DATA LINK LAYERS:

MEDIUM ACCESS PROTOCOLPART 1: MEDIUM ACCESS PROTOCOL , CHANNELIZATION, SCHEDULING,

RANDOM ACCESS

Overview

SWITCHED NETWORKS

Provide interconnection between users by means of transmission lines,

Transfer information across such networks requires routing table to direct the information from source to destination .

The addressing scheme is switched networks is typically hierarchical task.

BROADCAST NETWORKS

Because all information is received by all users, routing is not necessary.

A nonhierarchical addressing scheme is sufficient to indicate which users the information is destined.

However, broadcast networks require a medium access control protocol to orchestrate the transmissions from the various users.

CHARACTERISTSICS OF BROADCAST CHANNEL

When only one node has data to send, that node has a throughput of R

bps

When M nodes have data to send, each of these nodes has a throughput

of R/M bps. This need not necessarily imply that each of the M nodes

always has an instantaneous rate of R/M, but rather that each node

should have an average transmission rate of R/M over some suitably

defined interval of time

The protocol is decentralized: that is there is not master node that

represents a single point of failure for the network

The protocol is simple, so that it is inexpensive to implement

What we will learn?

PART I

Introduction to multiple access information

Random Access – MAC, ALOHA, CSMA-CD

Scheduling

Channelization

Delay performance

PART II

Overviews of LAN,

LAN Standards

LAN bridges

PART I: MEDIUM ACCESS

CONTROL PROTOCOL

PREVIOUSLY: DATA LINK LAYER

Point-to-point Link

Consists of a single sender at one end of the link and single

receiver at the other end of the link

Framing techniques to indicate boundaries

Retransmission algorithms to provide a reliable service. (ARQ)

Stop and Wait ARQ

Go Back N ARQ

Selective Repeat ARQ

Flow Control to regulate the rate of data transfer from transmitter to the receiver

BROADCAST LINK

Can have multiple sending and receiving nodes all connected to the same, single,

shared broadcast channel.

Broadcast – when any one node transmits a frame, the channel broadcasts the frame

and each of the other nodes receive a copy.

Example: Ethernet and LAN

Thus?. Additional Issues

How to efficiently share the access to the medium

How to coordinate the access of multiple and receiving nodes to a shared broadcast channel

Multitapped Bus

MULTIPLE ACCESS COMMUNICATIONS

Transmit when ready

Crash!!

Collision can occur; need retransmission strategy

MULTIPLE ACCESS COMMUNICATIONS

Shared wireless

medium (2.4HGz

radio)

Collision!

Packet drop

MULTIPLE ACCESS COMMUNICATIONS

The major problem with multi-access is allocating the channel between the

users; the nodes do not know when the other nodes have data to send

› Need to coordinate transmissions

› To avoid collision

multiple access protocol – Medium Access Control Protocol

› distributed algorithm that determines how nodes share channel, i.e., determine

when node can transmit

Medium Access Control

Channelization Scheduling Random access

MULTIPLE ACCESS TECHNIQUES

Partition medium

/channel

(TDMA, FDMA)

Dedicated

allocation to users

E.g. Satellite

transmission,

Cellular Telephone

Polling: take turns

Reservation Protocol

Request for slot in

transmission

schedule

E.g. Token ring,

Wireless LANs

Loose coordination

(no partition, allow

collisions)

Send, wait, retry if

necessary

E.g. Aloha, Ethernet

MAC: CHANNELIZATION

Channelization

› Semi-static bandwidth allocation of portion of shared medium to a given user

Approaches

› Frequency Division Multiple Access (FDMA)

Frequency band allocated to users

Broadcast radio & TV, analog cellular phone

› Time Division Multiple Access (TDMA)

Periodic time slots allocated to users

Telephone backbone, GSM digital cellular phone

› Code Division Multiple Access (CDMA)

