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Computer Networking – Physical Layer FAQ
What problems should the physical layer solve? What are the main tasks of the physical layer protocol?
As we all know, the physical layer is concerned with transferring data over the computer’s transport medium, not the specific transport medium.
There are many kinds of hardware devices and transmission media in computer network, and there are many kinds of communication methods. Therefore, the physical layer is required to mask these differences as much as possible, so that the data link layer does not feel these differences. The data link layer only needs to consider how the protocols and services of the layer are performed.
What is the difference between “bit/SEC” and “code/SEC”?
Symbol per second, also known as Baud, is a unit of code transmission rate, while bit per second is a unit of information transmission rate. The two have a certain relationship in quantity. In practice, a code element often has more than one bit of information. The relationship is as follows: Maximum information transfer rate = maximum symbol transfer rate * log2 (n element system)
Take an example. Assuming an ideal low communication channel with a bandwidth of 3000Hz, the maximum symbol transfer rate is 6000Baud, and in 8-bit system, the maximum message transfer rate is 18000 bit/s.
Suppose a telephone channel with a 3KHz bandwidth is used to transmit 64k bit/SEC data. What high SNR should this channel have?
64000 = 3000 * log2(1+S/N)
SNR S/N = 2^ 21.3-1 = 2,581,895
A total of four users carried out CDMA communication. The chip sequence of the four users is:
A:(-1 -1 -1 +1 +1 -1 +1 +1 )
B:(-1 -1 +1 -1 +1 +1 +1 -1)
C:(-1 +1 -1 +1 +1 +1 -1 -1)
D:(-1 +1 -1 -1 -1 -1 +1 -1)
The sequence of the received code slice is :(-1 + 1-3 + 1-3 +1 +1). Which users sent the data? Is it a 1 or a 0?
Because A times S over n is equal to 8/8 is equal to 1
B times S over n is minus 8/8 is minus 1
C times S over n is equal to 0
D times S over n is equal to 8/8 is equal to 1
So A sends 1, B sends 0, C doesn’t send, D sends 1.
Introduce some English abbreviations
FDM: Frequency division multiplexing, which is to divide the frequency band resources of the line into several sub-bands to form multiple sub-channels. It allows users to occupy different bandwidth resources at the same time.
TDM: Time division multiplexing (TDM). The frequency band resources of a line are allocated to different users in turn according to time. Allows users to occupy the same bandwidth resources at different times.
STDM: Statistical time division multiplexing, which differs from TDM in that it does not allocate fixed time slots, but dynamically allocates time slots on demand.
WDM: Wavelength division multiplexing, which divides the wavelength optical band into multiple wavelength ranges, and each user occupies one of these ranges for transmission.
DWDM: Dense wavelength division multiplexing (DWDM), which can multiplex dozens or more optical carrier signals on an optical fiber.
CDMA: code slice sequence
SONET: Synchronous fiber optic network.
SDH: Synchronous digital system
Stm-1: STM-1 is a synchronous transmission module with a rate of 155.520Mbps, which is the most basic module of SDH signal.
Oc-48: indicates that the network transmission speed is 1Gbps.