LAN Media Access
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Transcript of 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
April 4, 2003 Data Communications and Networks: LAN Media Access
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Overview
• OSI Data Link Layer
• LAN Standards
• LAN Access Methods
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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)
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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
April 4, 2003 Data Communications and Networks: LAN Media Access
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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