1M. Veeraraghavan

MAC schemes

Prof. Malathi Veeraraghavan

2M. Veeraraghavan

Consider wireless links

• Naturally broadcast medium– One transmitter sends data; multiple receivers

can receive the signal and obtain the data– Need a MAC (Medium Access Control)

protocol to share the “naturally broadcast” wireless medium

Endpoint

Endpoint

Endpoint

3M. Veeraraghavan

Shared links in wired domain

• Distance limitation between farthest hosts – Shannon’s capacity; SNR; attenuation

outbound

inbound

Multipoint drops: potential interference on inbound line – polling; e.g. multidrop telephone lines

Hub or opticalpassive star coupler

Host HostHost

Hubs/Optical passive star couplers: any data received on one line is broadcast to all other lines

4M. Veeraraghavan

Shared links in wired domain

• Ring networks:– ring adapters pass

through the signal in cut-through fashion

– hosts pick up data destined for themselves

• Multitapped shared buses– in coaxial cable

transmisison systems users can inject a signal that propagates in both directions of the medium

Host

ringadapter

Host

Host

Host

Host

point-to-pointtransmissionlinks

5M. Veeraraghavan

Classification of MAC protocols

MAC protocols

Fixed-assignment schemes

Random-access schemes

Demand assignment schemesCircuit-switched

(e.g., FDMA, TDMA)

Connectionlesspacket-switched(e.g., Ethernet,802.11)

Connection-orientedpacket-switched(e.g., CDMA, polling)Channelization

6M. Veeraraghavan

FDMA (Frequency Division Multiple Access)

• Similar to broadcast radio and TV, assign a different carrier frequency per call

• Modulation technique determines the required carrier spacing

• Each communicating wireless user gets his/her own carrier frequency on which to send data

• Need to set aside some frequencies that are operated in random-access mode to enable a wireless user to request and receive a carrier for data transmission

7M. Veeraraghavan

TDMA(Time Division Multiple Access)• Each user transmits data on a time slot on

multiple frequencies• A time slot is a channel• A user sends data at an accelerated rate (by

using many frequencies) when its time slot begins

• Data is stored at receiver and played back at original slow rate

8M. Veeraraghavan

Frequency vs. time

Fre

quen

cy

Time

CarrierFDMA

Time

Fre

quen

cy

TDMA

Time

Fre

quen

cy

Hybrid FDMA/TDMA

Basic principle of communication: Two regions in the time-frequency plane with equal areas can carry the same amount of information

9M. Veeraraghavan

Control aspect

• How is the fixed assignment of a frequency and/or timeslot made?– Who makes it?– How long does an endpoint get to hold the

frequency/timeslot?

• Need a control channel on which stations can compete using a random-access protocol to obtain a channel assignment– Set aside control frequencies and/or timeslots

10M. Veeraraghavan

Use of fixed assignment schemes on one shared link

• All endpoints/hosts receive/transmit on common frequencies– many if FDMA is used

• Use control channels to allocate frequency/timeslot for user communication between one pair– Release when done

• In Ethernet hub, baseband transmission – 1 frequency – random access not fixed-assignment

Endpoint

Endpoint

Endpoint

Hub or opticalpassive star coupler

Host HostHost

11M. Veeraraghavan

On wireless links

• Previous slide shows what’s referred to as an “adhoc” architecture

• More common is to use an “infrastructure” based architecture– basestation or access point present

12M. Veeraraghavan

Concept of upstream vs. downstream

PSTN

Base stations/cell sitesReversechannels

Forward channels

Downstream

Upstream

Internet

Access points

• Send data between endpoints via a basestation or AP – Can be placed at a height allowing all endpoints to see the basestation/AP– Endpoints that cannot “hear” each other communicate via the basestation/AP– MAC problem only in upstream direction

13M. Veeraraghavan

Example of FDMA scheme: Advanced Mobile Phone System (AMPS)

• FDMA/FDD– Spectrum allocation by FCC: A and B allocations to

different providers

A B

825 8451 667

A B

870 8901 667

channelsKhzMhz

6663020 Original

Reverse Forward

A B

825 845

1023667

A

824 849

991 1 799

BA A B

870 890

1023667

A

869 894

991 1 799

BA channelsKhzMhz

8323025 Extended

14M. Veeraraghavan

Duplex techniques

• Separates signals transmitted by base stations from signals transmitted by terminals– Frequency Division Duplex (FDD): use

separate sets of frequencies for forward and reverse channels (upstream and downstream)

– Time Division Duplex (TDD): same frequencies used in the two directions, but different time slots

15M. Veeraraghavan

Impact of FDD

• In AMPS, endpoints cannot “hear” each other– receivers at endpoints (phones) tuned only to

downstream frequencies– adhoc mode essentially disabled!

