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Medium Access Control (MAC) in Computer Networks: Random Access vs. Controlled Access, Study notes of Computer Systems Networking and Telecommunications

University of California - DavisComputer Systems Networking and Telecommunications

An overview of medium access control (mac) protocols in computer networks, focusing on random access and controlled access methods. Topics include aloha, csma, csma/cd, and their respective algorithms and performance. The document also discusses the advantages and disadvantages of contention access.

Typology: Study notes

Pre 2010

Uploaded on 07/30/2009

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EEC 189Q
ComputerNetworks
Medium Access Control (MAC)
Random vs. controlled access
Random Access
Aloha, CSMA, CSMA/CD
Reading: Chapter 6.2-6.4
Chuah, Fall 2004 2
Medium Access Sublayer
Medium Access Control (MAC)
Protocols
--- distributed algorithm that arbitrate
access to a common shared channel
among a population of users, i.e.,
determine when a node can
transmit
Physical
Network
Medium Access
Sublayer
Data Link
Chuah, Fall 2004 3
Communication Links
Ā§Point to point link
-One sender, one receiver
-Dedicated channel
Ā§Multi-access broadcast link
-Multiple senders, multiple potential receivers
-Shared media
Chuah, Fall 2004 4
Remarks on MAC Sublayer
Ā§MAC is not important on point-to-point links
Ā§MAC is only used in broadcast or shared channel
networks
-communication about ā€œsharingā€ must use the channel
itself!
Ā§Examples:
-Packet-switched Radio Network (Aloha)
-Ethernet IEEE 802.3 (CSMA/CD)
-Token Ring IEEE 802.5, FDDI (Token Passing)
-Cellular, Satellite, Wireless LAN (MACAW)
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EEC 189Q

Computer Networks

Medium Access Control (MAC)

Random vs. controlled access

Random Access

Aloha, CSMA, CSMA/CD

Reading: Chapter 6.2-6.

Chuah, Fall 2004 2

Medium Access Sublayer

Medium Access Control (MAC)

Protocols

--- distributed algorithm that arbitrate

access to a common shared channel

among a population of users, i.e.,

determine when a node can

transmit

Physical

Network

Medium Access Sublayer

Data Link

Chuah, Fall 2004 3

Communication Links

Ɵ Point to point link

  • One sender, one receiver
  • Dedicated channel

Ɵ Multi-access broadcast link

  • Multiple senders, multiple potential receivers
  • Shared media

Chuah, Fall 2004 4

Remarks on MAC Sublayer

Ɵ MAC is not important on point-to-point links

Ɵ MAC is only used in broadcast or shared channel

networks

  • communication about ā€œsharingā€ must use the channel

itself!

Ɵ Examples:

  • Packet-switched Radio Network (Aloha)
  • Ethernet IEEE 802.3 (CSMA/CD)
  • Token Ring IEEE 802.5, FDDI (Token Passing)
  • Cellular, Satellite, Wireless LAN (MACAW)

Chuah, Fall 2004 5

Broadcast Links: Multiple Access

Single shared communication channel

Ɵ Only one can send successfully at a time

Ɵ Two or more simultaneous transmissions

  • Interference

Ɵ How to share a broadcast channel

  • Humans use multi-access protocols all the time

Chuah, Fall 2004 6

Contention Access Protocols

Ɵ General Characteristics

  • Single channel/medium shared by a large number of

hosts

  • No coordination between hosts
  • Control is completely distributed

Ɵ How does it compared to complete control with

switching?

Chuah, Fall 2004 7

Design Goals

Ɵ Fully decentralized

Ɵ Fairness among users

Ɵ High efficiency

Ɵ Low delay

Ɵ Fault tolerance

Chuah, Fall 2004 8

Taxonomy of MAC Protocols

Chuah, Fall 2004 13

Pure Aloha Algorithm (contā€™d)

2. Listen to the broadcast

Ɵ Assume the receiver rebroadcasts the received signal, so the

sender can find out if its packet was destroyed just by

listening to downward broadcast one round-trip time after

sending it

3. If packet was destroyed, wait a random amount of

time, and send it again

Ɵ prevent the same packet from colliding over and over again

Chuah, Fall 2004 14

Contention Period in Pure Aloha

Ɵ Just send: no waiting for beginning of slot

Ɵ If first bit of a new packet overlaps with last bit of a packet

almost finished, both packets are destroyed.

t : one packet transmission time ( L/R )

t 0 -t t 0 t 0 +t

Vulnerable period: 2t

Chuah, Fall 2004 15

Slotted Aloha

Ɵ How to reduce vulnerability/contention period?

Ɵ Time is divided into equal size slots

Ɵ Nodes transmit at the beginning of a slot

  • Packets must be transmitted within a slot.

