Pointto Point WAN Summary

Point-to-point WAN leased lines and their associated data-link protocols use another set of terms and concepts beyond those covered for LANs. Table 4-7 lists the terms.

Table 4-7 WAN Terminology




The imposition of time ordering on a bit stream. Practically, a device tries to use the same speed as another device on the other end of a serial link. However, by examining transitions between voltage states on the link, the device can notice slight variations in the speed on each end and can adjust its speed accordingly.


The lack of an imposed time ordering on a bit stream. Practically, both sides agree to the same speed, but there is no check or adjustment of the rates if they are slightly different. However, because only 1 byte per transfer is sent, slight differences in clock speed are not an issue. A start bit is used to signal the beginning of a byte.

Clock source

The device to which the other devices on the link adjust their speed when using synchronous links.


Data service unit/channel service unit. Used on digital links as an interface to the telephone company in the United States. Routers typically use a short cable from a serial interface to a DSU/CSU, which is attached to the line from the telco with a similar configuration at the other router on the other end of the link.


Telephone company.

Four-wire circuit

A line from the telco with four wires, comprised of two twisted-pair wires. Each pair is used to send in one direction, so a four-wire circuit allows full-duplex communication.


A line from the telco that allows transmission of data at 1.544 Mbps.


Similar to a T1, but used in Europe. It uses a rate of 2.048 Mbps and 32 64-kbps channels.

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