16M. Veeraraghavan

Hexagonal cell frequency plan

R D

• D: Distance between a base station and the nearest base station that uses the same channels• R: Radius of a cell• Reuse distance = D/R• Channel plan: method of

assigning channels to cells to guarantee a minimumreuse distance between cells thatuse the same channel

reqI

S

I

S

reqR

D

R

D

which is the minimum reuse distance for which

17M. Veeraraghavan

Reuse factor

• Divide available channels into N groups• N: reuse factor; select N such that cells assigned the same

frequencies will have a D:R ratio greater than (D:R)req

• For hexagons, reuse factor N is given by

2

3

1

reqR

DN

• Practical values of N – range from 3 to 21

– most commonly used: 7 (D/R = 4.6)

18M. Veeraraghavan

Second example of fixed-assignment scheme: IS136 NA-TDMA

• NA-TDMA is a hybrid FDMA/TDMA scheme• Therefore each frequency will have time slots that

are shared by multiple calls• Typical: three calls share one frequency• NA-TDMA is three times as efficient• Same frequency allocation as for AMPS• Carriers are 30khz apart

19M. Veeraraghavan

The TDMA aspect: frames and time slots

• Every frame is 40ms long and consists of 6 time slots

• 1.9ms offset: allows a terminal to perform full-duplex communications without transmitting and receiving simultaneously

– done to avoid a duplexing filter that separates strong transmit signal from weak receive signal

6 1 652 3 4 1 2 3 4

6 1 652 3 4 1 2 3 4

1.9ms

5

40ms

45 Mhz or

80 Mhz

base station to mobile

mobile to base station

20M. Veeraraghavan

Data rate of a carrier (frequency)

• What is the data rate of a carrier (frequency)– Each time slot carries 324 bits

– Data rate per carrier (frequency)skb

framems

frametimeslotstimeslotbits/6.48

/40

/6/324

• Four types of channels– A full-rate channel occupies two time slots per frame

– data rate: 16.2kb/s– can have three times as many calls as in AMPS– per frame: 1, 2, 3, 1, 2, 3, 1, 2, 3,....

– Half-rate channel (8.1kbps), double full-rate channel (32.4kbps) triple full-rate channel (48.6kbps)

– Voice calls carried within 1 full-rate channel (7.95kbps speech codec rate; channel coding rate 13kbps)

21M. Veeraraghavan

What about CDMA?

• CDMA is not a fixed-assignment scheme• Concept of CDMA soft capacity

– Allows more conversations to be packed in at the cost of quality of service

– Many more codes can be assigned than there are frequencies/timeslots

• My classification: CDMA is a demand-assignment scheme– postpone study to a wireless class

22M. Veeraraghavan

Random access MAC protocols

• Comparable to connectionless packet-switching

• No reservations are made; instead a wireless endpoint simply starts sending data packets

• What can happen?– Collision

• Need to avoid collisions or detect collisions and retransmit

– What’s the cost of being too careful to avoid collisions?• Utilization will be sacrificed

23M. Veeraraghavan

Various random-accessMAC schemes

• ALOHA: just send & wait for ACK

• Slotted ALOHA: send in slots

• CSMA: sense carrier, but wait for ACK

• CSMA/CD: detect collisions instead of waiting for ACK

• CSMA/CA

24M. Veeraraghavan

ALOHA

• Simplest scheme• True free-for-all. When a node needs to send, it

does so. It listens for an amount of time equal to the maximum round trip delay plus a fixed increment. If it hears an acknowledgment, fine; otherwise it resends after waiting a random amount of time. After several attempts, it gives up.

• Low delay if light load• Max. utilization: 18%

25M. Veeraraghavan

Slotted ALOHA

• Competition to send only occurs at the start of each slot (equal to X)

• Vulnerable period is X (not 2X as in ALOHA)

• What is maximum throughput?

26M. Veeraraghavan

CSMA

• Carrier Sense Multiple Access– sense carrier– if idle, send– wait for ack

• If there isn’t one, assume there was a collision, retransmit

• Vulnerable period: one tprop

27M. Veeraraghavan

Different techniques

• 1-persistent: – if busy, constantly sense channel– if idle, send immediately– if collision is detected, wait a random amount of time before retransmitting

• Non-persistent:– sense channel when station has a packet to send– if busy, wait a random amount of time before sensing again;– if idle, transmit as soon as it is idle– collisions reduced because sensing is not immediately rescheduled– drawback: more delay

• p-persistent: combines 1-persistent goal of reduced idle channel time with the non-persistent goal of reduced collisions.