Ɵ If collision, retransmit later

Success (S), Collision (C), Empty (E) slots

Chuah, Fall 2004 16

Contention Period in Slotted Aloha

t : one packet transmission time ( L/R ) = slot duration

Vulnerable period: t

t 0 -t t 0 t 0 +t

Packet could arrive in the

middle of a slot

Potential collision with

packets sent within

the same slot

Chuah, Fall 2004 17

Performance of Aloha Protocols

G = offered load = Np

Pure Aloha

Slotted Aloha

S = throughput = ā€œgoodputā€

(success rate)

Chuah, Fall 2004 18

Slotted vs Pure Aloha

Ɵ Synchronous system

Ɵ Have to wait till start of

next slot

Ɵ Packets overlap

completely or not at all

Ɵ Maximum throughput

0.37 (1/e)

Ɵ Simpler, no

synchronization

Ɵ No waiting for beginning

of slot

Ɵ Packets may overlap

partially

Ɵ Maximum throughput

0.18 (1/2e)

Chuah, Fall 2004 19

Carrier Sense Multiple Access

Ɵ Aloha is inefficient (and rude)

  • Doesnā€™t listen before talking

Ɵ CSMA: Listen before transmit

  • If channel idle, transmit entire packet
  • If busy, defer transmission
    • How long should we wait?
  • Human analogy: donā€™t interrupt others

Ɵ Can carrier sense avoid collisions completely?

Chuah, Fall 2004 20

Assumptions with CSMA Networks

Ɵ Constant length packets

Ɵ No errors, except those caused by collisions

Ɵ Each host can sense the transmission of all other

hosts

Ɵ The propagation delay is small compared to the

transmission time.

Chuah, Fall 2004 25

Brain Teaser

Ɵ Can we use 1-Persistent in Satellite Network

Ɵ Satellite system has LONG propagation delay

  • time taken to sense the channel can be as long as 270 ms
  • vulnerability period = 540ms (1/2 a second)

=> CSMA doesnā€™t make sense here

Chuah, Fall 2004 26

1-Persistent CSMA

Ɵ Even if propagation delay is zero, there will still

be collisions.

Ɵ Why?

  • Nodes are greedy! They send packets as soon as they

ā€œhearā€ that the channel is idle!

Chuah, Fall 2004 27

Persistent and Non- persistent CSMA

Ɵ p-persistent

  • If idle, transmit with probability p
  • If busy, wait till it becomes idle
  • If collision, wait random amount of time

Ɵ Non-persistent

  • If idle, transmit
  • If busy, wait random amount of time

Chuah, Fall 2004 28

CSMA/CD Algorithm

Ɵ Sense the channel

  • If idle, transmit immediately
  • If busy, wait until the channel becomes idle

Ɵ Collision detection

  • Abort a transmission immediately if a collision is

detected

  • Try again later after waiting a random amount of time.

Chuah, Fall 2004 29

CSMA/CD

In CSMA protocols:

Ɵ If two colliding stations will transmit their complete

packet, wasting the channel for the entire packet time.

CSMA with collision detection (CD)

Ɵ Listen while talking

Ɵ Stop transmitting when collision detected

  • Compare transmitted and received signals

Ɵ Human analogy

  • Polite conversationalist

Ɵ Worst case time to detect a collision?

Chuah, Fall 2004 30

Worst Case Collision Detection Time

Chuah, Fall 2004 31

Performance of CSMA/CD

Ɵ Assume time -slotted channel

Ɵ Probability that exactly one node transmits in a

given time slot

max

s

s p p p

N N

when p = 1/N

Ɵ Average number of time slots A wasted before a

packet is transmitted successfully

A = s.0 + (1-s)(1+A)

Ɵ When s = 0.4 , A = 1.

Chuah, Fall 2004 32

Performance of CSMA/CD (contā€™d)

Ɵ Relevant parameters

  • cable length, signal speed, frame size, bandwidth

Ɵ More accurately (from simulation), channel

efficiency under heavy load

Efficiency = 1/ (1 + 5.4 a)

framesize

bandwidth

signalspeed

cablelength

TRANS

PROP

a = = Ɨ

Chuah, Fall 2004 37

Ethernet Frame Format

Dest addr

64 48 32

Preamble CRC Src addr

Type Body

48 16

Chuah, Fall 2004 38

Ethernet Addressing

Ɵ 48-bit addresses

Ɵ All adaptors receive all packets

  • Dropped if address do not match

Ɵ Broadcast address

  • All 1ā€™s: received and processed by all stations

Ɵ Multicast addresses

  • First bit is 1

Chuah, Fall 2004 39

IEEE 802.3 Parameters

Ɵ 1 bit time = time to transmit one bit

  • 10 Mbps Ƌ 1 bit time = 0.1 Ī¼s

Ɵ Maximum network diameter ā‰¤ 2.5km

  • Maximum 4 repeaters

Ɵ Worst case collision detection time: 51.2 Ī¼s

Ɵ Slot time

  • 51.2 Ī¼s = 512 bits = 64 bytes

Chuah, Fall 2004 40

Ethernet Summary

Ɵ 1-persistent CSMA/CD

Ɵ 51.2 Ī¼s to seize the channel

Ɵ Collision not possible after 51.2 Ī¼s

Ɵ Minimum frame size of 64 bytes

Ɵ Binary exponential backoff

Ɵ Works better under light load

Ɵ Delivery time non-deterministic

Chuah, Fall 2004 41

Pros and Cons of Contention Access

Advantages:

Ɵ Short delay for bursty traffic

Ɵ Simple

Ɵ Flexible to fluctuations in the

number of hosts

Ɵ Fairness

Disadvantages:

Ɵ Low channel efficiency

Ɵ Not good for continuous

traffic

Ɵ Cannot support priority traffic

Ɵ High variance in transmission

delays