Mac layer

Medium Access Sublayer

EEC 189Q Computer Networks

Medium Access Control (MAC) Protocols

Network

Medium Access Control (MAC) Random vs. controlled access Random Access Aloha, CSMA, CSMA/CD

Data Link Physical

Medium Access Sublayer

--- distributed algorithm that arbitrate access to a common shared channel among a population of users, i.e., determine when a node can transmit

Reading: Chapter 6.2-6.4 2

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Communication Links §

Remarks on MAC Sublayer

Point to point link

§

- One sender, one receiver - Dedicated channel §

§

MAC is not important on point-to-point links MAC is only used in broadcast or shared channel networks - communication about “sharing” must use the channel itself!

Multi-access broadcast link - Multiple senders, multiple potential receivers - Shared media

§

Examples: -

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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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Broadcast Links: Multiple Access

Contention Access Protocols

Single shared communication channel § Only one can send successfully at a time § Two or more simultaneous transmissions

§

- Single channel/medium shared by a large number of hosts - No coordination between hosts - Control is completely distributed

- Interference §

How to share a broadcast channel - Humans use multi-access protocols all the time

§

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Fully decentralized

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Fairness among users

§

High efficiency

§

Low delay

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Fault tolerance

How does it compared to complete control with switching?

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Design Goals

§

General Characteristics

Taxonomy of MAC Protocols

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Random Access

Controlled Access

Stations contend for channels Overlapping transmissions (collisions) can occur

§ §

§

- No collisions - Allocation: static or dynamic

- Carrier sensing?

- Collision detection?

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Protocols

§

Stations reserve or are allocated channel

Protocols - Static channel allocation • Time division multiple access - Demand adaptive channel allocation

- Aloha - Slotted Aloha - Carrier Sense Multiple Access: Ethernet

• Reservation protocols • Token passing (token bus, token ring)

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Pure Aloha §

§

Pure Aloha Algorithm

Originally developed for packet radio communications at the campuses of U. of Hawaii in 1970 -

§

1. Users transmit whenever they have something to send

between a central computer & various data terminals

If more than one node transmits at the same time => Collision!

If there is a collision, both nodes need to retransmit

All nodes transmit on one frequency, fo Central node relays packets on the other frequency, f1 Central Node

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f1

Central Node

fo

X node 1 Chuah, Fall 2004

node 2

node 3

node n 11

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Pure Aloha Algorithm (cont’d)

Contention Period in Pure Aloha

2. Listen to the broadcast §

§

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

§

t: one packet transmission time (L/R)

3. If packet was destroyed, wait a random amount of time, and send it again §

Just send: no waiting for beginning of slot If first bit of a new packet overlaps with last bit of a packet almost finished, both packets are destroyed.

prevent the same packet from colliding over and over again

t0 -t

t0

t0 +t

Vulnerable period: 2t 13

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Slotted Aloha

§

How to reduce vulnerability/contention period?

Time is divided into equal size slots Nodes transmit at the beginning of a slot

§

If collision, retransmit later

§ §

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Contention Period in Slotted Aloha t: one packet transmission time (L/R) = slot duration Packet could arrive in the middle of a slot

- Packets must be transmitted within a slot.

Potential collision with packets sent within the same slot

t0 -t

t0 +t

Vulnerable period: t

Success (S), Collision (C), Empty (E) slots Chuah, Fall 2004

t0

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Performance of Aloha Protocols

Slotted vs Pure Aloha

S = throughput = “goodput” (success rate)

§ § 0.4 § 0.3 §

Slotted Aloha 0.2

§ § §

Simpler, no synchronization No waiting for beginning of slot Packets may overlap partially Maximum throughput 0.18 (1/2e)

Pure Aloha 1.0

1.5

2.0

G = offered load = Np 17

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Assumptions with CSMA Networks

Aloha is inefficient (and rude)

§

- Doesn’t listen before talking §

CSMA: Listen before transmit

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- If channel idle, transmit entire packet - If busy, defer transmission • How long should we wait?

- Human analogy: don’t interrupt others Can carrier sense avoid collisions completely?

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Carrier Sense Multiple Access

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§

0.1

0.5

§

Synchronous system Have to wait till start of next slot Packets overlap completely or not at all Maximum throughput 0.37 (1/e)

§ § §

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Constant length packets No errors, except those caused by collisions Each host can sense the transmission of all other hosts The propagation delay is small compared to the transmission time.

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CSMA Protocols §

1-Persistent CSMA §

There several types of CSMA protocols

Sense the channel - If busy, keep listening to the channel and transmit immediately when the channel becomes idle.

- If idle, transmit a packet immediately.

