Chapter 3: WAN Switching

In Depth

Switches are not only used in LAN networks; they are also used extensively in wide area networks (WANs). Chapters 1 and 2 gave you an overview of LAN switching. Well, WAN switching is the same in some ways and completely different in others.

In an Ethernet switching environment, the switch utilizes Carrier Sense Multiple Access with Collision Detection (CSMA/CD). The switch or host sends out a packet and detects if a collision occurs. If there is a collision, the sender waits a random amount of time and then retransmits the packet. If the host does not detect a collision, it sends out the next packet. You may think that if the switch or host is set to full−duplex, there will be no collision—that is correct, but the host still waits between sending packets.

In a Token Ring switching environment, a token is passed from one port to the next. The host must have possession of the token to transmit. If the token is already in use, the host passes the token on and waits for it to come around again. All stations on the network must wait for an available token. An active monitor, which could be any station on the segment, performs a ring maintenance function and generates a new token if the existing token is lost or corrupted.

As you can see, both Token Ring and Ethernet switching require the node to wait. The node must wait either for the token or for the frame to reach the other nodes. This is not the most efficient utilization of bandwidth. In a LAN environment, this inefficiency is not a major concern; in a WAN, it becomes unacceptable. Can you imagine if your very expensive T1 link could be used only half the time? To overcome this problem, WAN links utilize serial transmission.

Serial transmission sends the electric signal (bits) down the wire one after another. It does not wait for one frame to reach the other end before transmitting the next frame. To identify the beginning and the end of the frame, a timing mechanism is used. The timing can be either synchronous or asynchronous. Synchronous signals utilize an identical clock rate, and the clocks are set to a reference clock. Asynchronous signals do not require a common clock; the timing signals come from special characters in the transmission stream.

Asynchronous serial transmissions put a start bit and a stop bit between each character (usually 1 byte). This is an eight−to−two ratio of data to overhead, which is very expensive in a WAN link.

Synchronous serial transmissions do not have such high overhead, because they do not require the special characters; they also have a larger payload. Are synchronous serial transmissions the perfect WAN transmission method? No; the problem lies in how to synchronize equipment miles apart. Synchronous serial transmission is only suitable for distances where the time required for data to travel the link does not distort the synchronization.

So, first we said that serial is the way to go, and now we’ve said that serial has either high overhead or cannot travel a long distance. What do we use? Well, we use both, and cheat a little bit. We use synchronous serial transmission for a short distance and then use asynchronous for the remaining, long distance. We cheat by putting multiple characters in each frame and limiting the overhead.

When a frame leaves a host and reaches a router, the router uses synchronous serial transmission to pass the frame on to a WAN transmission device. The WAN device puts multiple characters into each WAN frame and sends it out. To minimize the variation of time between when the frames leave the host and when they reach the end of the link, each frame is divided and put into a slot in the WAN frame. This way, the frame does not have to wait for the transmission of other frames before it is sent. (Remember, this process is designed to minimize wait time.) If there is no traffic to be carried in a slot, that slot is wasted. Figure 3.1 shows a diagram of a packet moving from LAN nodes to the router and the WAN device.

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