Asynchronous transfer mode ATM (Asynchronous Transfe
ATM (Asynchronous Transfer Mode)
With the rapid development of Internet and multimedia technologies, more and more multimedia content such as images, audio, and video on the Web, users need to have a higher access rate. However, the existing circuit switching and packet switching are difficult to handle broadband high-speed switching tasks. For circuit switching, when the data transmission rate and its sudden changes are very large, the control of switching becomes very complicated; for packet switching, when the data transmission rate is high, the processing of protocol data units at each layer becomes very Large overhead, unable to meet the delay requirements of highly real-time services. Asynchronous transfer mode (ATM) is a new switching technology based on circuit switching and packet switching. It can exchange broadband information well.
1 ATM cell format and rate
The basic carrier that ATM transmits information is ATM cell. ATM cells use a fixed length of 53B, of which 48B is data, and 5B is added as a letterhead. In the process of cell exchange, the cell is mainly processed with reference to the content of the header. The letterhead content is slightly different in UNI and NNI, as shown in Figure 1.
GFC (Generic Flow Contr01): general flow control, only used for UNI interface, currently useless, set to 0000.
VPI (Virtual Path IdenTIfier): Virtual channel identification, which aggregates several virtual channels on an interface to form a virtual channel (VP), and uses virtual channels as the basic unit of network management. VPI is 8b in UNI and 12b in NNI.
VCI (Virtual Channel IdenTIfier): Virtual channel identifier, which identifies the virtual channel in the virtual channel, and VPI / VCI together identify a virtual connection.
PTI (Payload Type): Payload type indication, used to indicate the type of payload (data carried in the data field) in the cell
CLP (Cell Loss Priority): Cell loss priority, used for congestion control. When the network is congested, cells with CLP equal to 1 are first discarded.
HEC (Header Error Contr01): Cell error control, used to detect errors in the header, and can correct the 1 b difference in the header. Another function of HEC is for cell delimitation. The correlation between the HEC field and his previous 4B can be used to identify the letterhead position. The function of HEC is realized in the physical layer.
2 ATM switching principle
Unlike ordinary IP transmission, which is not connection-oriented, ATM is a connection-oriented switching method. The ATM switch is completed based on the cell based on the information in the cell header. An ATM switch may use only the VPI part of the cell header, or only the VCI part, or both parts to decide how to forward the cell. The working process is roughly: the ATM switch receives the marked VPI / VCI field from the specific input port and the cell indicating that it belongs to the specific virtual circuit, and then checks the routing table to find out which output port to forward the cell And set the VPI / VC! Value of the output cell. Just like the telephone call example, it is very useful to use only the VPI field of the cell header for mass exchange of ATM cells.
ATM uses virtual connection technology to separate the logical subnet from the physical subnet. Similar to circuit switching, ATM first selects a path, establishes a virtual channel between two communication entities, separates routing and data forwarding, makes the control in the middle of transmission simpler, and solves the bottleneck of routing. Set up two levels of addressing for virtual channels and virtual channels. Virtual channels are composed of a group of virtual channels multiplexed between two nodes. The main management and switching functions of the network are concentrated at the level of virtual channels, which reduces network management and network control. Complexity. Multiple virtual channels can be established on a link. The data units transmitted on a path are all transmitted on the same physical line, and maintain their order, so it overcomes the shortcomings of disordered reception in packet switching, ensures data continuity, and is more suitable for multimedia data transmission. Among the various components of the letterhead, VPI and VCI are the most important. The two parts together form the routing information of a cell, which indicates where the cell comes from and where. For this reason, these two parts are often referred to as VPI and VCI. ATM switching is based on the VPI and VCI on each cell to decide which output line to send them to.
Each ATM switch establishes a comparison table. For each VPI and VCI of each switching port, there is an entry in the corresponding table. When VPI and VCI are assigned to a certain channel, the comparison table will give a corresponding output port of the switch and the VPI and VCI values ​​used to update the header.
When a certain cell reaches the switch, the switch will read the VPI and VCI values ​​of the cell header and compare it with the routing table. When the output port is found, the VPI and VCI of the letterhead are updated, and the cell is sent to the next distance.
In an ATM environment, how to use VP and VC? VP is like a pipe or channel that can carry many VCs (up to 65 000), it can be a virtual line from switch to switch, or it can be across ATM All lines of the network from terminal to terminal. In addition to the largest private LAN or WAN, 65 000 VCs are sufficient today. In fact, it does not require so many VCs to support complex VPs. Many ATMLAN transmission points only support one virtual channel, namely
VPI = 0. When only one VP is supported, it is not used as an end-to-end connection, so there is no requirement for VC to be in a given VP, so that VC can connect to any group of stations without being affected by VP. Usually data is transmitted in a VC.
On the other hand, in a typical situation, a switch must support hundreds or thousands of different VPs, and at most millions of different VCs. Generally, customer systems want to provide their users with a dedicated VP through the network. The VP can connect any two end-to-end users in the network. If the VP uses this method, it is called a virtual channel connection (VPC) or called A "virtual channel path (VP Channel)". He can carry "Permanent Virtual Circuits (PVC, Permanent Virtual Circuits)" and "Switched Virtual Circuits (SVC, Switched Virtual Circuits)". as shown in picture 2.