Code allocated to users

Cellular phones, 3G cellular

MAC: CHANNELIZATION

PRO

Highly efficient for constant-bit rate traffic

Preferred approach in

Cellular telephone networks

Terrestrial & satellite broadcast radio & TV

CONS Inflexible in allocation of bandwidth to users with different requirements

Inefficient for bursty traffic

Does not scale well to large numbers of users

Average transfer delay increases with number of users M

MAC : ChannelizationTime Division Multiple Access (TDMA)

access to channel in "rounds"

each station gets fixed length slot (length = pkt trans time) in each round

unused slots go idle

example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle

1 3 4 1 3 4

6-slot

frame

6-slot

frame

time

MAC: ChannelizationFrequency Division Multiple Access (FDMA)

channel spectrum divided into frequency bands

each station assigned fixed frequency band

unused transmission time in frequency bands go idle

example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle

frequency

bands

FDM cable

Satellite communication: Shared radio using two frequency

bands, one for transmitting and one for receiving

MAC: ChannelizationEg: Satellite Transmission

uplink fin downlink fout

MAC: ChannelizationEg: Cellular Network

uplink f1 ; downlink f2

uplink f3 ; downlink f4

Cellular telephony: Two frequency bands

shared by a set of mobile users

MAC: ChannelizationEg: Cellular Network

Cellular networks use frequency reuse

Band of frequencies reused in other cells that are sufficiently far that

interference is not a problem

Cellular networks provide voice connections which is steady stream

Techniques implemented in cellular network

FDMA used in LTE, WiMax

TDMA used in GSM,EDGE

CDMA used in UMTS (3G),HSPA+

MAC: Random Access Protocol

when node has packet to send

› transmit at full channel data rate R.

› no a priori coordination among nodes

two or more transmitting nodes ➜ “collision”,

Each node involved in the collision repeatedly retransmit its frame (packet) until its frame gets through without collision.

Doesn’t necessarily retransmit the frame right away. Instead it waits a random delay before retransmitting the frame.

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, CSMA/CA

MAC: Random Access Protocol

MAC: Random Access ProtocolALOHA

Simplest solution: just do it

A station transmits whenever it has data to transmit

If ACK arrived –done

ELSE

If more than one frames are transmitted, they interfere with each other (collision) and are lost

If ACK not received within timeout, then a station picks random backoff time (to avoid repeated collision)

Station retransmits frame after backoff time (after completely transmitting its collided frame)

tt0t0-X t0+X t0+X+2tprop

t0+X+2tprop + B

Vulnerable period Time-out

Backoff period BFirst transmission Retransmission

All frames consist of exactly L bits

Time is divided into slots of size L/R seconds (that is, a slot equals the time to

transmit one frame)

Nodes start to transmit frames only at the beginning of slots.

The nodes are synchronized so that each node knows when the slots begin.

If two or more frames collided in a lot, then all the nodes detect the collision

event before the slots ends

MAC: Random Access Protocol Slotted ALOHA

MAC: Random Access Protocol Slotted ALOHA

Time is divided into “slots” of one packet duration

E.g., fixed size packets

When a node has a packet to send, it waits until the start of the next slot to

send it

Requires synchronization

If no other nodes attempt transmission during that slot, the transmission is

successful

Otherwise “collision”

Collided packet are retransmitted after a random delay

Let p be the probability, that is, a number between 0 and 1

When the node has a fresh frame to send, it waits until the beginning of the next

slot and transmits the entire frame in the slot

If there isn’t a collision, the node has successfully transmitted its frame, and thus

need not to consider retransmitting the frame.

If there is a collision, the node detects the collision before end of the slot. The

node retransmits its frame in each subsequent slot with probability p until the

frame is transmitted without a collision.

P – retransmits which occurs with probability p

(1-p) – skip the slot, and retransmit at the next slot

ADVANTAGES & DISADVANTAGES

ADVANTAGES

Superior to fixed assignments when there is a large number of burst stations.

Adapts to varying number of stations

DISADVANTAGES

Theoretically proven throughput maximum of 18.4%

Requires queuing buffers for retransmissions of packet.

Video: Pure ALOHA

ALOHA Model

Definitions and assumptions

X frame transmission time (assume constant)

S: throughput (average # successful frame transmissions per X seconds)

G: load (average # transmission attempts per X sec.)

Psuccess : probability a frame transmission is successful

successGPS

XX

frame

transmission

Prior interval

Any transmission that

begins during

vulnerable period

leads to collision

Success if no arrivals

during 2X seconds

Abramson’s Assumption

What is probability of no arrivals in vulnerable period?

