local area networks area networks new.pdf · differentiate lan physical and logical topologies ......

Local Area Networks

2

Objectives

Describe how different forms of LANs originated and how they evolved

Differentiate LAN physical and logical topologies

Identify LAN addressing issues and the role of MAC addresses

Describe the role of LAN segmentation and its impact on performance

Compare and contrast Ethernet, Token Ring, and FDDI LAN models

Describe the role of VLANs and LANE configurations in networking schemes

3

Overview

LAN decisions (configuration, speed, O/S, access, etc.) are made by businesses (LAN owners)

WAN links are owned by public carriers

“Despite the traditional classification

of LANs by span, a more relevant

classification is link ownership.”

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Overview

Two basic LAN classifications Dedicated-server (server-centric or client-server)

Servers function only as servers with specialized functions (printing, database, websites, etc.)

One server must be a file-server

Used by vast majority of businesses!

Peer-to-peer Each station is a functional equal of every other station

Any computer can access files from any other computer

Any computer can be a server (i.e., take on special functions)

5

LAN Hardware and Software

“LAN hardware and software are the concern

of the two lowest layers of the

Open Systems Interconnection Model (OSI)

and TCP/IP model architectures:

The two lower layers handle all the protocols and

specifications needed to run the LAN

Higher layers get involved only when interconnecting LANs

Layer 1, the physical layer, and

Layer 2, the data link layer.”

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Network Interface Card (NIC) Hardware/firmware combination containing almost all

of the LAN protocols

Contains port(s) to accommodate medium (e.g., CAT 6 copper, fiber, etc.)

Can provide device LAN address

Required for each node on the LAN; a node is a device Directly connected to the LAN

Directly addressable by the LAN

A device must have a NIC to be a LAN node

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Medium Access Control (MAC) address Physical address—different for each NIC

Defined (and assigned) by IEEE

Hard-coded by manufacturer

Flat addresses contain no location or sequencing

48 bits long First 24 bits—IEEE Organizationally Unique Identifier (OUI)

Second 24 bits—manufacturer ID [224 = 16,777,216 addresses]

Stored in read-only memory (ROM) on the NIC

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Network operating system (NOS)

Mediates between

LAN workstations

LAN resources

LAN processes

Computer operating system (OS)

Mediates individual workstation

resources

Full-blown NOS:

MS Windows Server

Novell Netware

Partial NOS:

(newer) Windows

Mac

UNIX

Linux

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NOS functions Contains redirector that determines whether actions

are local (for workstation) or network

Incorporates LAN protocols

Enables LAN software to use LAN hardware

Controls server operations

Manages network storage, disk access, and memory

Provides LAN management tools for administrators

The Channel Allocation Problem

• Static Channel Allocation in LANs and MANs

• Dynamic Channel Allocation in LANs and MANs

The traditional (phone company) way of allocating a single channel is Frequency

Division Multiplexing. (See Figure) FDM works fine for limited and fixed number

of users.

Now divide this channel into N subchannels, each with capacity C/N.

Inefficient to divide into fixed number of chunks. May not all be used, or may

need more. Doesn't handle burstiness.

Static Channel Allocation in LANs and MANs

Dynamic Channel Allocation in LANs and MANs

1. Station Model.

2. Single Channel Assumption.

3. Collision Assumption.

4. (a) Continuous Time.(b) Slotted Time.

5. (a) Carrier Sense.(b) No Carrier Sense.

Possible underlying assumptions include:

Station Model -

Assumes that each of N "stations" (packet generators) independently

produce frames. The probability of producing a packet in the interval dt

is dt where is the constant arrival rate. That station generates no

new frame until that previous one is transmitted.

Single Channel Assumption -

There's only one channel; all stations are equivalent and can send and

receive on that channel.

Collision Assumption -

If two frames overlap in any way time-wise, then that's a collision. Any

collision is an error, and both frames must be retransmitted. Collisions

are the only possible error.

Dynamic Channel Allocation in LANs and MANs (2)

Continuous Time -

There's no "big clock in the sky" governing transmission.

Time is not in discrete chunks.

