Multiple access - PowerPoint PPT Presentation

1 / 20
About This Presentation
Title:

Multiple access

Description:

... called ALOHA was developed for a wireless system at the University of Hawaii. ... The implication of the random delay after a collision is that the various remote ... – PowerPoint PPT presentation

Number of Views:90
Avg rating:3.0/5.0
Slides: 21
Provided by: alanwi8
Category:

less

Transcript and Presenter's Notes

Title: Multiple access


1
Multiple access
  • A single shared broadcast communications channel
    for a multi-point network.
  • Distributed algorithm determines manner in which
    stations access the channel - that is, when or
    how a station is permitted to transmit.
  • Communication about sharing must use the channel
    itself.
  • Medium Access Control (MAC)
  • Three categories of access control protocols
  • Random access
  • Controlled access
  • Channelization

2
Random Access
  • Stations contend for medium
  • Collisions (overlapping frame transmissions by
    two or more stations) can occur.
  • Methods needed for
  • Identifying when, and when not, to attempt
    transmission.
  • Detecting collisions
  • Resolving collisions

3
Multiple Access with ALOHA
  • In the 1970s, a protocol called ALOHA was
    developed for a wireless system at the University
    of Hawaii.
  • Multiple remote stations, plus one base station.
  • Frame transmissions are made at one frequency
    from a remote station to the base station the
    base station re-broadcasts frames on another
    frequency.
  • Contention is for access to the base station.
  • Access procedure
  • Frames can be sent at any time to the base
    station.
  • If no acknowledgment is received within 2 times
    the maximum propagation delay, assume that the
    frame is lost and retransmit after waiting a
    random amount of time.

4
Multiple Access Collisions
  • If more than one station broadcasts on the base
    station access frequency at the same time a
    collision interference will destroy the frames.
    The base station sends no acknowledgements when
    this occurs.
  • The implication of the random delay after a
    collision is that the various remote stations
    will choose (hopefully!) different times to try
    again and another collision will be avoided.
  • Two collision strategies
  • avoid them
  • detect their occurrence

5
Carrier Sense Multiple Access (CSMA)
  • In this approach, each station is responsible for
    monitoring the state of the transmission medium
    before and during frame transmission.
  • If the transmission medium is in use already, a
    station cannot send a frame. If the medium is
    clear, transmission can begin.
  • Due to propagation delays, collisions can still
    occur.
  • When a collision occurs, it must be resolved.

6
Monitoring the Transmission Medium
  • When the medium is detected to be in use at each
    station, that station cannot transmit.

7
Collision due to propagation delay
  • A station transmitting over the shared medium
    must continue to monitor the transmission for a
    collision.
  • This is known as CSMA/CD
  • CD collision detection

8
Ensuring Collision Detection
  • Any transmission has to be long enough that a
    collision can be detected at any station.
  • Transmission length must be at least twice the
    propagation delay between the most widely
    separated stations.

9
Resolving a collision
  • When a collision is detected, do the following
  • Send a jamming signal.
  • Stop transmission.
  • This is called backoff.
  • Choose a random number r between 0 and d, and
    after a delay of r, check if the medium is clear.
  • If the medium is clear, transmit
  • If the medium is in use, wait until free.
  • If a collision occurs on a subsequent attempt to
    send the same frame, repeat steps 1, 2, and 3.
  • Keep track of the number of attempts if a preset
    maximum attempt number (often 15) is reached,
    abort.
  • Exponential backoff the maximum delay time is
    doubled between transmission retries.

10
Collision avoidance
  • CSMA/CA
  • CA collision avoidance
  • When a sender wishes to transmit
  • Ensure that the medium has been clear for at
    least a minimum inter-frame gap time.
  • Wait for a random interval after the inter-frame
    gap.
  • Send the frame, and start a timer.
  • If no acknowledgement is received before the
    timer expires, count a transmission attempt, and
    backoff.
  • If the backoff limit is reached, abort.
  • In some cases, when the sender detects an
    inter-frame gap, it may choose to send only with
    a fixed probability otherwise, it will wait for
    another inter-frame gap.
  • This will reduce the probability of collisions.

11
Why CA in wireless
  • Generally, collision avoidance is used in
    wireless applications.
  • Collision detection requires that all stations be
    able to detect a collision.
  • Planning can rely on maximum cable length,
    well-defined signal strength at each host,
    spacing of hosts, etc.
  • With wireless stations, some stations may not
    receive sufficient signal strength to detect a
    collision
  • Moving stations
  • Stations may be temporarily obstructed
  • More noise on transmission medium
  • Since there is no guarantee that collsions can be
    detected, collision avoidance is used instead.

12
Controlled Access
  • Objective ensure that only one station has the
    right to send at any time.
  • Methods
  • Reservation a station must reserve a time slot
    before sending.
  • Polling a primary station asks each secondary
    station in turn if it wishes to transmit a data
    frame or not.
  • Token passing a special frame called a token
    is passed from host to host.
  • Only the host with the token is permitted to send
    a data frame.
  • If a host has no data to send, it must pass the
    token immediately to another host.
  • Alternative approach a host can hold the token
    for a small maximum time interval before passing
    the token.

13
Channelization
  • Techniques from physical layer multiplexing can
    be used by the data link layer
  • Frequency division multiple access (FDMA)
  • Each station is allocated its own frequency.
  • Frequency division multiplexing (FDM) used at
    physical layer.
  • Time division multiple access (TDMA)
  • Each station is allocated its own time slot for
    transmission uses TDM at physical layer.
  • Code division multiplexing (CDMA)
  • Use of encoding schemes to multiplex several
    signals.

14
Code Division Multiple Access
  • Each station is assigned a sequence of numbers,
    referred to as a chip.
  • Examples
  • A 1, 1, 1, 1
  • B 1, -1, 1, -1
  • C 1, 1, -1, -1
  • D 1, -1, -1, 1
  • The chips sequences are carefully selected.

15
Transmitting using CDMA
  • Encoding rule for data stream
  • Data bit 0 encode as -1
  • Data bit 1 encode as 1
  • No data to send encode as 0
  • Transmission
  • Stations A, B, C, D each take their next data bit
    to send, encode it as -1, 1, or 0 and multiply
    that code by each number in the chip sequence to
    obtain a 4-tuple.
  • The four 4-tuples are added together and the sum
    is transmitted.
  • The values will be the range -4 to 4, so 9
    levels of physical layer coding are needed.

16
CDMA multiplexing
17
Decoding CDMA
  • The input to the demultiplexer is a 4-tuple of
    values between -4 and 4.
  • Each station takes the four values, and
    multiplies the values by the chip sequence.
  • The resulting values are then summed to obtain a
    single value. The result will always be -4, 4,
    or 0.
  • Divide the result by 4 to get a value -1, 1, or
    0.
  • Decode this result to a data bit of 0, 1, or no
    data.

18
CDMA Demultiplexing
19
CDMA chip sequences
  • The chip sequences are chosen to be orthogonal
  • If we multiply a sequence by -1, every element is
    complemented. Note that this means that real
    data bits are complements of each other.
  • For a set of N sequences, if the inner product of
    two sequences is determined, A A is N,A (-A)
    is -N, and A B is 0 otherwise.

20
Organization note
  • Before this slide material for midterm.
  • After this slide will not be on midterm
Write a Comment
User Comments (0)
About PowerShow.com