2.6.1 multiplexing

时分复用
The capacity of the outgoing channel is divided into N logical channels 逻辑信道, and data in each of N incoming channels is placed in a designed outgoing logical channel.
the higher the rate, the larger the capacity allocated to it at that
time. As a result, the capacity of the outgoing channel needs to be only as large as the sum of the average data rates of the incoming channel, which, for bursty traffic, may be much smaller than the sum of the
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2.6.1 Multiplexing
Characteristics.
It is easy to implement. The overhead 开销 due to extra framing bits 额外成帧比特数 is small The utilization of the outgoing channel may vary a great deal depending on the burstiness 突发性 of the incoming data streams. The capacity of the outgoing channel must be as large as the sum of the capacity of the N incoming channels. As a result. The utilization of the outgoing channel will be low or high accordingly as the utilization of the incoming channel is low or high. TDM leads to high utilization if the incoming data is not bursty. Thus TDM is ideal for constant bit rate traffic.
A logical channel occupies every Nth slot. There are thus N logical
channels.
The first logical occupies slots 1, N+1, 2N+1,….; and so on.
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2.6.1 Multiplexing
Time-division Multiplexing Statistical Multiplexing
Frequency-division Multiplexing Code-division Multiplexing
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2.6.1 Multiplexing
Time-Division Multiplexing (TDM)
Utilization is equal to 100% only if the instantaneous data rate is always equal to the channel. Multiplexing. Is the process by which information bits from N incoming channels are transferred into bit streams on one outgoing channel. De-multiplexing 解多路复用. Is the reverse process: the information bits on one incoming multiplexed channel are separated 分解 and transferred onto N channels
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2.6.1 Multiplexing
Implement method.
Time on the outgoing channel is divided into fixed-length intervals called frames 帧. Frames are delimited 定界 by a special bit sequence called framing pattern 帧识别模式. Time in each frame is further subdivided into N fixed-length intervals called slots 时隙 . Each frame consists of a sequences of slots: slot 1, slot 2,…slot N.
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2.6 Basic Network Mechanisms
2.6.1 Multi.
多路复用
Multiplexing: combines data streams of many users into one
We explain several important techniques for multiplexing N incoming channels onto one outgoing channel.
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2.6.1 Multiplexing
User A a a t
Statistical TDM or SM
①
B
b
b
t ② a b b c c d a #3 t ③ t ④ #1 #2
C
c
c
D
d
t
3 SM Frames
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2.6.1 Multiplexing
The capacity allocated to each incoming channel varies with time, depending on its instantaneous data rate 瞬时传输率:
Frequency
The station in the TDM frame is fixed
ABCDABCDABCDABCD …
TDM Frame TDM
TDM
TDM
TDM
Time
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2.6.1 Multiplexing
Frequency
The station in the TDM Frame is fixed
In a 1Mbps channel, a bit time is 1 us long. Each bit time may be occupied by an information or data bit, or it may be empty. Then the channel is busy or idle at that time.
peak data rates 峰值传输速率.
Hence, the capacity of the outgoing channel may be smaller
than the sum of the incoming channel capacities.
We call the ratio of the total incoming capacity to the total outgoing capacity the multiplexing gain 复用增益 .
A channel 信道 is a communication link of fixed capacity measured in
bits per second (bps). Each second of time on the channel is divided into a number of intervals called bit times 比特时间, the number being equal to the channel capacity.
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2.6.1 Multiplexing
The average data rate divided by channel capacity, or the fraction of time the channel is busy, is the channel utilization 信道利用率.
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2.6.1 Multiplexing
TDM may be wasteful
User
TDM
A B C
D a a
t
①
b
b c c d
t ② ab t ③ #1 bc #2 c #3 a #4 d
t
④
t
4 TDM Frames
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2.6.1 Multiplexing
Statistical Multiplexing (SM)
Chapter 2 Network services and Layered Architectures
2.6 Basic Network Mechanisms
Multiplexing Switching Error control Flow control Congestion control Resource allocation
统计复用
The most effective in the case of bursty input data.
The data bits in each incoming channel are read into separate FIFOs. The multiplexer reads each buffer in turn until the buffer empties. It is customary to call the data read in one turn a data packet.