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Tham khảo tài liệu 'kỹ thuật truyền số liệu 18', giáo dục - đào tạo, cao đẳng - đại học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | VPTER 18 Frame Relay 18.1 INTRODUCTION Recently the nature of the demands on WANs has changed dramatically. Previous WAN technologies such as T-lines or X.25 were not responding to the needs of the user. Users were looking for higher data rates lower cost efficient handling of bursty data transmissions and less overhead. Frame Relay is a virtual-circuit technology that provides low-level physical and data link layers service in response to the following demands Higher Data Rate at Lower Cost. In the past many organizations used a WAN technology such as a leased line or X.25 to connect single computers. The data rate was relatively low. Today most organizations use high-speed LANs and want to use WANs to connect these LANs. One solution is to use T-lines but these lines provide only point-to-point connections not many-to-many. Creating a mesh network out of T-lines is very expensive. For example to connect six LANs we need 15 T-lines. On the other hand we need only six T-lines to connect the same six LANs to a Frame Relay network. Frame Relay provides the same type of service at lower cost. Figure 18.1 shows the difference. Although Frame Relay originally was designed to provide a 1.544-Mbps data rate equivalent to a T-l line today most implementations can handle up to 44.376 Mbps equivalent to a T-3 line . Bursty Data. Some services offered by wide area network providers assume that the user has a fixed-rate need. For example a T-l line is designed for a user who wants to use the line at a consistent 1.544 Mbps. This type of service is not suitable for the many users today that need to send bursty data. For example a user may want to send data at 6 Mbps for 2 seconds 0 Mbps nothing for 7 seconds and 3.44 Mbps for 1 second for a total of 15.44 Megabits during a period of 10 seconds. Although the average data rate is still 1.544 Mbps the T-l line cannot accept this type of demand because it is designed for fixed-rate data not bursty data. Bursty data requires what is
