Section 3 Multiple Access Communications Networks

Multiple Access Networking

Professor Izhak Rubin

[email protected]

Electrical Engineering Department

UCLA

2014-2015 by Professor Izhak Rubin

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Information Sources

Analog; Digital Real-time Store and Forward

1 2 3 4 1 2 3 4 1 2 3 4

Real-Time Transmission of a Stream

Time Frame=125 microsec Time Frame Time Frame

Store & Forward Transmission of a Stream

Examples: Voice Video Imaging Facsimile Data

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Multiplexing

Definition: Sharing communication channel (service) resource among collocated stations (clients)

Stations are said to be collocated when there is a low-delay low-cost mechanism for scheduling controlling and coordinating their use of the shared resource

Note: Messages fed into the Mux reside in a common buffer facility; scheduler then orderspackets for transmission across channel using designated slots.

MUX DeMUX

Communication Channel

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Multiplexing Methods

1. Fixed Assigned: channel resource dedicated to a link flow Frequency Division Multiplexing (FDM)

Time Division Multiplexing (TDM)

Wavelength Division Multiplexing (WDM)

2. Demand Assigned: channel resource allocated on demand Asynchronous Time Division Multiplexing

(ATDM) Statistical Multiplexing

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Time Division Multiplexing (TDM)-1

1 2 3 4 1 2 3 4 1 2 3 4

Time Frame Time Frame Time Frame

Time t

MUXed Comm Channel

Rx Tx

Station 3

Messages Received Across Channel

Message Arrivals For Tx

Queue Queue

Tx in slot 3 of every frame

MUX DeMUX


Control process, or via net management, used to allocate a slot for transmissions from station-i to station-j. Assignment is fixed, or (slowly) programmable.

Rx in specified slots

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Time Division Multiplexing (TDM)-2

1 2 3 4 1 2 3 4 1 2 3 4


Time t

MUXed Comm Channel

Rx Tx

Station 3



Queue Queue

Head of the line packets are destined to stations 4,3,2

From slot 3 in each frame

MUX DeMUX


Control process, or via net management, used to allocate a slot for transmissions from station-i to station-j. Assignment is fixed, or (slowly) programmable.

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Frequency Division Multiplexing (FDM) - 1

Frequency Band 2

Frequency Band 3

Frequency Band 4

Frequency Band 1 MUXed

Comm Channel

Frequency

Rx Tx

Station 2



Queue Queue

Received in specified Band

MUX DeMUX


Control process, or via net management, used to allocate band-k for transmissions from station-i to station-j. Assignment is fixed, or (slowly) programmable.

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Frequency Division Multiplexing (FDM) - 2

Frequency Band 2

Frequency Band 3

Frequency Band 4

Frequency Band 1 MUXed

Comm Channel

Frequency

Rx Tx

Station 3



Queue Queue

Head of the line message is destined to station 2 and is transmitted in Band 2

Received in Band 3

MUX DeMUX


Control process, or via net management, used to allocate band-k for transmissions from station-i to station-j. Assignment is fixed, or (slowly) programmable.

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Asynchronous Time Division Multiplexing (ATDM) Statistical Multiplexing

Time t

Rx Tx

Station 3



Queue Queue

Head of the line packets are destined to stations 4,3,2; packet headers included.

From any slot; packets header destination ID= Station 3

MUX DeMUX


MUXed Comm Channel

Queue Service Discipline: FCFS (FIFO) or Priority

Packets are transmitted across available

time slots. Packet includes a header identifying source-destination stations.

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Voice Digitization PCM (Pulse Code Modulation):

Time Sampling at Nyquist Rate = 2 x BW = 2 x 4KHz = 8000 samples / sec = 125 microsec /sample

Amplitude Quantization = 8bits/sample Data Rate = 8000 x 8 = 64 K bps ADPCM: 32 Kbps (4bits/ sample);

Other compression schemes: 8K 16K bps

Transmission of Voice Messages Across Communication Channels: Tx a single sample every 125 microsec across a deidcated channel (at 64 Kbps for

PCM voice)

Example: Four 64 Kbps voice streams multiplexed across a 256 Kbps Channel; 8 bits/segment

sample; slot time = 8 / 256K =31.25 microsec; Frame Time = 125 microsec (TDM)

1 2 3 4 1 2 3 4 1 2 3 4

Time Frame=125 microsec Time Frame Time Frame

31.25 microsec


Voice Digitization Packet Voice Assemble K samples into a packet (segment). Transmit the packet

across the communications channel. Use replay buffer at the receiver to smooth out statistical transport delay variations.

