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Multiple Access Protocols (Contd.)


To achieve true mobility, portable computers need to use radio ... Based on Simulation studies of MACA a new algorithm referred to as MACAW has been proposed ... – PowerPoint PPT presentation

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Title: Multiple Access Protocols (Contd.)

Multiple Access Protocols (Contd.)
  • Dr. R. K. Rao

Wireless LAN Protocols
  • To achieve true mobility, portable computers need
    to use radio (or Infra Red) signals for
  • A system of portable computers that communicate
    by radio can be regarded as a Wireless LAN
  • These LANs have different properties than
    conventional LANs and require special MAC layer

Wireless LAN Protocols
  • A common configuration for a wireless LAN is an
    office building with Access Points (base
    stations) strategically placed around the
  • All Access Points (base stations) are wired
    together using copper or fiber.
  • If the transmission power of the Access Points
    (base stations) and Portables is adjusted to have
    a range of 3 to 4 meters, then each room becomes
    a single cell, and entire building becomes a
    large cellular system

Wireless LAN Protocols
  • Unlike the cellular telephone system, each cell
    has only one channel, covering the entire
    available bandwidth. Typically its bandwidth is
    1 to 2 Mbps
  • In order to understand the Wireless LAN protocol,
    let us make the assumption that
  • all radio transmitters have the same fixed range

Wireless LAN Protocols
  • When a receiver is within range of two active
    transmitters, the resulting signal will be
    garbled and useless
  • In some Wireless LANs, not all stations are
    within range of one another, which leads to a
    variety of complications
  • In indoor Wireless LANs, the presence of walls
    between stations can have a major impact on the
    effective range of each station.

Hidden Station Problem
  • What happens if we try CSMA technique (just
    listen for other transmissions and only transmit
    if no one else is doing so) in Wireless LANs?
  • Consider 4 wireless stations as illustrated in
    the Figure 1. It does not matter whether these
    are portable or access points
  • The radio range is such that A and B are within
    each others range and can potentially interfere
    with one another.

Hidden Station Problem
  • A wireless LAN with Station A transmitting Figure

Hidden Station Problem
  • Station C can also potentially interfere with
    both Stations B and D, but not Station A.
  • Suppose that A is transmitting to B. If C senses
    the medium, it will not hear A because A is out
    of range, concludes falsely that it can transmit
  • If C does start transmitting, it will interfere
    at B, wiping out the frame from A.

Hidden Station Problem
  • What matters in Wireless LANs is the interference
    at the receiver and not at the sender
  • The problem of a station not being able to detect
    a potential competitor for the medium, because
    the competitor is too far away is referred to as
    the Hidden Station Problem

Exposed Station Problem
  • A wireless LAN with Station A transmitting Figure

Exposed Station Problem
  • Consider the reverse situation wherein B is
    transmitting to A.
  • If C senses the medium, it will hear an ongoing
    transmission and falsely conclude that it may not
    send to D, when in fact such transmission would
    cause bad reception only in the zone between B
    and C, where neither intended receivers is
  • This situation is referred to as the Exposed
    Situation Problem

Wireless LAN Protocols
  • The problem is that before starting a
    transmission, a station really wants know whether
    or not there is activity around the receiver.
  • CSMA merely tells the station whether or not
    there is activity around the station sensing the

An Analogy
  • Imagine an office building with every employee
    equipped with a wireless portable computer.
  • Suppose that Sandra wants to send a message to
  • Sandras computer senses the local environment
    and detecting no activity, starts sending message
  • However, there may still be a collision in
    Miltons office because a third party may
    currently be sending to him from a location so
    far from Sandra that her computer cannot detect it

Multiple Access Collision Avoidance (MACA)
  • Used as the basis for IEEE 802.11 wireless LAN
  • The basic idea is to stimulate the receiver into
    outputting a short frame, so stations nearby can
    detect this transmission and avoid transmitting
    themselves for a duration of the upcoming data
    frame (large)

Collision Avoidance Scheme
The MCCA Protocol
  • A is sending an RTS to B
  • B responds with a CTS back to A

CSMA with Collision Avoidance
Multiple Access Collision Avoidance (MACA)
  • A starts by sending an RTS (Request To Send)
    frame to B this short frame (30 bytes) contain
    the length of the data frame that will eventually
  • B responds with CTS (Clear To Send) frame CTS
    frame contains the data length (copied from the
    RTS frame).
  • Upon receipt of the CTS frame from B, A begins

Multiple Access Collision Avoidance (MACA)
  • Any station hearing the RTS is close to A and
    must refrain from transmission long enough for
    the CTS to be transmitted back to A without
  • Any station hearing CTS is close to B and must
    remain silent during the upcoming data
    transmission, whose length it can tell by
    examining the CTS frame

Multiple Access Collision Avoidance (MACA)
  • In the Figure, C is within the range of A but not
    within the range of B.
  • C, therefore, hears the RTS but not CTS from B.
  • As long as C does not interfere with the CTS, it
    is free to transmit while the data frame is being
  • In contrast, D does not hear RTS but hears CTS
  • Hearing CTS tips it off that it is close to a
    station that is about to receive a frame, so it
    defers from sending anything until that frame is
    expected to be finished.

