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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

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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%

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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%

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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 >> ALOHA schemes Three types: nonpersistent, 1-persistent and p-

persistent CSMA

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Nonpersistent CSMA

1. If medium is idle, transmit; otherwise, go to 2

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: 

1. If medium idle, transmit; otherwise, go to step 22. 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

1. If medium idle, transmit with probability p, and delay one time unit with probability (1 – p) Time unit is typically maximum propagation delay

2. If medium busy, listen until idle and repeat step 13. 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 > 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 < 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

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CSMA/CD

With CSMA, collision occupies medium for duration of transmission

With CSMA/CD, stations listen whilst transmitting1. If medium idle, transmit, otherwise, step 22. If busy, listen for idle, then transmit3. If collision detected, stop frame transmission and

send jam signal then cease transmission4. 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 >> Tprop) With random backoff, unlikely to collide on next tries To ensure backoff maintains stability, IEEE 802.3 and

Ethernet use binary exponential backoff

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Ethernet uses CSMA/CD

adapter doesn’t 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 K*512 bit times and returns to Step 1

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Ethernet’s 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 K=1023, 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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ExampleExampleSuppose 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?

SolutionSolution

Worst case: Min frame size = 512 bitsTime for complete bit emission = 512 + 64Time for collision detection = 500 + 499 = 999Since 576 < 999, collision not detected by A!

A B

500 bits

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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

<data rate><Signaling method><Max segment length>

10Base5 10Base2 10Base-T 10Base-F

Medium Thick Thin UTP 850nm Coaxial Coaxial fibre

Signaling Baseband Baseband BasebandManchester ManchesterManchester On/Off

Topology Bus Bus Star StarNodes 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 > 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 < 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

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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 you’re the only one transmitting shouldn’t 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