Example: ADPCM voice packetized into packets which contain 128 samples/packet = 4 x 128 = 512 voice bits/packet + 64 bits OH/packet = 576 bits/packet. Inter-packet generation time = 128x125microsec = 16 msec. Source loading rate = 1packet/16 msec = 62.5 packets/sec =62.5 x 576 = 36 kbps.

Communication Network

Source

Packet Buildup Buffer Tx Replay Buffer Rx

Other Sources

Ex: Packet Buildup time = 16 msec Packet Transmit Time / 256K Link= 576/256K =2.25 msec

Ex: End-to-End Network transfer Delay = 10 350 ms

Ex: 10- 50 msec


Voice Digitization Packet Voice (cont.)

Multiple Access Methods: Under a Fixed Assignment Circuit Switched System, each source

is allocated (via TDMA or FDMA methods) a 36 kbps channel Under a Packet Switched method, a demand assigned method

is used. For example, under a FCFS (First Come First Served) statistical

multiplexer, assuming a voice activity factor of 50%, 4 packetized voice streams are multiplexed across a communications channel operating at rate of R=4x36Kbpsx50%=72kbps. Or, across a 256 kbps channel one can multiplex 256K / 18K = 14 voice streams.

Note: each packet contains an header identifier; packets will have to be discarded when more than 256K/36K = 7 streams are active.


Multiple Access Networking

Definition: Sharing communication channel (service) resource among distributed stations (clients)

Stations are said to be distributed when there is no low-delay low-cost mechanism for scheduling, controlling and coordinating their use of the shared resource

Methods: Fixed Assignment

Frequency Division Multiple Access (FDMA)

Time Division Multiple Access (TDMA)

Demand Assignment Reservation Based

DA /FDMA; DA/TDMA

Polling Based Centralized Polling Token Passing Polling

Random Access ALOHA random Access Carrier Sense Multiple Access

(CSMA) CSMA with Collision Detection

(CSMA/CD)


Categorization of Medium Access Control Procedures

Fixed Assigned (FA) Demand Assignment (DA) Random Access (RA)

Signaling (SIG) /Control Techniques

T F C

S

Information Transmission Method

T F C

S

SIG IT

T F C

S

SIG/Control IT

T F C

S

Over (T,F,C,S)

Fixed Adaptive Channel Sensing Signaling/Control Channel

FA

FA, RA

Per-station Signaling/Control Access Scheme

Polling Reservation

Res/Assignment Access

FA, RA

Poll/Response Access

DA


Frequency Division Multiple Access (FDMA)

Parameters: Channel Data Rate (R)

[bps] and Bandwidth W [Hz]

Band Tx Rate (R(B)) [bps] and BW W(B) [Hz] BW Index of Utilization = R(B)/W(B) [bps/Hz]

No. of Bands = N(B) Others: Channel BW

Overhead Others: Band Overhead Others: Buffer Capacity

Frequency Band 2

Frequency Band 3

Frequency Band 4

Frequency Band 1 Shared

Comm Channel

Rx Tx

Station 2



Queue Queue

Transmit in Band 2 Receive from any band or across designated bands

Control channel, or via net management, used to allocate band-k for transmissions from station-i to station-j. Assignment is fixed, or (slowly) programmable.


Time Division Multiple Access (TDMA)

1 2 3 4 1 2 3 4 1 2 3 4 Time t

Shared Comm Channel

Rx Tx

Station 2



Queue Queue

To Slot 2 in a frame From any slot


Parameters: Channel Data Rate (R) [bps] Slot Length (T(S)) [sec] No. of Slots/Frame = N(SF) Frame Duration = T(F)=T(S)

N(SF) [sec] Segment Length = L(S) [bits] Others: Frame Overhead, Frame

Synch Preamble Others: Slot Overhead Others: Buffer Capacity

Control channel, or via net management, used to allocate time slots in each frame for transmissions from station-i to station-j. Assignment is fixed, or (slowly) programmable.


Demand Assigned / TDMA

1 2 3 4 1 2 3 4 1 2 3 4 Shared Comm Channel

Rx Tx

Station 3



Queue Queue

To Slot 2 in a frame From any slot


Signaling subsystem: Reservation (order wire) channel set by controller by using announced slots.

S1

S3 S4

S2

Multiple Access Network Slot allocations made by controller (or in a distributed manner) in response to requests made by stations.


