Exchange Technology-introducing the evolution of Exchange Technology

With the development of communication technology, the relationship between transmission, reuse and exchange is getting closer and closer, so that they are gradually inseparable. In recent years, people have used the term "Transfer Mode" to describe the transmission, reuse, and exchange in the communication network. The transfer mode can be divided into synchronous transfer mode ATM) and asynchronous transfer mode ATM.

ATM uses space or time-based Fixed-channel circuit switching. For example, in the current telephone network, after a call is established, both parties occupy a fixed pair of 64 kbit/s time slot channels ).

ATM uses a high-speed group switching mode that allocates time slots on demand and reuses statistics.

Both X.25 and frame relay on the existing data network adopt the Statistical Multiplexing group exchange technology. The interface speed is variable and the channel utilization is high, but the delay is too large, which is not suitable for real-time communication; however, the circuit switching technology on the telephone network has low latency and is suitable for real-time communication, but the interface speed is fixed and the channel utilization is low. Both of them are not suitable for transmitting and exchanging comprehensive business information such as voice, images, and data. Combined with the advantages of group switching and circuit switching, ATM provides an ideal Transfer Mode for B-ISDN.

1. ATM cells　

The basic feature of ATM is that transmission, multiplexing, and switching are all fixed-length cell units. That is, all the information transmitted from the sender to the receiver is in the form of credit.

An ATM Cell consists of the header H) and the Information Field INF, net load. The header includes the address CIDR Block, VC1, and other necessary control information. The valid information field can carry different types of user information, including digital voice, active images, and data.

ATM cells can be divided into user information cells, signaling cells, idle cells, and operation and maintenance cells.

2. ATM layered structure　

1) user layer

The main function of the ATM user layer is to provide users with broadband integrated services such as high-speed data, voice, and image.

2) Adaptation Layer　

The main function of the ATM adaptation layer is to adapt data, audio, video and other signals from the user plane to form the data format required by the ATM layer. Each signal must be adapted to an ATM network.

Generally, different types of services have different network requirements. For example, services can be divided into services that support fixed bit rate CBR and VBR based on the source business model. Based on the end-to-end timing relationship, businesses can be divided into continuous businesses and sudden businesses. According to the connection mode, businesses can be divided into connection-oriented businesses and connection-free businesses. Therefore, ATM defines several different AAL types to support different types of services. Currently, the defined AAL service classes include AAL 1-5. Among them, AAL5 supports all types of AAL services and is widely used.

3) ATM layer

The basic function of the ATM layer is to generate cells. It receives the 48-byte net load from the AAL layer and attaches the corresponding 5-byte cell header to form a cell to be transmitted to the physical layer.

4) Physical Layer　

The ATM physical layer is the bottom layer of the ATM model. It consists of the transport convergence layer and the physical media sub-layer. The ATM physical layer is responsible for the line encoding of the ATM cells and submits the cells to the physical media. The transport aggregation layer receives cells from the ATM layer, assembles them into appropriate formats, and then transmits them to the physical media sub-layer. When no information is transmitted, the transmission aggregation layer inserts idle cells to keep the cell stream continuous. At the receiving end, the transport aggregation layer extracts cells from the bit stream from the child layer of the physical media, verifies the cell header, deletes idle cells, and transmits valid cells to the ATM layer.

3. Virtual Channel VP) and virtual channel VC)　

ATM adopts the connection-oriented virtual circuit group switching technology, that is, when the sending end needs to communicate with the receiving end, the control part establishes an end-to-end virtual circuit by transmitting the signaling cells of both sending and receiving sides before the communication is established. After the virtual circuit is established, the ATM adopts two-level routing mode, that is, the ATM virtual circuit consists of virtual channel VP and virtual channel VC. For the ATM Backbone Network, there may be thousands of vcpus entering the switch. If each VC is connected to VCC, maintaining an information port will inevitably increase the burden on the switch, therefore, the same physical route VC is grouped into a VP connected to the VPC in the ATM ). The ATM physical link supports multiple VPNs at the same time, and one VPNs have multiple VC connections at the same time.

The VP node device in the virtual channel is a VP switch or DXC device. Its function is to connect the vpcs with different CIDR blocks to form a virtual circuit between the source and destination vswitches; the VC node device in the virtual path is a VC switch or DXC device. Its role is to connect VCC of different VC1 segments to form an end-to-end virtual circuit. It should be noted that after the VCC is switched, it needs to be reorganized into a new VPC. Therefore, the VC node device contains the VP switching and VC switching mechanisms. This is similar to a DXC4/1 device that contains both 64 kbit/s link and 2 Mbit/s link, the VP node device is similar to a DXC4/4 device that only contains 2 Mbit/s links.

