ATM Tutorial: Physical Layer

I. ATM Physical Layer
ATM means asynchronous transfer mode ). This mode can be compared with the synchronous T1 line. In line T1, each 125us has a T1 frame generated. This rate is controlled by the main clock. The k time slot of each frame contains 1 byte data from the same source. T1 is synchronized. While ATM does not strictly require that cells come from different sources alternately, and each column of cells come from different sources. There is no special mode for cells to come from any different sources. Moreover, cell streams from a computer are not required to be continuous, and data cells can be separated. These intervals are filled by special idle cell idle cells.
ATM does not standardize the cell format for transmission. In fact, it indicates that only one cell can be sent, and that cells can be mounted to T1, T3, SONET, or FDDI Optical fiber LAN) lines. For the above examples, there are standard rules on how cells are encapsulated into the frames provided by these systems.
In the initial ATM standard, the main rate is 155.52 Mb/s, and there is also a 4 times higher than its rate of 622.08 Mb/s ). This speed is selected to be compatible with SONET, which is the frame separation standard used for optical fiber lines in the telephone system. Based on T344.736Mb/s) and FDDI100Mb/s), the ATM has also appeared.
ATM Transmission Media are often optical fiber cables, but coaxial cables within M or five twisted pair wires are also possible. The optical fiber can reach several kilometers. Each link is located between a computer and an ATM switch or between two ATM switches. In other words, an ATM link is point-to-point and has many senders and receivers on a single cable ). You can obtain the broadcast effect by letting cells enter the vswitch from one line and output from multiple lines. Each point-to-point link is unidirectional. For full duplex operations, two links are required, and each direction occupies one traffic.
The physical layer of ATM includes two sub-layers, namely, the physical media sub-layer PM) and the transmission convergence TC) Sub-layer. The physical media sub-layer provides bit transmission capabilities, compares specific time and line encoding, and specifies the physical media used, such as optical fiber, coaxial cable, and twisted pair wires) define its corresponding features. The main function of the transport convergence sub-layer is to implement conversion between bit streams and element streams.
For the output, the ATM layer provides the cell sequence. The PDM sub-layer performs the necessary encoding and sends them as bit streams. For input, the PDM child layer obtains the input bit from the network and submits a bit stream to the TC child layer. The cell boundary is not marked. The TC sub-layer is responsible for finding out where the cell starts and ends. However, this is not only difficult, but not theoretically feasible. Therefore, the TC Layer removes this function. Because the TC Layer manages frames, it is a data link function, so we will discuss it in chapter 3rd.
Ii. physical interfaces
ITU-T and ATM Forum physical interfaces are divided into three types, namely, based on SDH, based on cells and based on PDH. The following is an introduction from different perspectives:
Traditional digital signaling.
DS0 64 kbit/s
DS1 (T1) 1.544 Mbit/s
DS2 (T2) 6.312 Mbit/s (4 DS1, 96 DS0)
DS3 (T3) 44.736 Mbit/s (28 DS1, 672 DS0)
DS4 274.176 Mbit/s (4032 DS0)
Synchronous Transmission signaling (STS) of SONET)
STS-1 (OC-1) 51.84 Mbit/s
STS-3 (OC-3) 155.52 Mbit/s (3STS-1)
STS-12 (OC-12) 622.08 Mbit/s (12STS-1)
STS-24 (OC-24) 1244.16 Mbit/s (24STS-1)
STS-48 (OC-48) 2488.32 Mbit/s (48STS-1)
ANSI standard
STS-1 51.84 Mbit/s
STS-3c 155.52 Mbit/s
STS-12c 622.08 Mbit/s
DS3 44.736 Mbit/s
CCITT Standard
DS1 1.544 Mbit/s
E1 2.048 Mbit/s
DS2 6.312 Mbit/s
E2 8.448 Mbit/s
E3 34.368 Mbit/s
DS3 44.736 Mbit/s
E4 139.264 Mbit/s
STM-1 155.52 Mbit/s is the same as STS-3)
STM-4 622.08 Mbit/s is the same as STS-12)