– sense constantly if busy and the station needs to send a packet– if the channel is idle, transmit packet with probability p– with probability 1-p station waits an additional tprop before sensing again

28M. Veeraraghavan

CSMA/CD

• CSMA/CD: – In CSMA, if collision occurs, need to wait till

damaged frames have fully propagated. For long frames compared to propagation delay, this could lead to significant waste of capacity. So add collision detection.

– Rule: Frames should be long enough to allow collision detection prior to the end of transmission

29M. Veeraraghavan

Example of CSMA/CD: Ethernet

Ethernet (also 802.3) standardizes the 1-persistent CSMA/CD multi-access control protocol.

1. Each station listens before it transmits.

2. If the channel is busy, it waits until the channel goes idle, and then it transmits.

3. If the channel is idle it transmits immediately. Continue sensing.

4. If collision is detected, transmit a brief jamming signal, then cease transmission, wait for a random time, and retransmit.

• collision detection is not by waiting for an ACK

Ethernet uses baseband transmission – one frequency

30M. Veeraraghavan

Collisions in Ethernet

• The collision resolution process of Ethernet requires that a collision is detected while a station is still transmitting.

• Assume: max. propagation delay on the bus is a.

A Begins TransmissionA B

B Begins TransmissionA B

t0

t0+a-

31M. Veeraraghavan

Collisions in Ethernet

• Restrictions: Frame should be at least as long as 2aR, where R is the transmission rate of the link, and a is the max. one-way propagation delay

B Detects CollisionA B

t0+a

A Detects CollisionA B

t0+2a

Just Before Endof Transmission

32M. Veeraraghavan

Exponential Backoff Algorithm

• If a station is involved in a collision, it waits a random amount of time before attempting a retransmission.

• The random time is determined by the following algorithm:

• Set “slot time” to 2a.• After first collision wait 0 or 1 time unit.• After i-th collision, wait a random number

between 0 and 2 i-1 time slots.• Do not increase random number range if i=10.• Give up after 16 collisions.

33M. Veeraraghavan

Ethernet performance

• f: fraction of time the channel is busy• L: length of frame; R: data rate• a: one-way propagation delay• 2ae: average number of contention slots before

success in getting the medium; 2a is contention slot

718.2;2

eaeaR

LR

Lf

34M. Veeraraghavan

Ethernet frame format

CRCSrc.Addr. Data

IPdatagram

ARP req./reply

RARP req./reply

Type

66

2 18

PAD

46-1500

28

0800

0806

8035

Type

Type

Type

28 18

PAD

2

2

2

46-1500

4

Dst.Addr.

Example MAC address:04-3C-5A-11-26-78Each four-bit half of eachbyte is expressed inhexa-decimal notation

35M. Veeraraghavan

Wireless 802.11 LAN

• Uses CSMA/CA• Why CA and CD?

– Difficult to detect collisions in a radio environment

– Hidden station problem:• Two mutually far away stations A and C want to

send to B.• At A and C, channel appears idle• But collision occurs at B

36M. Veeraraghavan

Mechanisms for CA

• Use of Request-To-Send (RTS) and Confirm-to-Send (CTS) mechanism– When a station wants to send a packet, it first sends an RTS. The

receiving station responds with a CTS. Stations that can hear the RTS or the CTS then mark that the medium will be busy for the duration of the request (indicated by Duration ID in the RTS and CTS)

– Stations will adjust their Network Allocation Vector (NAV): time that must elapse before a station can sample channel for idle status

• this is called virtual carrier sensing

– RTS/CTS are smaller than long packets that can collide

• Use of InterFrame Spaces (IFS)

37M. Veeraraghavan

802.11 MAC

• IEEE 802.11 combines a demand-assignment MAC protocol with random access– PCF (Point Coordination Mode) – Polling

• CFP (Contention-Free Period) in which access point polls hosts

– DCF (Distributed Coordination Mode)• CP (Contention Period) in which CSMA/CA is used

CPCFP CFP

Super-frame

38M. Veeraraghavan

DCFDistributed Coordination Function

• This mode of 802.11 is a random access MAC• When a node needs to send data, it senses the medium. If

idle, wait for a period of DIFS and if the medium is still idle after DIFS, send immediately.