In a VP channel, system users can establish PVC and SVC without the system participating in any way, and even the system switch does not need to directly support SVC. The VP channel can provide a path to isolate different companies in the public network from each other. In this path using a public ATM server, a compound VP channel is needed to interconnect the outlets in the user network.
In a public ATM network environment, if the system does not provide the VP channel capability (some may not), the system can only provide PVC, because the switch cannot directly support SVC (some never support), and some systems do not want to support SVC (Because he complicates accounts between enterprises and increases the flow of confidential data). If there is no VP channel, the system is usually
Use VPI = 0 at the network endpoint to generate and end PVC. As shown in Figure 3.
In the public network, PVC is applied in advance by users and established by the system. PVC is very useful for external connection of "ATM network equipment" (such as Ethernet or FDDI converter with ATM, ATM hub). Many non-ATM signal sources can be returned to the designated point through a single PVC dynamic multiplexing. The use of PVC between ATM hosts can also limit the communication of predetermined endpoints. This is in line with the requirements in the public network.
In a proprietary network (LAN or WAN), since the terminal station can apply for establishing an SVC by itself, SVC is a preferable path for communication between sites. This is how most dedicated non-ATMLAN and WAN work today. Therefore, the occupied network ATM switch must directly support SVC. However, if the terminal station or the edge device does not support SVC or is not allowed to apply for SVC connection as required, then PVC is useful in the private network. The PVC must be established in advance by the network controller. However, since the path is predetermined, when the network fails, PVC is less advantageous than SVC. Therefore, in the private network, the virtual channel VP is not important or even needed, as shown in Figure 4.
3 ATM switch
In B-ISDN, ATM switches connect subscriber lines and trunk lines. Both ATM cells are transmitted on subscriber lines and trunk lines. The general model of ATM cell switch and its principle are shown in Figure 5. Its general model has some input lines and some output lines, which are usually equal in number (because the lines are bidirectional). One cell (if any) is taken from each input line in each cycle. Through the internal switching fabric (switching fabric), and gradually transmitted on the appropriate output line. From this perspective, ATM switches are synchronized. Moreover, he does not care about the content and form of the information. He simply splits the information into packets of the same length and adds a header to the packet so that the packet can reach its destination. The ATM letterhead has only a few functions, which allow it to be processed by the network without delay.
All ATM switches have two common goals: one is to exchange all cells with the lowest possible loss rate; the other is to never record cells on a virtual circuit. It can be said that the task of the ATM switch is to transfer the given ATM cell to the corresponding trunk or subscriber line based on the virtual channel identifier and virtual channel identifier on the ATM header. For example, user A is using the virtual channel VPI; 2. The virtual channel VCI = 1 is sending a picture to Beijing; at the same time, it is using VPI = 3 and VCI = 1 to send a segment of speech to Beijing; while still using VPI = 4 , VCI = 2 Receive data from Shenzhen. Then, the switch should forward the ATM cells received from subscriber line A with VPI = 2 and VCI = 1 to trunk line C, and transfer the ATM messages received from subscriber line A with VPI = 3 and VCI = 1 The cell is also transferred to the trunk C; at the same time, the ATM cell with VPI = 4 and VCI = 2 received from the trunk D is transferred to the user A, as shown in FIG. 6.
Because on B-ISDN, both the subscriber line and the trunk line transmit ATM cells, so for ATM switches, the trunk line and the subscriber line can be distinguished in many cases, so that an abstract ATM model can be obtained . A part of the lines connected to this switch model cc sends out ATM cells to this switch, so it is called the incoming line of this switch; another part of the line receives ATM cells from this switch, so it is called the outgoing line of this switch. The function of the ATM switch is to send them to the corresponding outgoing line according to the VPI and VCI of the ATM cell sent to them.
In order to complete the work of the above ATM cells, an ATM switch is generally composed of three basic parts: an incoming processing and an outgoing processing part, an ATM switching unit, and an ATM control part. Among them, the ATM switching unit completes the switching operation; the ATM control unit controls the operation of the ATM switching unit; the incoming processing section processes the ATM cells on each incoming line, making them suitable for sending to the ATM switching unit; outgoing The processing part processes the ATM cells sent by the ATM switching unit, making them suitable for transmission.
We know that on a communication line, a serial signal of one bit per bit is often transmitted, and in order to increase the speed in an ATM switching unit, it is often necessary to read several bits of parallel signal at a time. Therefore, functions such as serial / parallel conversion are always required in the incoming and outgoing processing components. In fact, in order to simplify the design of the exchange unit, we always put those things that can be handled at the incoming and outgoing lines into people and outgoing processing parts.
The main function of the switch is to provide a way to quickly and efficiently route cells from the input port to the output port. The ATM switching equipment will perform input processing, header conversion and output processing of a single cell. The cell header must be converted as required by the output port. To ensure that the cell enters the proper physical link, the switch must perform output processing on the believer.
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