Links and Multiple Access

1
Links and Multiple Access

• dedicated links
• point-to-point (single wire, e.g. PPP, SLIP)
• shared links
• broadcast (shared wire or medium e.g, Ethernet,
  802.11b, etc.)
• two or more simultaneous transmissions by nodes
  interference, only one node can send successfully
  at a time
• multiple access protocol
• distributed algorithm that determines how
  stations share channel, i.e., determine when
  station can transmit
• Ideal multiple access protocol
• When only one node wants to transmit, it can send
  at full rate R.
• When M nodes want to transmit, each can send at
  average rate R/M
• Fully decentralized
• no special node to coordinate transmissions
• no synchronization of clocks, slots
• Simple

2
Media Access Control Protocols

• Three broad classes
• Channel Partitioning
• divide channel into smaller pieces (time slots
  TDMA, frequency FDMA, code CDMA)
• allocate piece to node for exclusive use
• Random Access
• allow collisions (how to detect collisions?)
• recover from collisions (how to recover from
  collisions?)
• ALOHA / slotted ALOHA, CSMA, CSMA/CD, and CSMA/CA
• Taking turns
• tightly coordinate shared access to avoid
  collisions

3
Code Partitioning (CDMA)

• CDMA (Code Division Multiple Access)
• unique code assigned to each user i.e., code
  set partitioning
• used mostly in wireless broadcast channels
  (cellular, satellite, etc)
• 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)

4
CDMA Encode/Decode
5
CDMA two-sender interference
6
Random AccessPure (unslotted) ALOHA

• unslotted Aloha simpler, no synchronization
• pkt needs transmission
• send without awaiting for beginning of slot
• collision probability increases
• pkt sent at t0 collide with other pkts sent in
  t0-1, t01

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Pure Aloha (cont.)

• P(success by given node) P(node transmits) .
• P(no
  other node transmits in t0-1,t0 .
• P(no
  other node transmits in t0,t0 1
• p .
  (1-p)N-1 . (1-p)N-1
• P(success by any of N nodes) N p . (1-p)N-1.
  (1-p)N-1
• 
  choosing optimum p as N -gt infty ...
• 
  1/(2e) .18

Even worse !
8
Random Access Slotted ALOHA

• 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

• 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

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Slotted ALOHA
Success (S), Collision (C), Empty (E) slots

• 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
• clock synchronization

10
Slotted Aloha efficiency
Efficiency is the long-run fraction of successful
slots when there are many nodes, each with many
frames to send

• 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

At best channel used for useful transmissions
37 of time!
11
CSMA (Carrier Sense Multiple Access)

• CSMA listen before transmit
• Persistent (1-persistent)
• Most aggressive
• If channel sensed busy, defer transmission until
  the end of transmission
• If channel sensed idle, transmit entire frame
• Non-Persistent
• Less aggressive
• If channel sensed idle, transmit
• If channel sensed busy, wait a random period and
  then retry the sense/transmit process
• Human analogy dont interrupt others!
• P-Persistent
• Applies to slotted channels
• If channel sensed idle, transmit with probability
  P

12
CSMA collisions
spatial layout of nodes
collisions can occur propagation delay means
two nodes may not hear each others transmission
collision entire packet transmission time wasted
note role of distance and propagation delay in
determining collision prob.
13
CSMA/CD

• CSMA/CD carrier sensing, deferral as in CSMA
• collision detection
• collisions detected within short time
• colliding transmissions aborted, reducing channel
  wastage
• 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

14
IEEE 802.11 CSMA/CA

• avoid collisions 2 nodes transmitting at same
  time
• 802.11 CSMA - sense before transmitting
• dont collide with ongoing transmission by other
  node
• 802.11 no collision detection!
• difficult to receive (sense collisions) when
  transmitting due to weak received signals
  (fading)
• cant sense all collisions in any case hidden
  terminal, fading
• goal avoid collisions CSMA/C(ollision)A(voidance
  )

distributed inter frame space
short inter frame space
15
Wireless LANs Issues (CSMA)

• The range of a single radio may not cover the
  entire system
• Hidden station problem (A-gtB, C-gtB since C does
  not hear A, collision)
• Exposed station problem (B-gtA, C hears B, C wont
  send to D, reduced efficiency)

16
Multiple Access with Collision Avoidance

• IDEA having a short frame transmitted from both
  sender and receiver before the actual long data
  transfer
• A sends a short RTS (30 bytes) to B with length
  of L
• B responds with a CTS to A, whoever hears CTS
  shall remain silent for the duration of L
• A sends data (length L) to B
• Avoid data collision with small reservation frames

17
WLAN MACDistributed Coordination Function

• Distributed Control, Ethernet-like CSMA
• CSMA/CA (collision avoidance)
• Physical channel sensing
• Sense channel, transmit entire frame, retry if
  necessary
• Virtual channel sensing (MACAW)
• Add ACK frame

Short frame (30B) Contains data length
data length copied from RTS
Network Allocation Vector (quiet time)
18
Fragmentation for Throughputfragment burst

• Unreliable ISM bands
• Error rate p 10-4,
• success rate for full Ethernet frame (12,144 bit)
  lt30, (1-p)n
• Error rate p 10-6, 1 will be damaged.

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WLAN MACPoint Coordination Function (PCF)

• Central Control
• Base polls other stations
• Broadcast a beacon frame periodically (10ms to
  100ms) with system parameters (hopping sequence,
  dwell time, clock synchronization)
• Base determines the transmission priority
• QoS guarantee
• Can Coexist with DCF

RTS/CTS/ACK
Fragment burst
Short InterFrame Spacing
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Quiz

• If a node is transmitting a packet of size K, and
  transmission bandwidth is R. Given the
  propagation speed is Sp, and the length of the
  shared link is L, answer the following question
• under what condition will a transmitting node be
  able to apply collision detection during its
  transmission?

21
Quiz

• Which of the following are correct
• A. Channel partitioning MAC protocols share
  channel efficiently at high load
• B. Channel partitioning MAC protocols share
  channel efficiently at low load
• C. Random access MAC protocols share channel
  efficiently at high load
• D. Random access MAC protocols share channel
  efficiently at low load

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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!

23
Taking Turns MAC protocols

• Polling (e.g. PCF)
• 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)

• Bit Map Reservation
• Station reserves contention slots in advance
• concerns
• polling overhead
• latency

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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)

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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,Code Division, Frequency Division
• Random partitioning (dynamic),
• MA ALOHA
• MA/CD -- S-ALOHA
• CSMA
• persistent, non-persistent, p-persistent
• CSMA/CD used in Ethernet
• CSMA/CA used in WiFi
• Taking Turns
• polling from a central site, token passing