What Is Asynchronous Transfer Mode

Asynchronous Transfer Mode.

ATM is a transmission mode. In this mode, information is organized into cells. Since each cell containing information from a user does not need to appear periodically, this transmission mode is asynchronous.

ATM is a new network technology. It uses a cell-based asynchronous transmission mode and a virtual circuit structure to fundamentally solve the problems of real-time and bandwidth of multimedia. To achieve point-to-point transmission for virtual links, it usually provides 155Mbps bandwidth. It not only absorbs the circuit-connected connected service and service quality guarantee in traffic communication, but also maintains the flexibility of variable bandwidth and suitable for burst transmission in traditional networks such as Ethernet and FDDI, thus becoming applicable so far The most extensive network, the most advanced technology, and the best transmission effect. ^[1]

Chinese name: Asynchronous transfer mode

Foreign name: ATM

ATM Generation of ATM in ATM asynchronous transmission mode

Since Alexander Graham Bell invented the telephone in 1870, in order to effectively connect with an increasing number of telephone users, a telephone exchange network came into being. It has gone through the development process of manual exchange, electromechanical automatic exchange system and digital program control system, but the principle of circuit exchange has not changed. With the popularization of computers, the telephone network uses modems to carry out computer data transmission and data information exchange, and a public data network has emerged. Its typical representative is the X.25 packet switching network, which is a technology based on packet switching It has the advantage of high reliability of signal transmission, but due to the limitations of the modem speed and the switching technology itself, X.25 can only process low- and medium-speed data streams. Although the development of LAN (Local Area Network) technology is advancing by leaps and bounds, such as Ethernet, Token ring, Token bus, etc., the transmission rate has reached Gigabit, but the nature of the local area network itself has greatly limited the large-scale coverage and application of LAN. Data transmission within the enterprise cannot form the scale of a WAN.

It is not difficult to see that the traditional network generally has the following shortcomings: First, the dependence of the service, the general network can only be used for dedicated services, the public telephone network cannot be used to transmit TV signals, and X.25 cannot be used to transmit High-bandwidth images and language signals that require high real-time performance; second, no flexibility, that is, the possibility of business expansion is not high, and the quality of service of the original network is difficult to adapt to new services emerging in the future; third, Low efficiency, resources of one network are difficult to be shared by other networks.

With the continuous development of society and the continuous diversification of network services, people can use the network to do many things, such as sending and receiving letters, home office, Video on demand, and Internet telephony. This has become increasingly demanding on the Internet. Some people ca nt help but propose such a Idea: Can these network services that have different requirements for bandwidth, real-time performance, and transmission quality be implemented by a unified multimedia network to achieve true first-line communication? The answer is yes, this is the ATM network. Fortunately, semiconductor and fiber-optic technology provide a solid guarantee for the fast switching and transmission of ATM. CMOS processing capacity has reached two to three hundred megabytes, and ECL can reach 5 to 10G. SDH and SONET technologies provide large-capacity and reliable transmission, and the STM-I standard is 155.52M. ^[1]

ATM ATM asynchronous transfer mode ATM virtual connection

ATM is a connection-oriented technology. The logical connection of ATM is called virtual connection (Visual Connection). Virtual connection is the basic switching unit in ATM networks. Many operations in ATM (such as resource allocation, traffic control, service quality assurance, etc.) are performed on the basis of virtual connections. When an ATM terminal communicates, it must first establish a virtual connection between the terminals, and the user's data cells will be transmitted along the connection. This connection-oriented approach facilitates high-speed processing of cells by switching nodes. If the connectionless method is adopted, the switching node must complete some complex functions, such as destination address routing, etc. These overheads will undoubtedly affect the forwarding speed. In the connection-oriented mode of ATM, each virtual link has a unique identifier on a link to identify it and is recorded in the cell header. The switching node only needs to identify the virtual link identifier in the cell header, and then it can perform fast cell forwarding.

