MAC Protocols & High Speed LANs

Lesson 8 NETS2150/2850 1

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) 2

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   how to detect collisions specifies: how to recover from collisions (e.g., via delayed retransmissions)  Examples of random access MAC protocols:    ALOHA slotted ALOHA CSMA, CSMA/CD 3

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% 4

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% 5

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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 7

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 8

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 2 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 9

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 1 3. If transmission is delayed one time unit, repeat step 1  What is an effective value of p?

10

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 11

CSMA/CD

 With CSMA, collision occupies medium for duration of transmission  With CSMA/CD, stations listen whilst transmitting 1. If medium idle, transmit, otherwise, step 2 2. If busy, listen for idle, then transmit 3. If collision detected, stop frame transmission and send

jam signal

then cease transmission 4. After jam, backoff random time then start from step 1 12

CSMA/CD Operation

13

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 T frame T prop ) >> With random backoff, unlikely to collide on next tries  To ensure backoff maintains stability, IEEE 802.3 and Ethernet use

binary exponential backoff

14

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 15

Ethernet CSMA/CD algorithm

  After aborting, adapter enters

exponential backoff

: after the

m

th collision, adapter chooses a K at random from {0,1,2,…,2

m

-1}  If adapter detects another transmission while transmitting  aborts and sends

jam signal

Adapter waits Step 1

K*512

bit times and returns to 16

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} 17

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

18

IEEE 802.3 Frame Format

Ethernet is similar, but length is replaced by type Both has min frame size = 512 bits (64 octets) 19

IEEE Notation for 10 Mbps Ethernet



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

Medium Signaling Topology Nodes Thick Coaxial 100 Thin Coaxial 30 UTP Baseband Baseband Baseband Manchester Manchester Manchester Bus Bus Star 850nm fibre On/Off Star 33 20

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 21

100Mbps (Fast Ethernet)

100Base-TX 100Base-FX 100Base-T4

2 pair, STP MLT-3 2 pair, Cat 5 UTP MLT-3 2 optical fibre 4 pair, cat 3,4,5 4B5B, NRZI 8B6T,NRZ 22

100BASE-T Options

23

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 24

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)  T frame > T prop (legacy compatibility) Frame bursting – allows multiple short frames transmission  1000BaseT is standardised as IEEE 802.3ab

25

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 26

Gigabit Ethernet Medium Options

27

Cisco® High-end Switches

28

Gigabit Ethernet Configuration

29

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

30

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 31

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 32

10GbE Distance Options

33

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)!

34

“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 35

The Polling Scheme

  The master/central node “invites” slave nodes to transmit in turn Main concerns:    polling overhead latency single point of failure (master) 36

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 37

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 38

Ring Repeater States

39

IEEE 802.5 Frame Format

Data Frame Token Frame 40

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 41

Token Ring Operation

42

Fig. 16.18

LAN Performance Comparison

43

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 44

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 45

McGraw-Hill

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)

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 47

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 48

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 49