Abramson assumption: Effect of backoff algorithm is that frame arrivals are equally likely to occur at any time interval

G is avg. # arrivals per X seconds

Divide X into n intervals of duration D=X/n

p = probability of arrival in D interval, then

G = n p since there are n intervals in X seconds

n as )1(p)-(1

intervals]2n in arrivals 0[

seconds] 2Xin arrivals 0[

222n Gn

success

en

G

P

PP

Throughput of ALOHA

G

success GeGPS 2

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0

0.00

78125

0.01

5625

0.03

125

0.06

25

0.12

50.

25 0.5 1 2 4

G

S

Collisions are means

for coordinating

access

Max throughput is

rmax= 1/2e (18.4%)

Bimodal behavior:

Small G, S≈G

Large G, S↓0

Collisions can

snowball and drop

throughput to zero

e-2 = 0.184

EXERCISE

THE STATIONS ON A WIRELESS ALOHA NETWORK ARE A MAXIMUM OF 600KM

APART. IF WE ASSUME THAT SIGNALS PROPAGE AT 3 X 10³ m/s, WE FIND?.

A PURE ALOHA NETWORK TRANSMITS 200-BIT FRAMES ON SHARED CHANNEL

OF 200kpbs. WHAT IS THE THROUGHPUT IF THE SYSTEM (ALL STATIONS

TOGETHER) PRODUCED:

1000 FRAMES PER SECOND

500 FRAMES PER SECOND

250 FRAMES PER SECOND

SLOTTED ALOHA

Throughput of PURE ALOHA = 18.4%

Time is divided into frame sized slot

Transmission can start only at the beginning of the slot

By the frame(s) that arrive in the previous slot

Increased the maximum throughput to 36%

Since the sensitive period decreased from 2T to T

SLOTTED ALOHA

Time is divided into “slots” of one packet duration

› E.g., fixed size packets

When a node has a packet to send, it waits until the start of the next slot

to send it

› Requires synchronization

If no other nodes attempt transmission during that slot, the transmission is

successful

› Otherwise “collision”

› Collided packet are retransmitted after a random delay

MAC: Random Access Protocol Slotted ALOHA

Pros:

single active

node can

continuously

transmit at full

rate of channel

simple

Cons:

collisions, wasting slots

idle slots

clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

• The only ‘unwasted’ slots will be those in which exactly one node transmits.

• A slot in which exactly one node transmits is said to be a successful slot

• The efficiency of a slotted multiple access protocol is defined to be the long run

fraction of successful slots in the case when there are a large number of active nodes

each always having a large number of frames to send.

ADVANTAGES & DISADVANTAGES

ADVANTAGES

Allows a node to transmit continuously at the full rate, R, when that node is the only active node ( a node is said to be active if it has frames to send)

Highly decentralized, because each node detects collisions and independently decides when to retransmits

Works well when there is only one active node, but how efficient is it when there are multiple active nodes?.

DISADVANTAGES

When there are multiple active nodes, a certain fraction of the slots will have collisions and will therefore be ‘wasted’,

Another fraction of the slots will be empty because all active nodes refrain from transmitting as a result of the probabilistic transmission policy.

Throughput of Slotted ALOHA

Gnn

success

Gen

GGpG

GP

GPGPS

)1()1(

intervals]n in arrivals no[

seconds] Xin arrivals no[

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.0

156

…

0.0

312

5

0.0

625

0.1

25

0.2

5

0.5 1 2 4 8

Ge-G

Ge-2G

G

S0.184

0.368

MAC: CARRIER SENSE MULTIPLE

ACCESS (CSMA)

ALOHA protocols are quite like a boorish party goer who continues to chatter away regardless of whether other people are talking.

Important rules :

Carrier sensing - Listen before speaking. A node listens to the channel before transmitting. If a frame from another node is currently being transmitted into the channel, a node ten waits until it detects no transmissions for a short amount of time and then begin transmission.

Collision detection – If someone else begins talking at the same time, stop talking. A transmitting node listens to the channel while it is transmitting. If it detects that another node is transmitting an interfering frame, it stops transmitting and waits a random amount of time before repeating the sense-and-transmit-when-idle cycle.

