LAN Media Access

LAN Media Access

Lecture 6, April 4, 2003

Mr. Greg Vogl

Data Communications and Networks

Uganda Martyrs University

April 4, 2003 Data Communications and Networks: LAN Media Access

2

Sources

• Hodson Ch. 8.3-8.4, 9

• Stamper Ch. 7

• BITDCO lectures 8, 9, 15


3

Overview

• OSI Data Link Layer

• LAN Standards

• LAN Access Methods


4

OSI Layers and Sub-layers

• Data Link Layer– Logical Link Control– Media Access Control

• Physical Layer– Media Signalling (voltages, frequencies)– Bus Interface Unit– Communication Interface Unit– Medium


5

Data Link Layer Tasks

• Delineation of data (start, end, size)

• Error control (parity, CRC)

• Addressing (source, destination)

• Transparency (can send any data bits)

• Code independence (ASCII/EBCDIC)

• Media access (which device can transmit)


6

LAN Standards

• IEEE 802.3: Ethernet• IEEE 802.4: Token Bus• IEEE 802.5: Token Ring• IEEE 802.6: MAN• IEEE 802.7: Broadband• IEEE 802.11: wireless/cableless• IEEE 802.12: 100Mbps• ISO 9314: FDDI


7

Some Popular LAN Standards

• 10base5: 10 Mbps, baseband, thick coax, <500m• 10base2: 10 Mbps, baseband, thin coax, <200m• 10baseT: 10 Mbps, baseband, twisted pair• 100baseTX: 100 Mbps, baseband, twisted pair• 100baseFX: 100 Mbps, baseband, fibre optic• 1000baseSX: 1000 Mbps, baseband, fibre optic• 100VG-AnyLAN: twisted pair, CSMA/CD; token


8

Topologies, Protocols, Media

Topology Media Access Protocol Media

Bus CSMA/CD Ethernet Thin Coax

Thick Coax

Ring Token Passing Token Ring

FDDI

Thin Coax/UTP

Fibre

Star CSMA/CD

Switching

Ethernet

ATM

UTP

UTP or Fibre


9

Ethernet vs. Token Ring

Standard IEEE 802.3 IEEE 802.5

Speed, Mbps 10, 100, 1000 4, 16, 100

Medium TP, Coax, Fiber TP

Distance 185-2500m 366-4000m

Stations 100 or 30 260

NIC+Connectors $50/station $225/station


10

LAN Access Control Methods

• Carrier sense multiple access (CSMA)– p-persistent, CMSA/CD, CSMA/CA

• Token passing– Token ring, token bus, slotted ring

• Dedicated lines– Demand priority, fast switching


11

Carrier Sense Multiple Access

• Listen to the medium for a signal• If not busy, transmit• If another is transmitting, wait until later• If >1 transmit at once, collision/corruption• CSMA is a broadcast protocol

– Similar to a multiple-party phone line– All workstations check all messages– Ignored if address is not destination address


12

P-persistent CSMA

• If busy, wait until idle• If idle, do not immediately transmit• Maybe wait a delay interval or slot time• Transmit with probability p in each slot

– If p=1.0, probability of both colliding is 1.0– If p=0.5, probability of both colliding is .25

• Average delay = (1-p)/p slot times– Lower p gives fewer collisions but longer delay


13

Collision Detection

• Collision damages data, makes it unusable

• Time wasted if continuing during collision

• Listen while you talk

• Abort sending as soon as collision detected

• Also send short signal indicating a collision


14

Retransmission

• If nobody waits, another collision will occur

• Choose to wait a random interval– from 0 to 2n slot times

• If more collisions, wait up to 2x as long– from 0 to 2n+1 slots


15

Limits

Maximum propagation delay <5 microsec.Collisions are rare (typically 20-30x/day) Possibly many collisions if overloaded

Network performance will drop No guaranteed upper bound on access time• min. packet size and max. medium length

– so short msg. not sent before collision detected

• Limited number of devices per segment– Depends on hardware, OS, traffic


16

Collision Avoidance

• Similar to p-persistent CSMA

• Send reservation bits, wait, then send msg.– Need to wait for reservation to propagate

• Option to use slots with priorities – Some computers may have long delays– only needed when load is very heavy


17

Token Passing

• Need permission before transmitting– The one device with the token can transmit– All other devices without token must wait

• Round robin scheduling– Similar to TDM but each can give up its slot– If nothing to transmit, pass token to next device


18

Benefits and Limits

No collisions or retransmissionsNo random waiting

Maximum wait = token circulation time

Performance is deterministic, predictable, stableEven a large network with heavy load

Time wasted passing the token around ring Every node must wait before transmitting Longer delays for larger rings


19

Implementation

• Each station receives and retransmits

• Only the intended receiver reads message

• Token = preamble+control field+ postamble

• To claim/free the token, invert its control bit

• Differential Manchester encoding for synch.

• Receiver attaches ACK to end of message

• Sender receives own message with ACK


20

Token Monitor

• One station is designated the active monitor

• Monitor checks that the token is circulating

• If token is lost, the monitor makes new one

• If monitor fails, a new monitor is chosen


21

Token Bus

• Physically a bus, logically a ring

• Stations sequenced by their addresses

• Each station knows previous & next address

• Station with highest address gets token first

• Procedures needed to add/remove stations

• Entire token data structure passed at once

• Usually uses broadband; real-time uses


22

Network Interface Card

• Manages Media Access Control

• MAC address– Unique for every NIC– 48 bits, 6 bytes– Grouped in 6 fields of 2 hex characters– E.g. 00:00:C0:76:5A:26– Right three bytes are unique card address


23

IEEE 803.2 Ethernet Frame

• 8 bytes: Preamble and start frame delimiter

• 2/6 bytes: Destination address

• 2/6 bytes: Source address

• 2 bytes: Length

• 0-1500 bytes: Data

• 46-0 bytes: Pad (to assure at least 64 bytes)

• 4 bytes: 32-bit CRC

LAN Media Access

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