Data communication and network note-virtual circuit network: Frame Relay and ATM

Document directory

• Frame Relay
• ATM

Virtual circuit switching has been introduced in previous articles, see: http://blog.csdn.net/todd911/article/details/9069447

Here we will introduce the two middle-Wan technology using virtual circuit switching: Frame Relay and ATM.

Frame Relay

Frame relay is a virtual circuit wide area network. Designed to meet the new Wan needs in 1980s and 1990s.

Frame Relay has the following features:

1. Work at a high rate (1.544mbps and the latest 44.376 Mbps.

2. Only work on the physical layer and data link layer

3. Allow burst data. For example, a user may want to send data at a speed of 6 Mbps for 2 seconds without sending data for 7 seconds, and send data at 3.44mbps for 1 second. A total of MB of data is sent within 10 seconds, the average speed is 1.544 M.

4. The frame size is 9000 bytes, which is suitable for all LAN frames.

5. less cost than traditional Wan. From the user's point of view, the cost is small.

6. There is error detection on the data link layer, but there is no traffic or error control.

Is a simple Frame Relay Network:

A good feature of frame relay is that it provides Congestion Control and Quality of Service (QoS), which will be introduced later.

ATM

Asynchronous transfer mode (ATM) is a cell relay protocol designed by the ATM Forum and adopted by the ITU-T. The combination of ATM and SONET (for an introduction to SONET, see previous article: http://blog.csdn.net/todd911/article/details/9324615) will allow high-speed Interconnection between networks around the world.

Hybrid Network

Before ATM, data communication at the data link layer is based on frame switching and frame network. frames of different protocols have different sizes and complexity. As you can imagine, changing the frame size leads to unpredictable traffic. A complex software system must be integrated to manage frames of different sizes. A large amount of frame header information must be read, each bit is counted and assigned to ensure the integrity of each frame.

Another problem is how to provide stable rate transmission when the frame size is unpredictable and greatly changed. In order to maximize the benefits from the broadband count, the traffic must be distributed and reused to the shared channel. Imagine that in the future, there will be different demands freely (and frame design) when the frame in the network is reused on the same link (for example), what will happen? What happens when line 1 uses a large frame (data frame) while line 2 uses a very small frame (audio and video frame?

Because frame X arrives first, the multiplex goes first to the path. frame a must wait until the entire x bit enters the path before it can follow, the absolute size of frame X causes the abnormal latency of frame a. The same imbalance may affect all frames from line 2. Because audio and video frames are usually very small, they are transmitted together with traditional data frames, which often leads to an unacceptable delay of such types of frames, making the Shared Frame link unavailable for voice and video information.

Cell network

Many problems related to the frame interconnection network can be solved by using the cell network concept. A cell is a fixed data unit. In a cell network, cells are used as the basic unit of data exchange. All data is loaded into the same cell, these cells can be transmitted in a completely predictable and unified manner. When frames of different sizes and formats arrive at the cell network from the branch network, they are divided into multiple small data units of the same size and loaded into cells. These cells and other cells are multiplexing and routed through the entire cell network.

It shows that there are two links of the multiplexing to send cells rather than frames, frame X is divided into three cells: X, Y, and Z, the first cell in link 1 is sent before the first cell in link 2. The cells in these two links are intertwined, and no cell bears a long delay.

Asynchronous TDM

ATM uses asynchronous time division multiplexing to process cells from different channels, which is why it becomes an asynchronous transmission mode. It uses a fixed time slot (the size of a cell ). The ATM multiplexing uses a cell from any input channel to fill a time slot. If the channel does not send any cell, the time slot is empty.

ATM has advanced congestion control and service quality, which will be discussed later.