ATM ATM asynchronous transfer mode ATM technology

As the name suggests, ATM (Asynchronous Transfer Mode) is an asynchronous transfer mode, which is a standard formulated by the International Telecommunication Union ITU-T. In fact, in the mid-1980s, people have begun experiments on fast packet switching and established various models with different names In Europe, the corresponding technology is called asynchronous time division multiplexing (ATD) in image communication. In the United States, the high-speed data communication is called fast packet switching (FPS). Named Asynchronous Transfer Mode (ATM) technology, it is recommended to be the information transmission mode of the broadband integrated service data network B-ISDN.

ATM cells are fixed-length packets with a total of 53 bytes divided into two parts. The first 5 bytes are letterheads, which mainly complete the addressing function; the next 48 bytes are information segments, which are used to load information from different users and different services. All digital information, such as voice, data, and images, must be cut, and the cells encapsulated into a uniform format are transmitted in the network, and restored to the required format at the receiving end. Because ATM technology simplifies the exchange process, eliminates unnecessary data verification, and uses an easy-to-handle fixed cell format, the ATM exchange rate is much higher than traditional data networks, such as x.25, DDN, Frame Relay, etc. In addition, for such a high-speed data network, the ATM network uses some effective service traffic monitoring mechanisms to monitor user data on the Internet in real time to minimize the possibility of network congestion. Different "privileges" are given to different services, such as the highest real-time privilege for voice, the highest privilege for correctness of general data file transmission, and the network allocates different network resources to different services, so that different services can achieve "peace" in the network Coexist ".

ATM general access method The picture on the left is the general network access method of ATM. The router directly connected to the network can be a router that supports the ATM protocol or a host with an ATM card, or an ATM subnet. On a physical link, multiple virtual circuits that carry different services, such as voice, image, and file transmission, can be established at the same time. ^[1]

Asynchronous transfer mode is an important milestone in the development of modern high-speed broadband information transmission and exchange technology. ATM technology is a unified broadband communication network service proposed on the basis of combining the technical advantages of various existing transmission switching modes. Therefore, ATM has actually designed the technical content of many aspects such as information reuse, exchange, and transmission, forming a series of advanced, strictly standardized, but also very complex technology and protocol systems. In theory, the advanced nature of ATM technology makes it a transmission mode that can be applied to network services and application information, but the complexity of the protocol has led to the existence of ATM in supporting the development and deployment of terminal services and applications. obstacle. On the other hand, a series of key technologies involved in the development of ATM technology have played a huge role in promoting the development of future broadband communication networks (especially the broadband Internet based on the IP protocol). The basic principles, information reuse, quality of service (QoS) guarantee, flow control, and congestion control technologies in ATM have been widely used in modern Internet technologies.

ATM ATM Asynchronous Transfer Mode ATM Business Introduction

ATM simplified protocol layering diagram Let's take a look at the simplified protocol layering diagram of ATM

ATM uses AAL1, AAL2, AAL3 / 4, AAL5, and multiple adaptation layers to adapt to four different user services of A, B, C, and D. The business description is as follows:

Class A-Fixed Bit Rate (CBR) services: ATM Adaptation Layer 1 (AAL1), supporting connection-oriented

Services, whose bit rate is fixed, common services are 64Kbit / s voice services, fixed bit rate

Leased circuits for uncompressed video communications and private data networks.

Class B-Variable Bit Rate (VBR) service: ATM Adaptation Layer 2 (AAL2). Supports connection-oriented

For services, the bit rate is variable. Common services are compressed packet voice communications

And compressed video transmission. This service has a delayed physical interface, the reason is

The receiver needs to reassemble the original uncompressed voice and video information.

Level C-Connection-oriented data service: AAL3 / 4. This service is a connection-oriented service, suitable for

For file transfer and data network services, the connection is established before data is transmitted

of. It is variable bit rate, but it is not an interface transfer delay.

Class D-connectionless data services: Common services are datagram services and data network services. in

The connection will not be established until data is passed. Both AAL3 / 4 or AAL5 support this service.

Note: . Due to the complexity of the AAL3 / 4 protocol technology, AAL5 was proposed to support C-level services.

. For each level of business, we can also subdivide, we will not repeat them here. ^[1]

ATM ATM asynchronous transfer mode ATM application example

LANE refers to LAN Emulation Over ATM, that is, the simulation of the LAN on the ATM network.

