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Multiple Access. ECS 152A. Concepts. Multiple access vs. multiplexing Multiplexing allows several transmission sources to share a larger transmission capacity. Often used in hierarchical structures.

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concepts
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
multiple access protocols
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 time

multiple 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
ideal multiple access protocol
Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1. 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/M

3. Fully decentralized:

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

4. Simple

mac protocols a taxonomy
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
channel partitioning mac protocols fdma
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.

time

frequency bands

fdma example
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!
channel partitioning mac protocols tdma
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.
code division multiple access cdma
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”)
cdma encode decode

d0 = 1

1

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1

1

d1 = -1

1

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M

Di = SZi,m.cm

m=1

M

d0 = 1

d1 = -1

CDMA Encode/Decode

channel output Zi,m

Zi,m= di.cm

data

bits

sender

slot 0

channel

output

slot 1

channel

output

code

slot 1

slot 0

received

input

slot 0

channel

output

slot 1

channel

output

code

receiver

slot 1

slot 0

multiplexing

(a)

(b)

A

A

A

A

Trunk

group

B

B

B

MUX

MUX

B

C

C

C

C

Multiplexing
  • Sharing network resources
    • Bandwidth, router buffer
  • The cost of deploying high bandwidth transmission line is more economical
  • Exploit the statistical behavior of users
frequency division multiplexing

(a) Individual signals occupy W Hz

A

f

W

0

B

0

f

W

C

B

A

f

C

f

0

W

(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
frequency division multiplexing18
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
analog carrier systems
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
time division multiplexing

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

A1

A2

t

0T

6T

3T

B1

B2

t

6T

3T

0T

C1

C2

t

0T

6T

3T

(b) Combined signal transmits 1 unit every T seconds

A2

B2

B1

C1

C2

A1

t

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

Time Division Multiplexing
  • 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
sonet digital hierarchy

DS1

Low-Speed

Mapping

Function

DS2

STS-1

CEPT-1

51.84 Mbps

DS3

Medium

Speed

Mapping

Function

STS-1

44.736

OC-n

STS-n

STS-3c

STS-1

Mux

E/O

Scrambler

CEPT-4

High-

Speed

Mapping

Function

STS-1

STS-1

139.264

STS-3c

STS-1

STS-1

High-

Speed

Mapping

Function

ATM

STS-1

150 Mbps

SONET Digital Hierarchy
statistical tdm
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
wave division multiplexing

Optical

MUX

Optical

deMUX

1

1

2

1

2

2.

m

Optical

fiber

m

m

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
wavelength division multiplexing
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
wdm operation
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
dense wavelength division multiplexing
Dense Wavelength Division Multiplexing
  • DWDM
  • No official or standard definition
  • Implies more channels more closely spaced that WDM
  • 200GHz or less
random access protocols
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
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

Slotted ALOHA
slotted aloha33
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

Slotted ALOHA
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

Slotted Aloha efficiency

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

At best: channel

used for useful

transmissions 37%

of time!

pure unslotted aloha
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]
pure aloha efficiency
Pure Aloha efficiency

P(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 !

csma carrier sense multiple access
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!
csma collisions
CSMA collisions

spatial layout of nodes

collisions can still occur:

propagation delay means

two nodes may not hear

each other’s transmission

collision:

entire packet transmission

time wasted

note:

role of distance & propagation delay in determining collision probability

csma cd collision detection
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
taking turns mac protocols
“Taking Turns” MAC protocols

channel 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” protocols

look for best of both worlds!

taking turns mac protocols42
“Taking Turns” MAC protocols

Token passing:

  • control token passed from one node to next sequentially.
  • token message
  • concerns:
    • token overhead
    • latency
    • single point of failure (token)

Polling:

  • master node “invites” slave nodes to transmit in turn
  • concerns:
    • polling overhead
    • latency
    • single point of failure (master)
summary of mac protocols
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