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Multiple Access Networking
Professor Izhak Rubin
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
Communication Channel
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 Frame Time Frame Time Frame
Time t
MUXed Comm Channel
Rx Tx
Station 3
Messages Received Across Channel
Message Arrivals For Tx
Queue Queue
Head of the line packets are destined to stations 4,3,2
From slot 3 in each frame
MUX DeMUX
Communication Channel
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
Messages Received Across Channel
Message Arrivals For Tx
Queue Queue
Received in specified Band
MUX DeMUX
Communication Channel
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
Messages Received Across Channel
Message Arrivals For Tx
Queue Queue
Head of the line message is destined to station 2 and is transmitted in Band 2
Received in Band 3
MUX DeMUX
Communication Channel
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
Messages Received Across Channel
Message Arrivals For Tx
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
Communication Channel
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
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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
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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.
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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)
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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
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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
Messages Received Across Channel
Message Arrivals For Tx
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.
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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
Messages Received Across Channel
Message Arrivals For Tx
Queue Queue
To Slot 2 in a frame From any slot
Time Frame Time Frame Time Frame
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.
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Demand Assigned / TDMA
1 2 3 4 1 2 3 4 1 2 3 4 Shared Comm Channel
Rx Tx
Station 3
Messages Received Across Channel
Message Arrivals For Tx
Queue Queue
To Slot 2 in a frame From any slot
Time Frame Time Frame Time Frame
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.
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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
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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
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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
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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
Time Successful Transmission Collision Successful Retransmission
An Slotted ALOHA System Operation
P1 P2 P3
P2
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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
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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)
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S vs. G Performance Curves
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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
Time Successful Transmission Collision Successful Retransmission
A CSMA System Operation
P1 P2
P3
P2
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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.
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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
Time Successful Transmission Collision Successful Retransmission
A CSMA/CD System Operation
P1 P2
P3
P2
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802.11 Medium Access Control (MAC) for Wireless Local Area Networks (WLANs)
Reference: website of IEEE 802.11
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Basic Access Protocol Features
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CSMA with Collision Avoidance (CSMA/CA)
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CSMA/CA + ACK Protocol
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Hidden Node Problem
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Hidden Node Provisions
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Frame Formats
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Address Field Description
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Power Management Approach
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Wireless LAN Infrastructure Network
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Roaming