more than one device transmits at the same time, the two transmissions will collide with each other and both transmissions will destroy each other.

When two or more of these devices attempts to access the network bus at the same time, some method must be used to prevent a collision (CSMA/CD). Historically, bus networks used coaxial cable as their medium of transmission. Token Bus, Ethernet (Thinnet and Thicknet) are common examples of bus topologies. Although some installations of Ethernet using coaxial cable still exist, all

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modern installations now use a hub and spoke or star topology.

HUB AND SPOKE (STAR)

Please note that this is not called a hub and spoke design because there is a network hub in the drawing. This drawing is to show how a star or hub and spoke network resembles the hub and spokes of a wheel. The Hub and Spoke topology refers to a network topology where there is a central connection point to which multiple devices are connected.

A network hub device is not the only device usable in this configuration. A switch may also be used and in some cases, a router. Ethernet utilizing twisted pair still considered a bus architecture from a logical standpoint; however, physically, an Ethernet network can be physically wired as a hub and spoke model.

RING

Ring topologies are similar to bus topologies, except they transmit in one direction only from station to station. Typically, a ring architecture will use separate physical ports and wires for transmit and receive.

Token Ring is one example of a network technology that uses a ring topology.

POINT TO POINT (Daisy Chaining)

Point to Point topologies are simplest and most straightforward. You must picture them as a chain of devices and another name for this type of connectivity is called daisy chaining. Most computers can 'daisy chain' a series of serial devices from one of its serial ports. Networks of routers are often configured as

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point-to-point topologies.

 

POINT TO MULTIPOINT

This is not quite the same as a hub and spoke configuration. In a hub and spoke topology, all transmissions from all devices pass through the hub--the hub broadcasts all communication from any single device to all other devices connected to it.

In a multipoint topology the hub can send to one or more systems based on an address. Frame Relay is the most common technology to implement this scheme, and it is typically used as a WAN technology. All the remote connection points are connected to a single Frame Relay switch or router port, and communication between sites is managed by that central point. In hub and spoke, all spokes or only one spoke hears a given transmission. In point to multipoint, any number of remote stations can be accessed.

Logical Network Topologies

Peer-to-Peer

A peer-to-peer network is composed of two or more self-sufficient computers. Each computer handles all functions, logging in, storage, providing a user interface etc. The computers on a peer-to-peer network can communicate, but do not need the resources or services available from the other computers on the network. Peer-to-peer is the opposite of the client-server logical network model.

A Microsoft Windows Workgroup is one example of a peer-to-peer network. UNIX servers running as stand-alone systems are also a peer-to-peer network. Logins, services and files are local to the computer. You can only access resources on other peer computers if you have logins on the peer computers.

Client - Server

The simplest client-server network is composed of a server and one or more clients. The server provides a service that the client computer needs. Clients connect to the server across the network in order to access the service. A server can be a piece of software running on a computer, or it can be the computer itself.

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One of the simplest examples of client-server is a File Transfer Protocol (FTP) session. File Transfer Protocol (FTP) is a protocol and service that allows your computer to get or put files to a second computer using a network connection. A computer running FTP software opens a session to an FTP server to download or upload a file. The FTP server is providing file storage services over the network. Because it is providing file storage services, it is said to be a 'file server'. A client software application is required to access the FTP service running on the file server.

Most computer networks today control logins on all machines from a centralized logon server. When you sit down to a computer and type in your username and password, your username and password are sent by the computer to the logon server. UNIX servers use NIS, NIS+ or LDAP to provide these login services. Microsoft Windows comptuers use Active Directory and Windows Logon and/or an LDAP client.

Users on a client-server network will usually only need one login to access resources on the network.

Distributed Services

Computer networks using distributed services provide those services to client computers, but not from a centralized server. The services are running on more than one computer and some or all of the functions provided by the service are provided by more than one server.

The simplest example of a distributed service is Domain Name Service (DNS) which performs the function of turning human-understandable names into computer numbers called IP addresses. Whenever you browse a web page, your computer uses DNS. Your computer sends a DNS request to your local DNS server. That local server will then go to a remote server on the Internet called a "DNS Root Server" to begin the lookup process. This Root Server will then direct your local DNS server to the owner of the domain name the website is a part of. Thus, there are at least three DNS servers involved in the process of finding and providing the IP address of the website you intended to browse. Your local DNS server provides the query functions and asks other servers for information. The Root DNS server tells your local DNS server where to find an answer. The DNS server that 'owns' the domain of the website you are trying to browse tells your local DNS server the correct IP address. Your computer stores that IP address in its own local DNS cache. Thus, DNS is a distributed service that runs everywhere, but no one computer can do the job by itself.

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