In order to connect the devices on both sides of the VP node with different CIDR blocks, the VPNs are marked with CIDR blocks. Similarly, the label route table of the VC1-VP1 stored in the VC node device.

4 ATM Switching Principle　

4.1 basic components of an ATM Switch　

Connect the user line and trunk line on the ATM switch. The transmitted data units are all ATM cells. Therefore, for an ATM switch, there is no need to distinguish between user lines and trunk lines in many cases, but only between the incoming and outgoing lines of the ATM. The task of an ATM switch is to send the cells to the corresponding outlets based on the input cell CIDR and VC1.

(1) Handling of incoming and outgoing lines

The entry processing component processes the incoming ATM cells so that they can be exchanged and synchronized and aligned. The outlet handling component processes the AATM cells sent by the switching unit to convert them into a form suitable for online transmission.

(2) Exchange Unit　

The structure of the exchange unit can be divided into two categories: Air Separation exchange and Time Division exchange. The task of switching a unit is to transfer the incoming ATM cells to the corresponding outlets according to the CIDR Block of the header. In addition, the ATM switch unit should have the copy function of the ATM cell to support multicast services.

(3) Control Unit

The task of an ATM control unit is to control the action of the switch unit. Because the signaling and operation, maintenance, and other information of the control switch unit actions are transmitted in the form of ATM cells, the ATM control unit should have the ability to receive and send ATM cells.

In the ATM switch unit, the control part searches for the address ing table based on the header address CIDR Block, modifies the write header address, and sends it to the corresponding port for output. The entire switching process is very simple. Hardware addressing and parallel switching can be used, greatly improving the switching speed of ATM cells.

4.2 VP switching and VC Switching　

In the VP switch, based on the destination VC switch connected by the VP), change the input cell csv1 value to the new csv3 value that can be directed to the destination, after changing the write header, the VC1 remains unchanged in the selected route ).

In the VC switch, change the VC1 value of the input cell to the new VC1 value that can be directed to the destination based on the destination Client Connected by the VC switch. Change the writing header to enter the new VP1 channel, therefore, you need to rewrite the CIDR block at the same time.

4.3 ATM user line　

An ATM user line is generally connected to an ATM access switch at a rate of 155 Mbit/s. On the ATM user line, cell transmission can also be divided into virtual channels and virtual channels, and has the user's proprietary vc1.

4.4 connection process

After the virtual circuit is established, the information to be transmitted is divided into 53-byte ATM cells, which are transmitted to the other party through the ATM network. If the sender sends more than one application information at the same time, different virtual circuits must be established to reach different destinations, and application information must be sent alternately.

A typical end-to-end full-connection VPHVP processing for an ATM network using an switched virtual circuit) is a VP switch DXC device, VPH selects routes based on the CIDR block, and VCHVC processes) is a VC switch, VCH selects routes based on VC1. The access switch is equivalent to the user switch in the telephone network.

5. Structure of the ATM branch network　

The ATM subnet network is composed of vswitches. In the ATM network structure example, the atm over sdh technology is used, that is, a transmission network composed of SDH Self-healing rings. The service node device is a VC switch, and the access point is an ATM access switch.

At present, the problem of ATM to desktop has not been solved, so the ATM service access is completed by the ATM access switch. The ATM access switch provides IP, FR, circuit simulation E1-E3, Ethernet, X.25.25 and other business interfaces through the ATM adaptation layer, provides high-speed Internet access, lan interconnection, voice, high-definition TVs, on-demand TVs, conference TVs, and other broadband services. ATM networks are generally called B-ISDN man.

As can be seen from the above, ATM has the advantages of group switching and circuit switching, and is suitable for transmitting a variety of real-time and non-real-time telecommunication services. However, A has problems such as complex signaling, difficult implementation, high price, and difficulty in transferring ATM cells to the desktop. In addition, ATM cells cannot pass through other data networks. Therefore, the ATM network can only exist in isolated islands. However, IP packets that follow TCP/IP can pass through any underlying data network, so the IP network rapidly develops into a world-wide network. At present, it is widely believed that IP networks are expected to replace Network A after QoS resolution, and integrate all current and future service networks into A world-wide broadband integrated service network.

Most of the current ATM networks are used as the underlying network of the IP network and exist in the form of a computer subnet. They provide IP services at Layer 3 and ATM services at Layer 2.

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