Slotted Time -

Alternatively, frame transmissions always begin at the start

of a time slot. Any station can transmit in any slot (with a

possible collision.)

Carrier Sense -

Stations can tell a channel is busy before they try it. NOTE -

this doesn't stop collisions.

Dynamic Channel Allocation in LANs and MANs (3)

This is where the sender listens before ejecting something on the wire.

Collision occurs when a station hears something other than what it sent.

PERSISTENT AND NONPERSISTENT CSMA:

1-persistent CSMA

Station listens. If channel idle, it transmits. If collision, wait a random

time and try again. If channel busy, wait until idle.

If station wants to send AND channel == idle then do send.

Success here depends on transmission time - how long after the

channel is sensed as idle will it stay idle (there might in fact be someone

else's request on the way.)

Carrier Sense Multiple Access Protocols

Nonpersistent CSMA (equivalent to 0-persistent CSMA)

Same as above EXCEPT, when channel is found to be busy,

don't keep monitoring to find THE instant when it becomes

free. Instead, wait a random time and then sense again.

Leads to

1) better utilization and

2) longer delays than 1 - persistent. (why?)

Carrier Sense Multiple Access Protocols(2)


p-persistent CSMA [For slotted channels.]

If ready to send AND channel == idle

then send with probability p,

and

with probability q = 1 - p defers to the next

slot.

Interpret the chart for these shown in the

Figure.

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for

various random access protocols.

CSMA WITH COLLISIONS DETECTION:

CSMA/CD - used with LANs.

When a station detects a collision, it stops sending, even if

in mid-frame. Waits a random time and then tries again.

What is contention interval -- how long must station wait

after it sends until it knows it got control of the channel?

It's twice the time to travel to the furthest station.


Ethernet MAC Sublayer Protocol

Collision detection can take as long as 2 .

CSMA with Collision Detection

CSMA/CD can be in one of three states:

contention, transmission, or idle.

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Ethernet: The Once and Future King

LAN protocols are designed for best effort delivery Data frames have a “good chance” of surviving

Receiver determines whether a frame has errors

Higher-layer protocols might provide more precise error detection and recovery

LANs do not guarantee error-free delivery!

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Ethernet—802.3 Was not the first [Arcnet was first in 1977]

Is currently the most widely installed

Is considered a contention protocol (stations contend for access)

Uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Station desiring access must listen

If a transmission is detected—carrier sensed—station waits

If no transmission is detected—bus is idle—station transmits

Simultaneous transmissions cause collisions

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Fig 9.1

CSMA/CD

If a collision

is detected

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The Ethernet frame Max frame size = 1,518 bytes [data = 1,500 bytes]

Min frame size = 64 bytes [data = 46 bytes]

5 data fields Destination address

Source address

Network protocol or data length (if < 1,518)

Data PDU (higher layer data)

Frame check sequence (error detection)

2 synchronization fields Preamble for frame synchronization

Start frame delimiter indicating frame start for receiver

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The Ethernet frame Max frame size = 1,518 bytes (data = 1,500 bytes)

Min frame size = 64 bytes (data = 46 bytes)

DataSynchronization

Fig 9.2

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Ethernet collision window (“slot time”) Length of time for frame to travel

from one end of the LAN to the other

Requires frame limits to work (64 byte min frame size)

For 10 Mbps

Key factors

Bit rate—time for a station to transmit a complete frame

Propagation speed—time for 1 bit to travel to the end of the bus

512 bit times = 512 bits/8 bits per byte = 64 bytes

Max length = 500 m

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Improving Traditional Ethernet

Bus and hub comparison

Fig 9.3

bus hub

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Bus and star cabling comparison (8 nodes)

Fig 9.4

bus

star

Node

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Ethernet (with media type indicators)

Advantages Reliability improved—bus disruptions don’t take down LAN

Management improved—simple network management protocol (SNMP) installed on hub

Maintenance improved—easier to add workstations

Disadvantages Physical stars require more cabling

Hub becomes single point of failure

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Thicknet

10BASE5 10 Mbps data rate

Baseband signaling over thick coaxial copper

Max segment length: 500 m

Up to 100 nodes

Up to 4 repeaters

Physical bus

Connected by medium attachment unit (MAU)

