MAC Protocols

1
MAC Protocols High Speed LANs

• Lesson 8
• NETS2150/2850

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Lesson Outline

• Random access MAC protocols
• Ethernet Implementations
• Ethernet (10 Mbps)
• Fast Ethernet (100 Mbps)
• Gigabit Ethernet - GbE (1 Gbps)
• 10 Gb Ethernet 10 GbE (10 Gbps)
• Round robin MAC protocol
• Token Ring (10 Mbps 100 Mbps)

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Random Access Protocols

• When node has frame to send
• transmit at full channel data rate R
• no a priori coordination among nodes
• two or more transmitting nodes ? collision
• random access MAC protocol specifies
• how to detect collisions
• how to recover from collisions (e.g., via delayed
  retransmissions)
• Examples of random access MAC protocols
• ALOHA
• slotted ALOHA
• CSMA, CSMA/CD

4
ALOHA

• Built for packet radio net across Hawaiian
  islands
• When station has frame, it sends immediately
• Wait for round trip time (RTT)
• RTT is time between send of frame and receive of
  ACK
• If receive ACK, fine. If not, retransmit
• If no ACK after repeated transmissions, give up
• Frame may be damaged by noise or by another
  station transmitting at the same time (collision)
• Max utilisation 18

5
Slotted ALOHA

• Time in uniform slots equal to frame transmission
  time
• All frames are same fixed size
• Need central clock (or other sync mechanism)
• Transmission begins at slot boundary
• Frames either miss or overlap totally
• Max utilisation 37

6
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7
Carrier Sense Multiple Access (CSMA)

• First listen for clear medium (i.e. carrier
  sense)
• If medium idle, transmit
• If two stations start at the same instant,
  collision
• Wait reasonable time (RTT plus ACK contention)
• No ACK then retransmit
• CSMA utilisation gtgt ALOHA schemes
• Three types nonpersistent, 1-persistent and
  p-persistent CSMA

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Nonpersistent CSMA

• If medium is idle, transmit otherwise, go to 2
• If medium is busy, wait for random time and
  repeat 1
• Random delays reduces probability of collisions
• However, capacity is wasted because medium will
  remain idle following end of transmission
• Even if stations waiting to access

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1-persistent CSMA

• To avoid idle channel time, 1-persistent protocol
  used
• Station wishing to transmit listens and obeys
  following 
• If medium idle, transmit otherwise, go to step 2
• If medium busy, listen until idle then transmit
  immediately (probability 1)
• 1-persistent stations are greedy
• If two or more stations waiting, collision is
  guaranteed!
• Gets sorted out after collision

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p-persistent CSMA

• Compromise that attempts to reduce collisions
• Like nonpersistent
• And reduce idle time
• Like 1-persistent
• If medium idle, transmit with probability p, and
  delay one time unit with probability (1 p)
• Time unit is typically maximum propagation delay
• If medium busy, listen until idle and repeat step
  1
• If transmission is delayed one time unit, repeat
  step 1
• What is an effective value of p?

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Value of p?

• n stations waiting to send
• At end of a transmission, expected/average number
  of stations attempting to transmit is
• np
• If np gt 1, higher chance of a collision
• Repeated attempts to transmit almost guaranteeing
  more collisions as retries compete with new
  transmissions
• Eventually, all stations trying to send
• Continuous collisions ? zero throughput
• So np lt 1 for expected peaks of n
• If heavy load expected, p small
• However, as p made smaller, stations wait longer
• At low loads, this gives very long delays

12
CSMA/CD

• With CSMA, collision occupies medium for duration
  of transmission
• With CSMA/CD, stations listen whilst transmitting
• If medium idle, transmit, otherwise, step 2
• If busy, listen for idle, then transmit
• If collision detected, stop frame transmission
  and send jam signal then cease transmission
• After jam, backoff random time then start from
  step 1

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CSMA/CDOperation
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Which Persistence Algorithm?