- 1-Persistent CSMA - Non-Persistent CSMA - P-Persistent CSMA §

If collision occurs

- Wait a random amount of time and start over again

§

Greedy algorithm - It’s called 1-persistent because the host transmits with probability of 1 whenever it finds the channel idle.

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The Effect of Propagation Delay

Contention Period §

A

B

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Recall contention period in - Pure Aloha: 2 x packet transmission time - Slotted Aloha: 1 packet transmission time

Sense channel: idle => transmit §

Contention period in CSMA - up to 2 x end-to-end propagation delay

Collision!

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Brain Teaser §

Can we use 1-Persistent in Satellite Network

§

Satellite system has LONG propagation delay

1-Persistent CSMA § §

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

Even if propagation delay is zero, there will still be collisions.

Why? - Nodes are greedy! They send packets as soon as they “hear” that the channel is idle!

=> CSMA doesn’t make sense here

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CSMA/CD Algorithm

Persistent and Non- persistent CSMA p-persistent

§

-

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§

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

Sense the channel - If idle, transmit immediately - If busy, wait until the channel becomes idle

Non-persistent

§

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§

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

Collision detection - Abort a transmission immediately if a collision is detected - Try again later after waiting a random amount of time.

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CSMA/CD

Worst Case Collision Detection Time

In CSMA protocols: § If two colliding stations will transmit their complete packet, wasting the channel for the entire packet time.

CSMA with collision detection (CD) § Listen while talking § Stop transmitting when collision detected - Compare transmitted and received signals §

Human analogy

§

Worst case time to detect a collision?

- Polite conversationalist

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Performance of CSMA/CD § §

( ) p(1 − p ) N 1

smax ≈ 0. 4 §

Performance of CSMA/CD (cont’d)

Assume time -slotted channel Probability that exactly one node transmits in a given time slot

s( p ) =

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Relevant parameters - cable length, signal speed, frame size, bandwidth

§

N −1

when p = 1/N

More accurately (from simulation), channel efficiency under heavy load

Efficiency

Average number of time slots A wasted before a packet is transmitted successfully

a=

A = s.0 + (1-s)(1+A) §

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= 1/ (1 + 5.4 a)

PROP cablelength bandwidth = × TRANS signalspeed framesize

When s = 0.4, A = 1.5 31

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Ethernet and IEEE 802.3

Backoff After Collision

1-persistent CSMA/CD § Carrier sense: station listens to channel first §

If idle, station may initiate transmission

§

Collision detection: continuously monitor channel

§

If collision, stop transmission

§

When collision occurs

§

If second collision occurs

- Wait random time [0,1] slots

- Listen before talking

- Wait random time [0,3] slots

- Talk if quiet §

- Listen while talking

§

Double range for each successive collision Called Exponential backoff

- One talker at a time

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Binary Exponential Backoff Algorithm

Binary Exponential Backoff (cont’d)

If collision

§

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§

- choose one slot randomly from 2k slots, {0,1,2,…,2k-1}, where k is the number of collisions the frame has suffered.

- One contention slot length usually ~ 512 bit times (minimum frame size = 64 bytes)

In Ethernet - Binary exponential backoff will allow maximum of 15 retransmission - If 16 backoffs occur, the transmission of the frame is considered a failure.

Can adapt to varying network load §

Example - if k = 1, choose from {0, 1} - if k = 2, choose from {0, 1, 2, 3} - if k = 10, choose from {0, 1, 2, ....... 1023}

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Ethernet Frame Format

Ethernet Addressing

§

48-bit addresses All adaptors receive all packets

§

Broadcast address

§

Multicast addresses

§

- Dropped if address do not match

64

48

Preamble

Dest addr

48 Src addr

16 Type

32 Body

- All 1’s: received and processed by all stations

CRC

- First bit is 1

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IEEE 802.3 Parameters §

Ethernet Summary

1 bit time = time to transmit one bit

§

- 10 Mbps è 1 bit time = 0.1 µs §

§

Maximum network diameter ≤ 2.5km

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- Maximum 4 repeaters § §

§

Worst case collision detection time: 51.2 µs Slot time

§ §

- 51.2 µs = 512 bits = 64 bytes

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§

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1-persistent CSMA/CD 51.2 µs to seize the channel Collision not possible after 51.2 µs Minimum frame size of 64 bytes Binary exponential backoff Works better under light load Delivery time non-deterministic

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Pros and Cons of Contention Access Advantages: § § § §

Short delay for bursty traffic Simple Flexible to fluctuations in the number of hosts Fairness

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Disadvantages: § § § §

Low channel efficiency Not good for continuous traffic Cannot support priority traffic High variance in transmission delays

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