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Multiple Access

ECS 152A

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Concepts

Multiple access vs. multiplexing Multiplexing allows several transmission

sources to share a larger transmission capacity. Often used in hierarchical structures.

Multiple access: two or more simultaneous transmissions share a broadcast channel. Often used in access networks

sometimes interchangeable Bandwidth (bps) vs. bandwidth (Hz)

bps: data rate Hz: frequency in physical carrier

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Multiple Access protocols Point-to-point vs. broadcast channel

Broadcast link can have multiple sending and receiving nodes all connected to the same, single, shared broadcast channel.

single shared broadcast channel two or more simultaneous transmissions by nodes:

interference collision if node receives two or more signals at the same

timemultiple access protocol distributed algorithm that determines how nodes

share channel, i.e., determine when node can transmit

communication about channel sharing must use channel itself! no out-of-band channel for coordination

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Ideal Multiple Access Protocol

Broadcast channel of rate R bps1. When one node wants to transmit, it can send

at rate R.2. When M nodes want to transmit, each can

send at average rate R/M3. Fully decentralized:

no special node to coordinate transmissions no synchronization of clocks, slots

4. Simple

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MAC Protocols: a taxonomy

Three broad classes: Channel Partitioning

divide channel into smaller “pieces” (time slots, frequency, code)

allocate piece to node for exclusive use

Random Access channel not divided, allow collisions “recover” from collisions

“Taking turns” Nodes take turns, but nodes with more to send can

take longer turns

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Channel Partitioning MAC protocols: FDMA

FDMA: frequency division multiple access 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

TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.

FDM (Frequency Division Multiplexing): frequency subdivided.

frequ

ency

bands time

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FDMA

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FDM of Three Voiceband Signals

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FDMA: example

AMPS (Advanced Mobile Phone System) The first cellular system in US Forward link 869-894 MHz Reverse link 824-847 MHz

Example: An operator with 12.5MHz in each simplex band. Bg is the guard band allocated at the edge of the allocated spectrum band. Bc is the channel bandwidth. Bg=10KHz Bc=30kHz N= (12.5M-2x10K)/30K =416 simultaneous users!

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Channel Partitioning MAC protocols: TDMA

TDMA: time division multiple access 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

TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.

FDM (Frequency Division Multiplexing): frequency subdivided.

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TDMA

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Example

GSM

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Code Division Multiple Access (CDMA) used in several wireless broadcast channels

(cellular, satellite, etc) standards unique “code” assigned to each user; i.e., code

set partitioning all users share same frequency, but each user

has own “chipping” sequence (i.e., code) to encode data

encoded signal = (original data) X (chipping sequence)

decoding: inner-product of encoded signal and chipping sequence

allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

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CDMA Encode/Decode

slot 1 slot 0

d1 = -1

1 1 1 1

1- 1- 1- 1-

Zi,m= di.cmd0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutput

channel output Zi,m

sendercode

databits

slot 1 slot 0

d1 = -1d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutputreceiver

code

receivedinput

Di = Zi,m.cmm=1

M

M

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CDMA: two-sender interference

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B

C

A

B

C

A(a)

B

C

A

B

C

A

MUXMUX

(b)

Trunkgroup

Multiplexing

Sharing network resources Bandwidth, router

buffer The cost of deploying

high bandwidth transmission line is more economical

Exploit the statistical behavior of users

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A CBf

Cf

Bf

AfW

W

W

0

0

0

(a) Individual signals occupy W Hz

(b) Combined signal fits into channel bandwidth

Frequency Division Multiplexing

The bandwidth is divided into frequency slots

Each frequency slot is allocated to a different user

FDM was first introduced in the telephone network

Other examples – broadcast radio and cable television

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Frequency Division Multiplexing

Useful bandwidth of medium exceeds required bandwidth of channel

Each signal is modulated to a different carrier frequency

Carrier frequencies separated so signals do not overlap (guard bands)

e.g. broadcast radio Channel allocated even if no data

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Frequency Division Multiplexing

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FDM System

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Analog Carrier Systems

AT&T (USA) Hierarchy of FDM schemes Group

12 voice channels (4kHz each) = 48kHz Range 60kHz to 108kHz

Supergroup 60 channel FDM of 5 group signals on carriers between 420kHz and

612 kHz Mastergroup

10 supergroups

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Time Division Multiplexing

(b) Combined signal transmits 1 unit every T seconds

tA1 A2

tB1 B2

tC1 C2

3T0T 6T

3T0T 6T

3T0T 6T

tB1 C1 A2 C2B2A1

0T 1T 2T 3T 4T 5T 6T

(a) Each signal transmits 1 unit every 3T seconds

Separate bit streams are multiplexed into a high-speed digital transmission line

Transmission is carried out in terms of frames which are composed of equal sized slots which are assigned to users

