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Fall 2005

LAN Technologies and Network Topology

Qutaibah MalluhiComputer Science and Engineering

DepartmentQatar University

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Agenda

LANs Topologies Bit Encoding Media Access Control Examples of LANs

– Ethernet emphasized

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Network Classification Terminology

Network technologies classified into three broad categories– Local Area Network (LAN)

– Metropolitan Area Network (MAN)

– Wide Area Network (WAN)

LAN and WAN most widely deployed

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Point-to-Point Networks

Computers connected by communication channels that each connect exactly two computers

Point-to-point network – Allows flexibility in communication hardware, packet

formats, etc. – Provides security and privacy because communication

channel is not shared Number of wires grows as square

of number of computers For N hosts,

No. Connections = (N2-N)/2 Adding a new computer requires

N - 1 new connections

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Local Area Network Developed in the 60’s and 70’s Interconnect a wide range of devices over short

distances, e.g., within the same floor, building or campus (typically up to 10 km diameter).

Key idea - reduce number of connections by sharing connections among many computers – Computers take turns – TDM– Reduce cost but ... attached computers compete for

use of shared connection– Must include techniques for synchronizing use

In practice – Local communication almost exclusively LAN – Long distance almost exclusively point-to-point

» E.g., ATM

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LAN (Cont.)

Typically, all hosts on a LAN share a common medium.– operate on a broadcast mode

High throughput, low delay Many LAN technologies and standards exist

– E.g., Ethernet and FDDI are popular ones– LAN standards are collectively known as the IEEE 802

standards– Must include techniques for synchronizing hosts

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Significance of LAN and Locality

LANs are most popular types of networks WHY?– Economical– Principle of locality

Principle of locality of reference helps predict computer communication patterns: – Spatial (or physical) locality of reference

» computers likely to communicate with other computers that are located nearby

– Temporal locality of reference » computers are likely to communicate with the same

computers repeatedly

Thus - LANs are effective because of spatial locality of reference, and temporal locality of reference

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Different LAN Types

LAN parameters– topology– shared medium (twisted pair, coaxial, fiber)– medium access control technique

» governs the access to the LAN transmission medium.

Topology– Specifies general “shape” of a network– Handful of broad categories– Three most popular:

» Star » Ring » Bus

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Star Topology

Central component of network known as hub Hub: repeats incoming signal to all outgoing links Each computer has separate connection to hub

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Star topology in practice

Previous figure is idealized

In practice:

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Ring Topology

No central facility

Connections go directly from one computer to another

In practice, there is a short connector cable from the computer to the ring

Fault tolerance with two rings

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Bus Topology

Single cable connects all computers Each computer has connector to shared cable Computers must synchronize and allow only one

computer to transmit at a time

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Why Multiple Topologies?

Each has advantages and disadvantages: – Ring ease synchronization; may be disabled if any

cable is cut – Star easier to manage and more robust; requires more

cables – Bus requires fewer cables; may be disabled if cable is

cut Industry is settling on star topology as most

widely used

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Ethernet Widely used LAN technology

– Invented at Xerox PARC (Palo Alto Research Center) in 1970s – Defined in a standard by Xerox, Intel and Digital - DIX

standard – Standard now managed by IEEE - defines formats, voltages,

cable lengths, ... Uses bus topology

– Single coax cable - the ether – Multiple computers connect to the ether

One Ethernet cable is sometimes called a segment – Limited to 500 meters in length – Minimum separation between connections is 3 meters

Speed– Originally 3Mbps (Obsolete)– Popular standard is 10Mbps (Classic Ethernet)– Fast Ethernet operates at 100Mbps – Now Gigabit and 10 Gigabit

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Ethernet Operation

One station transmits at a time Signal propagates across entire cable All stations receive transmission

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Manchester Encoding

Ethernet standard uses Manchester encoding Uses rising and falling edges to encode data

– Edge triggered hardware Falling edge to encode 0, rising edge to encode 1 Use preamble for synchronization Preamble consists of 64 alternating 1’s and 0’s

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Characteristics of Good Encoding Scheme

Self synchronization– Digital signal includes information about bit boundaries– Transitions at the beginning, middle or end of the signal are

used– A signal with the same voltage level for a long period of time

is bad No DC Component

– DC Component: Useless extra energy residing on the line– Signal with only positive voltage (unipolar) has a DC

component– Signal with positive and negative voltages (polar) reduce the

DC component Bit rate versus pulse rate (baud rate)

The first two (Self-Synch and No DC Comp) are provided by Manchester encoding

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Manchester Encoding Efficiency

Classical Ethernet uses Manchester encoding– Up to two signal transitions per-bit– 1 Gbps Requires baud rate of 2 G– Waste of bandwidth

Newer Faster networks use a more efficient block encoding schemes (e.g. 4b/5b encoding for fast Ethernet and 8b/10b encoding for Gb Ethernet)

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Block Coding Schemes

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Careful Selection of Valid Codes Consider 4b/5b coding Select a subset of the 5-bit

codes such that no more than three consecutive zeros are sent.