• IEEE 802.3 uses CSMA/CD 1-persistent!
• Both nonpersistent and p-persistent have
  performance problems
• 1-persistent (p 1) seems more unstable than
  p-persistent
• Greed of the stations
• But wasted time due to collisions is short (if
  Tframe gtgt Tprop)
• With random backoff, unlikely to collide on next
  tries
• To ensure backoff maintains stability, IEEE 802.3
  and Ethernet use binary exponential backoff

15
Ethernet uses CSMA/CD

• adapter doesnt transmit if it senses that some
  other adapter is transmitting, that is, carrier
  sense
• transmitting adapter aborts when it senses that
  another adapter is transmitting, that is,
  collision detection

• Before attempting a retransmission, adapter waits
  a random time, that is, random access

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Ethernet CSMA/CD algorithm

• If adapter detects another transmission while
  transmitting
• aborts and sends jam signal
• After aborting, adapter enters exponential
  backoff after the mth collision, adapter chooses
  a K at random from 0,1,2,,2m-1
• Adapter waits K512 bit times and returns to Step
  1

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Ethernets CSMA/CD (more)

• Jam Signal make sure all other transmitters are
  aware of collision 48 bits
• Bit time 0.1 ?s for 10 Mbps Ethernet for
  K1023, wait time is about 50 ms

• Binary Exponential Backoff
• Goal adapt retransmission attempts to estimated
  current load
• heavy load random wait will be longer
• first collision choose K from 0,1 delay is K
  x 512 bit transmission times
• after second collision choose K from 0,1,2,3
• after ten collisions, choose K from
  0,1,2,3,4,,1023

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Example
Suppose stations A and B are on the same 10 Mbps
Ethernet segment, and the propagation delay
between them is 500 bit times. In the worst case,
will A be able to detect a collision involving B?
500 bits
A
B
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IEEE 802.3 Frame Format
Ethernet is similar, but length is replaced by
type Both has min frame size 512 bits (64
octets)
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IEEE Notation for 10 Mbps Ethernet

• ltdata rategtltSignaling methodgtltMax segment lengthgt
• 10Base5 10Base2 10Base-T 10Base-F
• Medium Thick Thin UTP 850nm Coaxial Coaxial fib
  re
• Signaling Baseband Baseband Baseband Manchester
  Manchester Manchester On/Off
• Topology Bus Bus Star Star
• Nodes 100 30 - 33

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100Mbps Fast Ethernet

• Use same IEEE 802.3 MAC protocol and frame format
• 100BASE-TX uses STP or Cat 5 UTP
• 100BASE-FX uses optical fiber
• 100BASE-T4 can use Cat 3 UTP
• 100 Mbps over lower quality cables
• Uses 4 twisted-pair lines between nodes
• Data transmission uses three pairs in one
  direction at a time
• Star-wire physical topology
• Similar to 10BASE-T

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100Mbps (Fast Ethernet)

• 100Base-TX 100Base-FX 100Base-T4
• 2 pair, STP 2 pair, Cat 5 UTP 2 optical fibre 4
  pair, cat 3,4,5
• MLT-3 MLT-3 4B5B, NRZI 8B6T,NRZ

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100BASE-T Options
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Full Duplex Operation

• Traditional Ethernet half duplex
• Either transmit or receive but not both
  simultaneously
• With full-duplex, station can transmit and
  receive simultaneously
• 100-Mbps Ethernet in full-duplex mode,
  theoretical transfer rate 200 Mbps
• Must use switches
• Each station constitutes separate collision
  domain!
• In fact, no collisions

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Gigabit Ethernet - Differences

• Same frame format and MAC protocol as before
• Carrier extension is used for short frames
• At least 4096 bit-times long (cf. 512 for 10/100)
• ? Tframe gt Tprop (legacy compatibility)
• Frame bursting allows multiple short frames
  transmission
• 1000BaseT is standardised as IEEE 802.3ab

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Gigabit Ethernet Physical

• 1000Base-SX
• Short wavelength, multimode fibre
• 1000Base-LX
• Long wavelength, Multi or single mode fibre
• 1000Base-CX
• Copper jumpers lt 25m, shielded twisted pair (STP)
• 1000Base-T
• 4 pairs of Cat 5 UTP

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Gigabit Ethernet Medium Options
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Cisco High-end Switches
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Gigabit Ethernet Configuration
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10 Gigabit Ethernet - Uses

• High-speed, local backbone interconnection
  between large-capacity switches or server farm
• Campus wide connectivity
• Allows construction of MANs and WANs
• Connect geographically dispersed LANs between
  campuses
• Ethernet competes with ATM and other WAN
  technologies
• 10GbE provides substantial value over ATM
• 10GBaseT is standardised as IEEE 802.3ae