Demultiplexing is done by reading the data in the appropriate slot in each frame

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Time Division Multiplexing

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TDM System

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Low-SpeedMappingFunction

MediumSpeed

MappingFunction

High-Speed

MappingFunction

DS3

44.736

DS1

DS2

CEPT-1

CEPT-4

139.264

ATM

150 Mbps

STS-1

STS-1

STS-1STS-1STS-1

STS-1STS-1STS-1

STS-3c

STS-3c

OC-n

Scrambler E/O

51.84 Mbps

High-Speed

MappingFunction

MuxSTS-n

SONET Digital Hierarchy

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Statistical TDM

In Synchronous TDM many slots are wasted

Statistical TDM allocates time slots dynamically based on demand

Multiplexer scans input lines and collects data until frame full

Data rate on line lower than aggregate rates of input lines

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1

2

m

OpticalMUX 1

2

m

OpticaldeMUX

1 2.m

Opticalfiber

Wave Division Multiplexing

Optical-domain version of FDM Different information signals are modulated to different

wavelengths and the combined signals sent through the fiber Prism and difffraction gratings are used to combine/split

signals

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Wavelength Division Multiplexing Multiple beams of light at different frequency Carried by optical fiber A form of FDM Each color of light (wavelength) carries separate data channel 1997 Bell Labs

100 beams Each at 10 Gbps Giving 1 terabit per second (Tbps)

Commercial systems of 160 channels of 10 Gbps now available

Lab systems (Alcatel) 256 channels at 39.8 Gbps each 10.1 Tbps and Over 100km

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WDM Operation

Same general architecture as other FDM Number of sources generating laser beams at different

frequencies Multiplexer consolidates sources for transmission over single

fiber Optical amplifiers amplify all wavelengths

Typically tens of km apart Demux separates channels at the destination Mostly 1550nm wavelength range Was 200MHz per channel Now 50GHz

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Dense Wavelength Division Multiplexing DWDM No official or standard definition Implies more channels more closely

spaced that WDM 200GHz or less

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Random Access Protocols

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: slotted ALOHA ALOHA CSMA, CSMA/CD, CSMA/CA

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Slotted ALOHA

Assumptions all frames same size time is divided into

equal size slots, time to transmit 1 frame

nodes start to transmit frames only at beginning of slots

nodes are synchronized if 2 or more nodes

transmit in slot, all nodes detect collision

Operation when node obtains fresh

frame, it transmits in next slot

no collision, node can send new frame in next slot

if collision, node retransmits frame in each subsequent slot with prob. p until success

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Slotted ALOHA

Pros single active node can

continuously transmit at full rate of channel

highly decentralized: only slots in nodes need to be in sync

simple

Cons collisions, wasting

slots idle slots nodes may be able to

detect collision in less than time to transmit packet

clock synchronization

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Slotted Aloha efficiency

Suppose N nodes with many frames to send, each transmits in slot with probability p

prob that node 1 has success in a slot = p(1-p)N-1

prob that any node has a success = Np(1-p)N-1

For max efficiency with N nodes, find p* that maximizes Np(1-p)N-1

For many nodes, take limit of Np*(1-p*)N-1

as N goes to infinity, gives 1/e = .37

Efficiency is the long-run fraction of successful slots when there are many nodes, each with many frames to send

At best: channelused for useful transmissions 37%of time!

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Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization when frame first arrives

transmit immediately

collision probability increases: frame sent at t0 collides with other frames sent in [t0-

1,t0+1]

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Pure Aloha efficiencyP(success by given node) = P(node transmits) .

P(no other node transmits in [p0-1,p0] .

P(no other node transmits in [p0-1,p0]

= p . (1-p)N-1 . (1-p)N-1

= p . (1-p)2(N-1)

… choosing optimum p and then letting n -> infty ...

= 1/(2e) = .18 Even worse !

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CSMA (Carrier Sense Multiple Access)

CSMA: listen before transmit:If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission

Human analogy: don’t interrupt others!

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CSMA collisions

collisions can still occur:propagation delay means two nodes may not heareach other’s transmissioncollision:entire packet transmission time wasted

spatial layout of nodes

note:role of distance & propagation delay in determining collision probability

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CSMA/CD (Collision Detection)CSMA/CD: carrier sensing, deferral as in CSMA

collisions detected within short time colliding transmissions aborted, reducing channel

wastage collision detection:

easy in wired LANs: measure signal strengths, compare transmitted, received signals

difficult in wireless LANs: receiver shut off while transmitting

human analogy: the polite conversationalist

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CSMA/CD collision detection

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“Taking Turns” MAC protocolschannel partitioning MAC protocols:

share channel efficiently and fairly at high load

inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node!

Random access MAC protocols efficient at low load: single node can fully

utilize channel high load: collision overhead

“taking turns” protocolslook for best of both worlds!

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“Taking Turns” MAC protocolsPolling: master node

“invites” slave nodes to transmit in turn

concerns: polling overhead latency single point of

failure (master)

Token passing: control token passed

from one node to next sequentially.

token message concerns:

token overhead latency single point of failure

(token)

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Summary of MAC protocols

What do you do with a shared media? Channel Partitioning, by time, frequency or

code• Time Division, Frequency Division

Random partitioning (dynamic), • ALOHA, S-ALOHA, CSMA, CSMA/CD• carrier sensing: easy in some technologies (wire),

hard in others (wireless)• CSMA/CD used in Ethernet• CSMA/CA used in 802.11

Taking Turns• polling from a central site, token passing