Use a coding scheme that does not waste bandwidth (unlike Manchester Encoding)

Advantages– Synchronization

» No same voltage level for long period of time

– Error detection» Errors may generate a non-

valid 5 bit code– Higher bit rate (than

Manchester)

1101011010110001010010100100

1101111011110101011010110101

1110011100111001110011100110

1110111101111101111011110111

1011110111101110101101010011

1010010100

0100101001

1111011110

Code

101101011010100010

100111001110010001

10010100101000 0000

CodeDataData

1101011010110001010010100100

1101111011110101011010110101

1110011100111001110011100110

1110111101111101111011110111

1011110111101110101101010011

1010010100

0100101001

1111011110

Code

101101011010100010

100111001110010001

10010100101000 0000

CodeDataData

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CSMA/CD

No central control managing when computers transmit on ether

Ethernet employs CSMA to coordinate transmission among multiple attached computers

Carrier Sense with Multiple Access – Multiple access

» multiple computers are attached to shared media» each uses same access algorithm

– Carrier sense » computer wanting to transmit tests the media for carrier

before transmitting

Simultaneous transmission possible Collision

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Carrier-Sensing Effects

10 Mbps

Propagation delay

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CSMA/CD (cont’d) Even with CSMA, two computers may transmit simultaneously

– Both check ether at same time, find it idle, and begin transmitting – Window for transmission depends on speed of propagation in ether

Signals from two computers will interfere with each other – Overlapping frames is called a collision – Data from both frames is garbled

Ethernet uses CSMA + Collision Detection (CD) to coordinate transmission– Ethernet interfaces include hardware to detect transmission

» Monitor outgoing signal » Garbled signal is interpreted as a collision

– Listen to medium during transmission– Detect whether another station’s signal interferes– Back off from interference (wait random amount of time) and try

again

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Recovery from Collision

Computer that detects a collision sends special signal to force all other interfaces to detect collision

Computer then waits for ether to be idle before transmitting – If both computers wait same length of time, frames will

collide again – Standard specifies maximum delay, and both

computers choose random delay less than maximum After waiting, computers use carrier sense to

avoid subsequent collision

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Exponential Backoff

Even with random delays, collisions may occur Especially likely with busy segments Computers double delay with each subsequent

collision Reduces likelihood of sequence of collisions Also called binary backoff

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CSMA/CD Algorithm

Wait whileMedium is busy

to small value

Double

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Wireless LAN

Use radio signals at 900 MHz Data rate of 2 Mbps Shared medium - radio instead of coax In contrast with wired LAN, not all participants

may be able to reach each other – Low signal strength – Propagation blocked by walls, etc.

Can't depend on CD; not all participants may hear

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CSMA/CA

CSMA: Sense before sending. Only send if idle for IFS (Inter-Frame Space). O/W backoff.

Wireless uses collision avoidance rather than collision detection – Transmitting computer sends very short message to receiver – Receiver responds with short message reserving slot for

transmitter Response from receiver is broadcast so all potential

transmitters receive reservation

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Handling Collisions

Receiver may receive simultaneous requests – Results in collision at receiver – Both requests are lost – Neither transmitter receives reservation; both use

backoff and retry Receiver may receive closely spaced requests

– Selects one – Selected transmitter sends message – Transmitter not selected uses backoff and retries

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Physical and Logical Ring Synchronization

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Token Ring

Many LAN technologies that use ring topology use token passing for synchronized access to the ring

Ring itself is treated as a single, shared communication medium

Bits pass from transmitter, past other computers and are copied by destination

Hardware must be designed to pass token even if attached computer is powered down

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Token Passing Synchronization

Used with ring topology Guarantees fair access: IEEE 802.5 standards Token: Special small (a few bits) reserved (can

not appear in data) message Only computer holding the token can transmit

– Because there is only one token, only one computer will transmit at a time

– Hardware must regenerate token if lost Token gives computer permission to send one

frame – If all ready to transmit, enforces ``round-robin'' access – If none ready to transmit, token circulates around ring

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Token Ring Transmission

Station waits for token before sending Signal travels around entire ring Sender receives its own transmission

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Token Release Mechanisms

Release After Reception (RAR): – Each station reissues the free token only after

it receives the transmitted frame.– Used on lower speed token rings ( <=

4Mbps). Release After Transmission (RAT):

– Each station attaches a free token at the end of its frame

– Possible multiple frames propagate in a ring– Used on higher speed token rings (>=

16Mbps)

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Token Passing Ring Technologies

IBM token ring– Very widely used – Originally 4mbps, now 16Mbps

Fiber Distributed Data Interface (FDDI)– Operates at 100 Mbps

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FDDI

Fiber Distributed Data Interconnect (FDDI) – Uses ring token passing synchronization (RAT token release)– Uses fiber as transmission media – Transmits data at 100Mbps – Also suitable for MAN– Can attach 1000 stations, can be up to 200 km– Uses pairs of fibers to form two concentric rings

FDDI uses counter-rotating rings in which data flows in opposite directions ==> Reliability

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In case of fiber or station failure, remaining stations loop back and reroute data through the spare ring

All stations automatically configure loop back by monitoring the data ring

FDDI Self-healing

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ATM

Asynchronous Transfer Mode technology consists of electronic packet switches to which computers can connect

ATM switches form hub into which computers connect in a star topology

Computers get point-to-point connections - data from transmitter is routed directly through hub switches to destination

Transmits data at over 100Mbps Uses fiber optics to connect computer to switch Each connection includes two fibers

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ATM Switches

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IEEE 802.x LAN Standards

10 Mbps CSMA/CD (802.3) 100 Mbps CSMA/CD (802.3u) 1000 Mbps CSMA/CD (802.3z) Token Bus (802.4) Token Ring (802.5) Wireless LAN (IEEE 802.11) And many more……

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Summary Local Area Networks

– Designed for short distance– Use shared media

» Transmitting computer has exclusive use of communication medium

» computers must synchronize transmission– Many technologies exist

» Ethernet, Wireless, IBM Token Ring, FDDI, ATM Topology refers to general shape

– Bus– Ring– Star

Ethernet– CSMA/CD – Manchester encoding– Exponential Binary backoff

Synchronization by token passing in a ring– IBM token ring and FDDI