MAC: Random Access Protocol Carrier

Sensing Multiple Access (CSMA)

A

Station A begins

transmission at

t = 0

A

Station A captures

channel at t = tprop

A station senses the channel before it starts transmission

If busy, either wait or schedule backoff (different options)

If idle, start transmission

Vulnerable period is reduced to tprop (due to channel capture effect)

When collisions occur they involve entire frame transmission times

If tprop >X, no gain compared to ALOHA or slotted ALOHA

By listening before transmitting, stations try to reduce the vulnerability

period to one propagation delay

MAC: Random Access Protocol Carrier Sensing Multiple Access (CSMA)

Transmitter behavior when busy channel is sensed

› 1-persistent CSMA (most greedy)

Start transmission as soon as the channel becomes idle

Low delay and low efficiency

› Non-persistent CSMA (least greedy)

Wait a backoff period, then sense carrier again

High delay and high efficiency

› p-persistent CSMA (adjustable greedy)

Wait till channel becomes idle, transmit with prob. p; or wait one mini-slot time & re-sense with probability 1-p

Delay and efficiency can be balanced

Sensing

1-PERSISTEN CSMA

If the channel is busy, they sense the channel continuously, waiting until

the channel becomes idle.

As soon as the channel is sensed idle, they transmit their frames

If more than one station is waiting, a collision will occur

Stations involved in a collision perform the back-off algorithm

‘Greedy’ – attempting to access the medium as soon as possible.

Has a relatively high collision rate.

NON-PERSISTENT CSMA

Attempts to reduce the incidence of collisions

Stations with a frame to transmit sense the channel

If the channel is busy, the stations immediately run the back-off algorithm

and reschedule a future re-sensing time

If idle, the stations transmit

By immediately rescheduling a re-sensing time and not persisting, the

incidence of collisions is reduced.

Resulted longer delays

P-PERSISTENT CSMA

Combination of both 1-Persistent and Non-Persistent

Stations with a frame to transmit sense the channel, and if the channel is

busy, they persist with sensing until the channel becomes idle

If the channel is idle

With probability p, the station transmits its frame

With probability 1-p, the station decides to wait an additional propagation

delay tprop before again sensing the channel

MAC: Random Access Protocol CSMA with Collision Detection (CSMA/CD)

Monitor for collisions & abort transmission

› Stations with frames to send, first do carrier sensing

› After beginning transmissions, stations continue listening to the medium to detect collisions

› If collisions detected, a short jamming signal is transmitter to ensure all station knows there is a collision, abort transmission, reschedule random backofftimes, and try again at scheduled times

In CSMA collisions result in wastage of X seconds spent transmitting an entire frame

CSMA-CD reduces wastage to time to detect collision and abort transmission

Disadvantages of CSMA: Once a packet is sent, feedback occurs a roundtrip

time after the entire packet is transmitted

SUMMARY CSMA/CD OPERATION

FROM THE PERSPECTIVE OF AN

ADAPTER

The adapter obtains a datagram from the network layer, prepare a link-layer frame, and puts the frame adapter buffer

If the adapter sense that the channel is idle (that is there is no signal energy entering the adapter from the channel), it starts to transmit the frame. If on the other hand, the adapter sense the channel is busy, it waits until it senses no signal energy and then starts to transmit the frame

While transmitting, the adapter monitors for the presence of signal energy coming from other adapters using the broadcast channel.

If the adapter transmit the entire frame without detecting signal energy from other adapters, the adapter is finished with the frame. If, on the other hand, the adapter detects signal energy from other adapters while transmitting, it aborts the transmission.

After aborting, the adapter waits a random amount of time and then returns to step 2.

MAC: Random Access Protocol CSMA-CD

Application: Ethernet

First Ethernet LAN standard used CSMA-CD

1-persistent Carrier Sensing

R = 10 Mbps

tprop = 51.2 microseconds

512 bits = 64 byte slot

accommodates 2.5 km + 4 repeaters

Truncated Binary Exponential Backoff

After nth collision, select backoff from {0, 1,…, 2k – 1}, where k=min(n, 10)

MAC: Random Access Protocol Throughput for Random Access MACs

For small propagation delay, a: CSMA-CD has best throughput

For larger a: Aloha & slotted Aloha better throughput

0

0.2

0.4

0.6

0.8

1

0.01 0.1 1

ALOHA

Slotted ALOHA

1-P CSMA

Non-P CSMA

CSMA/CD

a

rmax

MAC: Random Access Protocol -

Summary

when node has packet 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, CSMA/CA

