chapter 12 multiple access figure 12.1 data link layer divided into two functionality-oriented...
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Chapter 12 Multiple Access
Figure 12.1 Data link layer divided into two functionality-oriented sublayers
Figure 12.2 Taxonomy of multiple-access protocols
12-1 RANDOM ACCESS12-1 RANDOM ACCESS
In In random accessrandom access or or contentioncontention methods, no station is superior to methods, no station is superior to another station and none is assigned the control over another. No another station and none is assigned the control over another. No station permits, or does not permit, another station to send. At each station permits, or does not permit, another station to send. At each instance, a station that has data to send uses a procedure defined by instance, a station that has data to send uses a procedure defined by the protocol to make a decision on whether or not to send. the protocol to make a decision on whether or not to send. Figure 12.3 Frames in a pure ALOHA network
Figure 13.3 ALOHA network
The earliest random-access method ,developed in 1970sDesigned to be used on wireless Local area Network at 9600bps
Based on Following rules
•Multiple Access
•Any station can send a frame when it has to send a frame
•Acknowledgment
•After sending a frame the station waits for an acknowledgment .If it does not receive the Ack within 2times maximum propagation delay ,It tries sending the frame again after a random amount of time
CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission Human analogy: don’t interrupt others!
CSMA collisions collisions can still occur:
propagation delay means two nodes may not hear each other’s transmission
collision: entire packet transmission time wasted
•Sense before transmit•The possibility of collision still exist because of the propagation delay
Figure 13.6 Persistence strategies
It defines the procedure for a station that senses a busy medium
•Station senses the line ,If the line is Idle station sends the frame immediately
•If the line is not Idle station waits for a random period of time and then senses the line again
This method has two variationsI-PERSISTANTStation senses the line ,If the line is Idle station sends the frame immediately(with a probability of 1) ,this method increases the chance of collisionp-PERSISTANTIf the line is Idle station sends the frame with probability p ,and refrain from sending with a probability 1-pStation runs a Random number between 1-100
Figure 12.11 Flow diagram for three persistence methods
Figure 12.15 Energy level during transmission, idleness, or collision
Figure 12.16 Timing in CSMA/CA
In CSMA/CA, the IFS can also be used to define the priority of a station or a frame.
In CSMA/CA, if the station finds the channel busy, it does not restart the timer of the contention window;
it stops the timer and restarts it when the channel becomes idle.
12-2 CONTROLLED ACCESS12-2 CONTROLLED ACCESS
In In controlled accesscontrolled access, the stations consult one another to find , the stations consult one another to find which station has the right to send. A station cannot send which station has the right to send. A station cannot send unless it has been authorized by other stations. unless it has been authorized by other stations.
ReservationPollingToken Passing
Topics discussed in this section:Topics discussed in this section:
Figure 12.18 Reservation access method
Figure 12.19 Select and poll functions in polling access method
Token passing:control token passed from one node to next sequentially.concerns:•token overhead •single point of failure (token)
Polling: master node “invites” slave nodes to transmit in turnconcerns:
•polling overhead •single point of failure (master)
12-3 CHANNELIZATION12-3 CHANNELIZATION
ChannelizationChannelization is a multiple-access method in which the is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, or available bandwidth of a link is shared in time, frequency, or
through code, between different stations.through code, between different stations.
Frequency-Division Multiple Access (FDMA)Time-Division Multiple Access (TDMA)Code-Division Multiple Access (CDMA)
Topics discussed in this section:Topics discussed in this section:
In FDMA, the available bandwidth of the common channel is divided into bands that are separated by
guard bands.
Figure 12.22 Time-division multiple access (TDMA)
In TDMA, the bandwidth is just one channel that is timeshared between different stations.
Figure 12.23 Simple idea of communication with code
In CDMA, one channel carries all transmissions simultaneously.
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”)
•Each station is assigned a code ,which is a sequence of numbers called CHIPS
•Chip period is always less than a bit period
•Chip rate is always an integral multiple of bit rate
Figure 12.24 Chip sequences
There is only one sequence flowing through the
channel ,the sum of the sequences But each
receiver can detect its data from the sum
In the following example Four stations sharing the
link during the 1-bit interval is shown below
Figure 12.27 Digital signal created by four stations in CDMA
Figure 12.28 Decoding of the composite signal for one in CDMA
The number of sequences in a Walsh table needs to be N = 2m.
The sequences are not chosen randomly They should be orthogonal to each other
Orthogonal sequences satisfies following properties If we multiply a sequence by –1,every element in the sequence is complemented If we multiply two sequences element by element and add the results , we get a number
called inner product If two sequences are same we get N (N=Number of sequences) ,if they are different we get 0 (A.A=N A.B=0)
A.(-A) = -N For the generation of the sequences ,WALSH TABLE method is used
Find the chips for a network witha. Two stations b. Four stations
Example 12.6
SolutionWe can use the rows of W2 and W4 in Figure 12.29:a. For a two-station network, we have [+1 +1] and [+1 −1].
b. For a four-station network we have [+1 +1 +1 +1], [+1 −1 +1 −1], [+1 +1 −1 −1], and [+1 −1 −1 +1].
Example 12.7What is the number of sequences if we have 90 stations in our network?
SolutionThe number of sequences needs to be 2m. We need to choose m = 7 and N = 27 or 128.We can then use 90 of the sequences as the chips.
Prove that a receiving station can get the data sent by a specific sender if it multiplies the entire data on the channel by the sender’s chip code and then divides it by the number of stations.
Example 12.8
SolutionLet us prove this for the first station, using our previous four-station example. We can say that the data on the channel D = (d1 c⋅ 1 + d2 c⋅ 2 + d3 c⋅ 3 + d4 c⋅ 4). The receiver which wants to get the data sent by station 1 multiplies these data by c1.
When we divide the result by N, we get d1 .
Orthogonal Codes of CDMA
In Our previous example At the Multiplexer the sequence S that comes out is S=-A-B+D
Station 1 is sending –A (-1 was multiplied by A) Station 2 is sending –B Station 3 is sending an empty sequence Station 4 is sending D
At the De multiplexer the sequence S is received Station 1 finds the inner product of S and A S.A=(-A-B+D).A=-4+0+0=-4
The result is divided by 4 (–4/ 4 = -1) , And –1 represents bit 0
Same is true for stations 2 ,3 and 4