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Thinnet

10Base2 10 Mbps data rate

Baseband signaling over pencil-thin coaxial copper


Up to 30 nodes

Up to 4 repeaters

Physical bus

Connected by NICs (MAU function moved to NICs)

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Ethernet (with media type indicators)

10BASE-T 10 Mbps data rate

Baseband signaling over twisted pair copper


Node limits dictated by ports available on hubs

Hubs could be repeaters (“active hubs”)

Physical star operating as a logical bus

Connected by hubs

Ethernet Cabling

The most common kinds of Ethernet cabling.

This is a 1-persistent CSMA/CD LAN. Originated in Aloha.

WIRES:

Ethernet Cabling (2)

Three kinds of Ethernet cabling.

(a) 10Base5, (b) 10Base2, (c) 10Base-T.


Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.

Repeaters - Multiple cables can be connected. From software point, a

repeater is transparent.


(a) Binary encoding, (b) Manchester encoding,

(c) Differential Manchester encoding.

After a collision, station waits 0 or 1 slot. If it collides

again while doing this send, it picks a time of 0,1,2,3

slots. If again it collides the wait is 0 to 23 -1 times.

Max time is 210 -1 (or equal to 10 collisions.) After 16

collisions, an error is reported.

Slot is determined by the worst case times; 500 meters

* 4 repeaters = 512 bit times = 51.2 microseconds.

Algorithm adapts to number of stations.

Binary Exponential Backoff Algorithm

Uses 10Base-T to each of the hosts. And a high speed backplane

between the connectors. Works because the assumption is that

many requests can be routed within the switch. Relieves congestion

on the hub.

Routing -

Local (on-switch) destinations are sent there directly. Off-switch are

sent to the backplane.

Collision Detection -

The connections on the switch

form their own LAN and do

collision handling as we've just

seen. The switch buffers the

transmission and ensures no

collisions occur.

Switched Ethernet

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Replacing the hub with a switch

How it works Switch connects workstations in pairs

Will not connect transmitting stations to a busy one

LAN no longer operates as a bus—no contention!

Advantages No collisions—each station has own link to switch

Compatibility with CSMA/CD is maintained

Multiple workstations can transmit simultaneously

Simple to upgrade—replace hub with switch

41


Fast Ethernet

100BASE-TX 100 Mbps data rate

Baseband signaling, cat 5 UTP

Max segment length: 100 m (span limit: 250m)

Node limits dictated by ports available on hubs

Hubs could be repeaters (“active hubs”)

Physical star operating as a logical bus

Connected by switches

100BASE-FX is

multimode fiber-optic version

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Fast Ethernet (100BASE-TX)

Advantages Speed boost (10 Mbps to 100 Mbps)

Backward compatible—10/100 Mbps on same LAN

Easy device upgrade

Upgrade switch

With CAT 5 UTP or STP, swap NICs

Disadvantages Maximum segment length is 100 m [total span limit: 250 m]

Switch is single point of failure

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Gigabit Ethernet

1000BASE-T

1000 Mbps data rate

Baseband signaling over cat 5 UTP

Max segment length: 100 m (span limit: 100 m)

Min frame size: 512 bytes (up from 64 byte)

Connected by switches

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Gigabit Ethernet (other classifications)

1000BASE-X 1000BASE-CX

Copper over twinax or quad cabling

Max span: 25 m

1000BASE-LX Fiber-optic (1,300 nm signals)

Max span: 550 m (multimode)

Max span:3,000 m (single-mode)

1000BASE-SX Fiber-optic (850 nm signals)

Max span: 550 m (multimode)

Max span 3,000 m (single-mode)

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10 Gigabit Ethernet

10GBASE-X 10 Gbps data rate

Full duplex signaling over fiber-optic media

7 versions 10GBASE-SR (short-range) and –SW (short-wavelength)

10GBASE-LR (long-range) and –LW (long-wavelength)

10GBASE-ER (extended-range) and –EW (extra-long wavelength)

10GBASE-LX4 (carries signals on 4 light wavelengths)

IEEE Standard 802.4:Token BusNeed a mechanism to handle real-time, deterministic requirements.