• If when the medium is sensed it is busy; then– wait for medium to be idle for a DIFS (DCF IFS) period – then decrement backoff timer until

• medium becomes busy again; freeze timer, OR• timer reaches 0; transmit frame

– if two stations have their timers reach 0; collision will occur; for every retransmission attempt, increase the contention window (CW), idle period after a DIFS, exponentially; 2i –1 starting with CWmin e.g., 7, 15, 31.

39M. Veeraraghavan

DCF mode transmission without RTS/CTS

source

destination

other

DIFSData

AckSIFS

NAV

Defer access

DIFSCW

Random backoff time

40M. Veeraraghavan

DCF MAC

• If medium was idle when sensed, send after waiting a DIFS period if medium is still idle after the wait

• If medium was busy when sensed, wait for it to become idle. Then wait for a DIFS; then wait for a random backoff period called contention window - CW (because many stations may be waiting when medium is busy; if they all send the instant the medium becomes idle, or after exactly DIFS, chances of collision are high); sense at the end of CW. If still idle, send data

41M. Veeraraghavan

What frequencies are used in 802.11?

• Frequency spectrum allocated for the ISM (Industrial, Scientific and Medical) unlicensed band– 2400 – 2483.5Mhz

• 83.5 Mhz of bandwidth (US: starts 2.402Ghz to 2.480 – so 79)

• 79 non-overlapping 1Mhz channels

• Can 79 access points be collocated in one physical space unit and each use random-access MAC protocol to share a given channel?– Answer: no

– Due to interference, only 26 can be collocated

– Another dimension: Frequency hopping is used instead of a single frequency per AP

42M. Veeraraghavan

Frequency Hopping (FH)

• What is FH?– Modulate the data signal such that it occupies different

frequency bands as transmission progresses • e.g., send a song over many FM radio channels with some

dwell time per channel

• Why not use a single frequency per AP?– Multipath fading affects narrow frequency bands so

that some channels offer very poor transmission

– In FH, time spent in each channel is small

43M. Veeraraghavan

802.11 FH PHY

• A channel hop occurs every 224 s

• 78 hopping patterns– Divided into 3 sets of 26 patterns each

– The sets are designed to avoid prolonged collision periods between hopping sequences in a set

– Hopping patterns collide 3 times on average, and 5 times in the worst case over a hopping cycle; each hop is a jump of a minimum of 6 channels

• Each 802.11 LAN must use a particular hopping pattern

• The hopping patterns allow for 26 networks to be collocated and still operate simultaneously

44M. Veeraraghavan

Demand assignment schemes

• Execute a call admission procedure to limit number of endpoints that simultaneously compete

• Allow endpoints to take advantages of silences in each other’s transmissions to maximize utilization

• Comparable to connection-oriented packet-switched networks

• Examples– CDMA– 802.11 polling scheme

45M. Veeraraghavan

Shared link as a LAN:relation between MAC protocols and LANs

• A shared link allows multiple end stations to hear a transmission from any station

• No node is serving as a “forwarding engine” for packets in a controlled fashion– hubs, passive star couplers, ring adapters, taps blindly

send data UNLIKE switches, routers, bridges

• This shared link concept works well as a local area network– if too large a network – with many hosts – each host

will get a small percentage of bandwidth

46M. Veeraraghavan

Shared links as “access” links

• Two reasons for using shared links on the access segment– individual endpoints (hosts/phones) generate

small quantities of data traffic– Costs should be kept low for end users

• Consequence: access links are often shared

• MAC protocols in the upstream direction

47M. Veeraraghavan

Shared link in the presence of basestations/APs?

• Is it still one shared link if basestations/APs forward data between two endpoints that cannot “hear” each other

– No, basestations/APs become forwarding engines, i.e., switches

– If a cell phone under one basestation calls another cell phone under the same basestation and the basestation allocates frequencies for both ends and forwards data bits

• Not different from a circuit switch forwarding bits received on one DS0 to another DS0

– Same thing when an AP uses destination addresses to rebroadcast data – it acts as a packet switch

48M. Veeraraghavan

Compare TDMA on an access link with TDM on an inter-switch link

• Similar in concept: sharing resources on one link among many users

• Difference: – Multiple senders on access link

– One sender in each direction on inter-switch link

Basestation

Endpoint EndpointEndpoint

Timeslot 1 Timeslot 2 Timeslot 3

Circuitswitch

CircuitswitchT1 line

carrying24 different DS0s(phone calls)

49M. Veeraraghavan

Summary

• Fixed-assignment schemes– FDMA, e.g. AMPS– TDMA, e.g., NA-TDMA

• Random-access schemes– Aloha, slotted ALOHA, CSMA– CSMA/CD, e.g. Ethernet– CSMA/CA, e.g. 802.11 (Wi-Fi)