Most data is transmitted on a LAN, such as an Ethernet network. Applying LANE technology on ATM network, we can interconnect the distributed networks in different areas and realize the function of LAN on WAN. For users, they are still in the traditional LAN area, and they do nt feel LANE at all. presence.

LANE technology mainly uses LANE Server. It can exist in one or more switches, or it can be placed in a separate workstation. LANE Server can be abbreviated as LES. Its main function is to perform MAC-to-ATM address translation. Because Ethernet uses MAC addresses, and ATM uses its own address scheme, LES address translation can connect LANE clients distributed at the edge of the ATM.

The picture below shows how LANE works

How LANE works 1. The LAN Switch receives a frame from the Ethernet terminal. The destination address of this frame is an Ethernet terminal on the other end of the ATM network. The LEC is the LANE Client (it resides in the LAN Switch), so it sends a MAC-to-ATM address translation request to the LES (LES resides in the ATM Switch).

2. LES sends multicast to other LECs on the network. How LANE works

3 The LEC containing the called MAC address in the address table responds to the LEC.

4 The LEC then broadcasts this response to other LECs.

5. The LEC sending the address translation request recognizes this response and obtains the destination ATM address, and then establishes an SVC to the destination LEC through the ATM network, and transmits data using ATM cells.

V. Introduction of Shanghai ATM Backbone Network Nodes

The national ATM backbone online sea node was established in April 1997, and the network has been operating stably since it was opened.

The network provides switched virtual circuit (SVC) and permanent virtual circuit services. The interface type supports BNC electrical ports and single and multi-mode optical fibers. The physical access rates are 2M, 34M, and 155M, which can meet the needs of any business. ^[1]

ATM asynchronous transmission mode characteristics

ATM is a technology about information transfer mode. Its most prominent feature is its asynchronous mode. ATM technology has the following characteristics: ^[2]

(1) Group-oriented approach

A small packet (cell) is used in ATM to carry user data. Various operations of ATM are designed around the exchange and processing of cells.

(2) Adopt asynchronous time division multiplexing

ATM uses asynchronous time division multiplexing. The user information occupies the bandwidth dynamically and each user statistically shares the transmission bandwidth, so it is suitable for burst services. In order to prevent resource access due to the asynchronous working mode from drawing into the Onuo individual, a large number of queues are used in ATM to swap out user information that is temporarily unavailable for service.

(3) Does not provide error control and flow control for segment-by-segment links

When an error occurs on the link (including the internal link in the switching node), the ATM switching node will not perform error recovery in any way. ATM assumes that the link quality in the network is very high, and the implementation of the error control function depends on the end-to-end protocol, which is the same as circuit switching. ATM switching nodes also do not support flow control on the link. The queues in the system may overflow due to bursts of information, resulting in cell loss. In order to prevent such loss, ATM provides preventive measures, that is, connection-oriented, and checks and allocates resources when the connection is established, so that the probability of such cell loss is controlled to a small extent.

(4) Simplified cell header function

Since there is no need for error control and flow control of the segment-by-segment link, the function of the ATM cell header is very small. The main function room identifies a virtual connection based on an identifier; the other function room checks for errors in the cell header to prevent errors. Cells lost or mis-inserted due to routing. Due to the limited function of the cell header, the processing of the switching node is very simple, it can run at a very high rate (several hundreds of megabits per second), and there is only a small processing and queuing delay.

(5) The payload length of the cell is small

In order to reduce the capacity of the internal buffer of the switching node and limit the queuing delay of information in these buffers, the payload (information field) in the ATM cell is relatively defined to ensure a small delay in service transmission And jitter.

Status of ATM asynchronous transmission mode

The national ATM backbone online sea node was established in April 1997, and the network has been operating stably since it was opened. 155M circuits have been opened directly with major bureaus in Beijing, Nanjing, Guangzhou, Hangzhou, Xi'an, Shenyang, and Wuhan, and a number of large-capacity circuits have been opened to other provincial capitals. The entire network has been fully connected, covering the whole country with bandwidth High, small delay, no bottlenecks, etc., is the best choice for network multimedia applications.