Multiple Access Collision Avoidance (MACA)
  • Station E hears both control messages, like D,
    must remain silent until the data frame is
  • Can collisions still occur? Yes
  • For example, B and C could both send RTS frames
    to A at the same time.
  • In the event of a collision, an unsuccessful
    transmitter waits a random amount of time and
    tries later (exponential backoff)

Media Access Protocol for Wireless LAN
  • Based on Simulation studies of MACA a new
    algorithm referred to as MACAW has been proposed
  • ACK frame is transmitted after each successful
    data frame.
  • A mechanism has been added for stations to
    exchange information about congestion to improve

  • Radio and infrared transmission is susceptible to
    noise and interference, so such transmission is
    not very reliable
  • Wireless links are prone to errors. High packet
    loss rate detrimental to transport-layer
  • Mechanisms needed to reduce packet loss rate
    experienced by upper layers

A simple Solution to Improve Reliability
  • When node B receives a data packet from node A,
    node B sends an Acknowledgement (ACK). This
    approach adopted in many protocols.
  • If node A fails to receive an ACK, it will
    retransmit the packet

Medium Access Control
  • MAC sublayer is responsible
  • for the channel access procedure
  • Frame formatting
  • Error checking
  • Fragmentation and reassembly
  • MAC management services are also defined to
    support roaming within an ESS and to assist
    stations in power management

IEEE 802.11 DCF
  • Distributed Coordination Function (DCF). DCF
    provides support for asynchronous data transfer
    on a best-effort basis.
  • Uses RTS-CTS exchange to avoid hidden terminal
  • Any node overhearing a CTS cannot transmit for
    the duration of the transfer
  • Uses ACK to achieve reliability
  • Any node receiving the CTS cannot transmit for
    the duration of the transfer

IEEE 802.11 DCF
  • CSMA/CA Wireless MAC protocols often use
    collision avoidance techniques, in conjunction
    with carrier sense mechanism
  • Carrier sense When a node wishes to transmit a
    packet, it first waits until the channel is idle
  • Collision avoidance once channel becomes idle,
    the node waits for a fixed period (e.g. DIFS)
    plus a randomly chosen duration before attempting
    to transmit

Elements of Queuing Theory
  • Objective is to understand basics of Queuing
    models and their application to networking
  • All models are an approximation
  • Exact solution is not necessary

Example 1 Network gateway analysis
  • Measurements at gateway show the following
  • pkts arrive at a mean rate of 125 pps (packet per
  • Gateway takes 2 millisecs to forward them
  • What is the probability of buffer overflow if the
    gateway has only 12 buffers (queue size)?
  • If we want to limit pkt loss (due to overflow) to
    10-6 , how many buffers do we need?

Example 2 Analysis of usage of LAN at a computer
  • The LAN contains 5 graphics workstations
  • Students arrive at an average rate of 10 per hour
  • A student spends 20 minutes at the workstation on
    an average
  • Complaint waiting times are too long
  • Is the complaint justified?

Example 3 System with loss
  • In an office there are 20 people. During the
    course of an 8 hour day, each person attempts on
    average 16 calls lasting 3 mins/call on an
  • There are 3 telephone lines in the office.
  • What is the probability of call being attempted
    when all three lines are busy? (prob. Of loss)

Model of a single-server queue
Elements of queuing system
  • Arrival process (interarrival time distribution)
  • Service time distribution
  • Number of servers
  • System capacity (Buffer Size)
  • Service discipline

Arrival process
  • Interarrival times assumed to be iid sequence
    (iid independent, identically distributed)
  • M exponentially distributed
  • D deterministic (constant)
  • G general interarrival distribution

Service process
  • Service times are assumed iid
  • Service times are independent of interarrival
  • M, D, G analogous to arrival process symbols

System Capacity (Buffer Size)
  • Maximum number of customers that can be present
    in system at any time
  • Includes both those in queue and those in service
  • If customer arrives when system is full, customer
    is blocked (discarded)
  • Example
  • In a cell if all channels are being used no new
    call is accepted

Service Discipline
  • Rule for choosing next customer to be served
  • FCFS or FIFO (first-in first-out)
  • LCFS or LIFO (last-in first-out)
  • Priority discipline
  • if higher priority customer arrives, a lower
    priority customer will be bumped from service

Description of a Queuing System
  • How do we specify the Queuing System?
  • M/M/1 Systems , FIFO, Infinite Buffer (simplest
  • G/G/m Systems, FIFO, Infinite Buffer (more
    complex system)