802.3 could contend forever and this is often not acceptable.

A ring, with stations taking turns is deterministic. Uses logical ring

on linear cable.

Mechanism -

All stations numbered; station knows # of its

neighbors.

A token, required in order to send, is initialized by the

highest number station.

A station, receiving the token, does a send if it has a

request, then sends the token to its logical (not

necessarily physical) neighbor.

Activation -

Stations can come and go on the bus, without breaking

mechanism.

Cabling -

Uses 75 ohm coax. Speeds are 1, 5, 10 Mbps.

IEEE Standard 802.4:Token Bus

A Token bus

Station has 4 possible priorities, 0, 2, 4, 6; station maintains 4 queues

for requests.

Within each station,

Token comes first to priority 6 queue. Sends occur

until nothing to send OR timer expires.

Token goes next to priority 4 queue. Sends occur

until nothing to send OR timer expires.

And so on . . . .

Proper setting of the various timers ensures that high priority

requests happen first.

The Token Bus MAC Sublayer Protocol

The frame format. Fields are:

Preamble - used to synchronize receiver clock.

Start/End Delimiter - contains a non-data (illegal) Manchester Encoding.

Frame control - shows control or data. shows priority of datapackets. flag requiring ACK from receiver. showstype of control frame (more later).

Destination Address - (same as 802.3) - usually 6 bytes.

Source Address - (same as 802.3) - usually 6 bytes.

Data - BIG - 8182 or 8174 bytes (note no length field)

Checksum - (Same as 802.3)

The Token Bus MAC Sublayer Protocol

The 802.4 frame format

50

Token Ring

Token ring – 802.5 Patented by Olof Söderblom in the late 60s—licensed to IBM

Practically no new installations

Speeds typically 4/16 Mbps (100 Mbps standard exists)

A round-robin protocol (stations take turns in order)

Most commonly configured as a physical star/logical ring

Each station is connected to a multistation access unit (MAU)

Logically, each station is connected point to point to a predecessor node and a successor node

A small packet (token) controls medium access

A station can transmit data only when it has the token

Only one token is in circulation at any time

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Token Ring

Fig 9.7

Not broadcast but point to point.

All digital rather than analog (such as used by 802.3 for collision detection.)

Chosen by IBM for its LAN; included by IEEE as Token Ring.

Calculate the number of bits on the ring at any one time:

At R Mbps, a bit is emitted every 1/R microseconds (µsecs).

At a speed of 200 m/µsec, each bit occupies 200/R meters ofthe ring. So a 1 Mbps ring, with circumference 1000 metershas only 5 bits on it at any one time.

In addition, there's a 1 bit delay at each station. (Data bit can be modifiedbefore being forwarded.)

Token is 3 bytes. Must be sufficient delay on the ring so that the whole token is

there. Why?? Stations may be powered down, etc. - no guarantee that stations are

adding delay. So may need to add artificial delay.

IEEE Standard 802.5: Token Ring


a) Ring network b ) Listen mode c) Transmit mode

• Arbitration -

Must hold the token in order to transmit.

• Listen mode -

Input just copied to output.

• Transmit mode -

Seize the token and put own data on ring. As sender's data comes back

around, it removes data. At end of transmission, stick token back on.

Receiver can ACK receipt by flipping a bit on end of packet.

Efficiency is excellent: At high usage, with many stations transmitting, they

get token one after the other.



Four stations connected via a wire center

Wires -

Shielded twisted pair/ 1 or 4 Mbps.

Differential Manchester encoding.

Reliability – Star-Shaped Ring --

56

Token Ring

The token ring frame

Three frame types

Token frame

Data frame

Control frame

Data and control frames have the same

format

57

Token Ring

The token ring frame

Fig 9.8

data and control frames

have the same format

The Token Ring MAC Sublayer ProtocolFrame Structure Components -

SFD, EFD Delimiters - have illegal encoding so not confused as data.

