数据教程课件Chapter 10 connecting networks 1.ppt

Chapter Ten Connecting Networks Connecting Networks 10.1 Interconnection Topologies Ch10.1 Lay 1 Connections 10.2Repeaters Repeaters (1) Repeaters (2) Ethernet Repeaters (3) Ethernet Repeaters (4)-Problem Hub Hubs Problems with Ethernet Hubs (2) Bridges Bridges Bridges (cont.) Extend LANs at data link layer. Perform error detection, frame formatting, and frame routing. Allow interconnecting different types of LANs Frames may need to be reformatted or broken into pieces. Reasons for using bridges Efficiency - filter out unnecessary traffic Interdepartmental communications Security Bridging different LANs Speed differences require buffers Suppose a bridge connects a fast LAN and a slow LAN. The bridge has to provide a large buffer space for frames from the fast LAN. Bridge delays can cause excessive timeouts Timers are used in flow control in data link layer. Longer delays in bridges will cause those timers to expire. Frame formatting differences Reformat a received frame: change headers … How to convert frames with the priority information in a token ring to frames in an Ethernet? Bridging Different LANs (cont.) Maximum frame sizes The frame size varies between different types of LANs. Solution 1 - Choose the smallest maximum frame size as the maximum frame size in the interconnected LANs. Solution 2 - Break the larger frames into small fragments and send them. This requires additional logic which can be achieved at a higher layer protocol (i.e. the network layer). Bridge Routing If B3 senses a frame from L2 whose dest is A B or E, B3 ignores. If B3 senses a frame from L2 whose dest is C D or F,B3 should accepts it and forwards it to L3. How does a bridge know when to accept and forward a frame? And where to forward? If F is moved to L4, how does the bridge know? Bridges maintain routing tables to know where to send frames. Fixed-Routing Bridges Assume that once the routing table is set up it does not change. They are not flexible. Transparent Bridges Can create a