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10GbE - Advantages

• No expensive, bandwidth-consuming conversion
  between Ethernet packets and ATM cells
• Network is Ethernet, end to end
• Optimizing operation and cost for LAN, MAN, or
  WAN 
• Variety of standard optical and STP interfaces
  specified for 10 GbE

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10 GbE Implementations

• Maximum link distances cover 300 m to 40 km
• 10GBASE-S (short)
• 850 nm on multimode fiber
• Up to 300 m
• 10GBASE-L (long)
• 1310 nm on single-mode fiber
• Up to 10 km
• 10GBASE-E (extended)
• 1550 nm on single-mode fiber
• Up to 40 km

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10GbE Distance Options
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Cisco 10GbE module

• Supports 10GBase-S/L/E/CX
• Up to 32 10-GbE ports
• 256 MB buffer per port
• Up to 400 million frames per sec (mfps)
• Supports jumbo frame size (up to 9216 octets)!

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Taking Turns MAC Protocols

• Involve a controlled access
• No collision!
• A station cannot send unless been authorised
• There are two main types
• Polling
• Token-passing

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The Polling Scheme

• The master/central node invites slave nodes to
  transmit in turn
• Main concerns
• polling overhead
• latency
• single point of failure (master)

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Token Ring

• Developed from IBM's commercial token ring
• Because of IBM's large presence, token ring has
  gained broad acceptance
• But, never achieved popularity of Ethernet!
• Currently, large installed base of token ring
  products
• Market share likely to decline

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Ring Operation

• Each repeater connects to two others via
  unidirectional transmission links
• Single closed path
• Data transferred bit by bit from one repeater to
  the next
• Repeater regenerates and retransmits each bit
• Frame removed by transmitter after one trip round
  ring

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Ring Repeater States
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IEEE 802.5 Frame Format
Data Frame
Token Frame
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IEEE 802.5 MAC Protocol-Token Passing

• A special frame (i.e. token) circulates
  continuously
• Station waits for the token
• Changes one bit in token to make it SOF for data
  frame
• Append rest of data frame
• Frame makes round trip and is absorbed by
  transmitting station
• Inserts new token when transmission has finished
• How long to hold token token holding time (THT)
• Under light loads, some inefficiency
• Under heavy loads, round robin

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Token RingOperation
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LAN Performance Comparison
Fig. 16.18
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Wireless LAN Overview

• A wireless LAN uses wireless medium
• Saves installation of LAN cabling
• Eases relocation and other modifications to
  network structure
• Popularity of wireless LANs has grown rapidly
• Role for the wireless LAN
• Manufacturing plants, stock exchange trading
  floors, warehouses
• Historical buildings
• Small offices where wired LANs not economical
• IEEE has specified this technology in 802.11
  standard

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IEEE 802.11 Wireless LAN

• 802.11b
• 2.4-2.5 GHz unlicensed radio spectrum
• up to 11 Mbps
• widely deployed, using base stations

• 802.11a
• 5-6 GHz range
• up to 54 Mbps
• 802.11g
• 2.4-2.5 GHz range
• up to 54 Mbps

• All use CSMA/CA for MAC protocol
• All have infrastructure and ad-hoc network
  versions

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Infrastructure Approach

• Wireless host communicates with an access point
• Basic Service Set (BSS) (a.k.a. cell) contains
• wireless stations
• one access point (AP)
• BSSs combined to form a distribution system (DS)

McGraw-Hill

• The McGraw-Hill Companies, Inc., 2004

47
Ad Hoc Approach

• No AP!
• Wireless stations communicate with each other
• Typical usage
• laptop meeting in conference room, car
• interconnection of personal devices
• battlefield
• IETF MANET (Mobile Ad hoc Networks) working
  group looks into this approach
• Special needs such wireless routing, security

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IEEE 802.11 MAC protocol

• Collision if 2 or more nodes transmit at same
  time as the wireless channel is shared
• CSMA makes sense
• get all the bandwidth if youre the only one
  transmitting
• shouldnt cause a collision if you sense another
  transmission
• Thus, it uses CSMA with collision avoidance
  (CSMA/CA)
• Not CD because detecting collision is difficult
  in wireless environment
• Two-handshaking used

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Summary

• Random access protocol
• CSMA/CD in 802.3 (Ethernet)
• Round Robin
• Token passing in 802.5 (Token Ring)
• Wireless LAN
• Read Stallings chapter 16
• Next Layer-3 ? Network layer