AC Access control, containing bits for:

The token bit - flip this bit and it’s a data preamble

Monitor bit,

Priority bits,

Reservation bits

Frame control Provides numerous control options.

Source/Destination addresses/checksum

same as 802.3 & 802.4.

The Token Ring MAC Sublayer Protocol

Frame Structure Components -Frame status

A bit - the intended receiver saw the packet

C bit - the receiver copied the packet into its buffers. Serves as

acknowledgment.

Priorities -Token gives priority of that token - a sender must wait for token

of correct priority. The access control byte (of the token or data

frame) has reservation bits. As frame goes by, a requester can

say it wants the token at that priority the next time around.

The Token Ring MAC Sublayer Protocol

61

LAN Segmentation

LAN segmentation

Goal Reduce congestion by grouping stations according to traffic

Approach Segments include workstations that often communicate with

On another

Common data source

Common resource

Each segment becomes a LAN in itself

Segments can later be interconnected to share resources

40 stations : 10 Mbps LAN 10 Mbps/40 = 250 Kbps

(2) 20 stations: 10 Mbps LAN `10 Mbps/20 = 500 Kbps

Segmentation in action

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LAN Segmentation

Bridge operation and bridge types What is a bridge?

A traffic monitor between two LANs

A filter to keep local traffic from crossing between LANs

A segment device that keeps local traffic off other LANs

Bridge address tables Track device addresses on both sides

How they are created distinguishes types of bridges

Types of bridges Manual bridge—addresses are manually loaded into a table

Learning bridge—automatically creates its own tables

Bridge can flood both sides of a LAN to learn what devices respond

Bridge can learn device addresses when new source addresses appear in frames

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LAN Segmentation

Using backbones to interconnect LANs

Instead of directly connecting LANs and bridges,

all interLAN links traverse the backbone

Backbones may be

Linked to LANs by bridges

Based on routers

LANs themselves

LAN stations connect to the backbone via

their LAN hubs or switches

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LAN Segmentation

Bridged backbone

Each server has

one port connection to

the backbone bus

One port connection to

its LAN switch

Bridge forwards only to

the bus frames from its

LAN destined for a

nonlocal LANFig 9.11

65

LAN Segmentation

Star-wired (collapsed) backbone

Each LAN switch is connected to a

router that sends frames according

to frame destination addresses

Backbone is considered to be

shrunk (collapsed) into the router

itself

Fig 9.11

If the router fails

the backbone fails

66

LAN Segmentation

Backbone LAN

Same as star-wired backbone except a LAN takes the place of

a router

Each connected LAN becomes a node on the backbone LAN

Fig 9.12

67

LAN Segmentation

FDDI (Fiber Distributed Data Interface)

Token-passing protocol

100 Mbps

Station separation up to 2 Km (1.25 mi)

on single-mode fiber

Originally used as MAN backbone

Superseded by higher speed Ethernet

68

LAN Segmentation

FDDI (Fiber Distributed Data Interface)

Fig 9.13

69

VLANs

Virtual LAN (VLAN)

802.3ac

Grouped by

Station characteristics

Switch characteristics

Frame protocols

Physical LAN memberships or links

are not changed

70

VLANs

Virtual LAN (VLAN) Benefits

Security

Traffic reduction

Flexibility

Cost savings

Caveats

Ease in setup does not presume well-designed

Be wary of too many members on too many physical LANs

Stations with occasional communications should not be members

Problems

Congestion

Network management difficulty

71

VLANs

Fig 9.14

72

VLANs

Attribute-based VLANs Configured by creating list mappings (access lists)

Switches discern which ports belong to which VLANs

Membership can be assigned Mostly manual

Partly manual

Mostly automatic

Protocol-based VLANs Membership determined on a frame-by-frame basis

Participation based on individual transmissions instead of port assignment

73

VLANs

Tagged Ethernet

Enables workstations to belong to several VLANS at same time

First 20 bytes are same as Ethernet frame

Four tag bytes are inserted between the source address and the

type/length field

Fig 9.15

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