Demand Assigned Multiple Access: Polling

Centralized Structure

Distributed Structure

Hub Polling

Token Passing

S1 S5

S4 S3 S2

Central Controller Station Responds to Poll if it is active

Polls in turn Every station

Cluster Controller

Communication Bus


Demand Assigned Multiple Access: Token Passing Polling

Upon Receiving a Token, the Station: If Idle, lets the token pass by If Busy, seizes the token, transmits its packets, regenerates the token and

puts it on the medium when its transmission is complete (or upon reaching its dwell time limit)

Examples of Token Passing Ring Networks: IBM Token Ring, IEEE 802.5 Token Ring; 4/16Mbps FDDI (Fiber Data Distribution Interface); 100 Mbps; 100 Km.

S1 S5

S4 S3 S2

Token Passing Network (with early token release)

Polls in turn Every station

Token

Packet (from s3) Performance of a polling network is determined by the relative value of the walk time = time it takes a token to poll all stations when no station has a packet ready for access


Random Access: ALOHA Scheme

Unslotted ALOHA Access Control Algorithm: A ready station transmits its packet across the channel The station determines the outcome of the transmission

If no other stations transmission overlaps packets transmission is successful

If other station transmission overlap collision; The station then retransmits its packet after a random retransmission delay

Communication Bus

Wireline Multiple Access Network

S1

S3 S4

S2

Radio Communications Wireless Network


Random Access: ALOHA Scheme

Slotted ALOHA Access Control Algorithm: A ready station transmits its packet across the channel at a start of a slot The station determines the outcome of the transmission:

If no other stations transmission overlaps packets transmission is successful If other station transmissions overlap collision; the station then retransmits its

packet (at a start of a slot) after a random retransmission delay

Time Successful Transmission Collision Successful Retransmission

An Unslotted ALOHA System Operation

P1 P2

P3

P2


An Slotted ALOHA System Operation

P1 P2 P3

P2


Throughput Performance Measures

Throughput = average number of successful packet transmissions per unit time

Normalized throughput (s) = average number of successful packet transmissions per slot

Slot duration = time to transmit a (max length) packet

Hence: 0 s 1.

Throughput Capacity = maximum achievable throughput (or normalized throughput) level


Throughput Performance under Random Access MAC

G = average number of packet transmissions per slot

Assume the channel process to be modeled as a Poisson process:

P(number of transmissions per slot = k) = exp(-G)Gk/k!, k=0,1,2,

Under slotted ALOHA protocol:

s = P(number of transmissions = 1) = G*exp(-G)


S vs. G Performance Curves


Random Access: Carrier Sense Multiple Access (CSMA) Scheme

CSMA Access Control Algorithm 1. A Ready station listens to the channel

If the channel is Busy, it listens again later If the channel is Idle, it transmits its packet

2. The station determines the outcome of the transmission: If no other stations transmission overlaps packets transmission is

successful If other station transmissions overlap collision; The station then

reschedules its next sensing of the channel to take place after a random back-off time

Go to step 1


A CSMA System Operation

P1 P2

P3

P2


CSMA: Performance Factor

Performance depends on the value of the acquisition factor a = t(a) / T; where T = average packet transmission time; t(a) = channel acquisition time = time taken by a ready station to initiate transmission, when allowed, and induce all other stations to detect it so that they avoid accessing the channel at this time.


Random Access: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Scheme

CSMA/CD Access Control Algorithm: 1. A ready station listens to the channel.

If the channel is Busy, it listens again later If the channel is Idle, it transmits its packet

2. The station determines the outcome of the transmission while it is transmitting its packet:

If no other stations transmission overlaps packets transmission is successful

If other station transmission overlap collision; The station aborts transmission at the collision detection time, it then reschedules its next sensing of the channel to take place after a random back-off time

Go to Step 1


A CSMA/CD System Operation

P1 P2

P3

P2

802.11 Medium Access Control (MAC) for Wireless Local Area Networks (WLANs)

Reference: website of IEEE 802.11


Basic Access Protocol Features


CSMA with Collision Avoidance (CSMA/CA)


CSMA/CA + ACK Protocol


Hidden Node Problem


Hidden Node Provisions


Frame Formats


Address Field Description


Power Management Approach


Wireless LAN Infrastructure Network


Roaming

Section 3 Multiple Access Communications Networks

Documents

frequency band

channel resource

stream time frame

channel message arrivals

tx queue queue tx

tx queue queue head

multiplexing definition

multiplexing methods