Comparison of MAC approaches

Aloha & Slotted Aloha

Simple & quick transfer at very low load

Accommodates large number of low-traffic bursty users

Highly variable delay at moderate loads

CSMA-CD

Quick transfer and high efficiency for low delay-bandwidth

product

Can accommodate large number of bursty users

Variable and unpredictable delay

MAC: Scheduling

Schedule frame transmissions to avoid collision in shared medium

More efficient channel utilization

Less variability in delays

Can provide fairness to stations

Increased computational or procedural complexity

Two main approaches

› Reservation

› Polling

MAC: Scheduling Reservation Systems

TimeCycle n

Reservation

intervalFrame

transmissions

r d d d r d d d

Cycle (n + 1)

r = 1 2 3 M

Stations take turns transmitting a single frame at the full rate R bps

Transmissions organized into cycles

Cycle: reservation interval + frame transmissions

Reservation interval has a minislot for each station to request reservations for frame transmissions

Cycle length (variable) based on the number of stations that have a frame to transmit.

MAC: Scheduling Reservations Systems

Centralized systems: A central controller accepts requests from stations and issues grants to transmit

› Frequency Division Duplex (FDD): Separate frequency bands for uplink & downlink

› Time-Division Duplex (TDD): Uplink & downlink time-share the same channel

Distributed systems: Stations implement a decentralized algorithm to determine transmission order

Central

Controller

tr 3 5 r 3 5 r 3 5 8 r 3 5 8 r 3

(a)

tr 3 5 r 3 5 r 3 5 8 r 3 5 8 r 3

8(b)

MAC: Scheduling Example

Initially stations 3 & 5 have reservations to transmit frames

Station 8 becomes active and makes reservation

Cycle now also includes frame transmissions from station 8

MAC: Scheduling Example: GPRS

General Packet Radio Service

Packet data service in GSM cellular radio

GPRS devices, e.g. cellphones or laptops, send packet data over radio and

then to Internet

Slotted Aloha MAC used for reservations

Single & multi-slot reservations supported

MAC: Scheduling Polling Systems

Reservation systems required stations to make reservation to gain access to the transmission medium

Polling system :- stations take turns accessing the medium

› Requires one of the nodes to be designated as a master node. The master node polls each of the nodes in a round-robin fashion.

› Master node sends a message to node 1, saying that node 1 can transmit up to some maximum number of frames.

› Centralized polling systems: A central controller transmits polling messages to stations according to a certain order

› Distributed polling systems: A permit for frame transmission is passed from station to station according to a certain order

› A signaling procedure exists for setting up order

Inbound line

Outbound lineHost

computer

Stations

MAC: SchedulingPolling

1 2 3M

Poll 1

Data from 1

Poll 2

Data from 2

Data to M

Host computer issues polling messages to each terminal,

providing it with permission to transmit on the inbound line

MAC: SCHEDULING TOKEN RING

No master node

A small, special-purpose frame known as a token is exchanged among the nodes in some fixed order.

When a node receives a token, it holds onto the token only if it has some frames to transmit; otherwise it immediately forwards the token to the next node.

If a node does have frames to transmit when it receives the token, it sends up to a maximum number of frames and then forward the token to the next node.

Drawback?. The failure of one node can crash the entire channel

Or a node accidentally neglects to release the token, then some recovery procedure must be invoked to get the token back in circulation.

Ring networks

MAC: SchedulingToken Passing

token

Station that holds token transmits into ring

tokenData to M

MAC: Scheduling Application Examples

Single-frame reinsertion

IEEE 802.5 Token Ring LAN @ 4 Mbps

Single token reinsertion

IBM Token Ring @ 4 Mbps

Multitoken reinsertion

IEEE 802.5 and IBM Ring LANs @ 16 Mbps

FDDI Ring @ 50 Mbps

All of these LANs incorporate token priority mechanisms

Comparison of MAC approaches

Reservation

› On-demand transmission of bursty or steady streams

› Accommodates large number of low-traffic users with slotted Aloha reservations

› Can incorporate QoS

› Handles large delay-bandwidth product via delayed grants

Polling

› Generalization of time-division multiplexing

› Provides fairness through regular access opportunities

› Can provide bounds on access delay

› Performance deteriorates with large delay-bandwidth product