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Leased Lines

A dedicated leased line is typically a point-to-point link interconnecting two sites. All the bandwidth on that dedicated leased line is available to those sites. This means that, unlike a packet-switched connection, the bandwidth of a dedicated leased line connection does not need to be shared among multiple service provider customers.

WAN technologies commonly used with dedicated leased lines include digital circuits, such as T1, E1, T3, and E3 circuits. These circuits use multiplexing technology to simultaneously carry multiple conversations in different 64-kbps channels. A single 64-kbps channel is called a Digital Signal 0 (DS0).

When one of these circuits comes into your location, it terminates on a device called a channel service unit/data service unit (CSU/DSU). Also, be aware that a customary Layer 2 protocol used on dedicated leased lines is PPP. A common connection type used to join to a CSU/DSU is an RJ-48C, which looks similar to an RJ-45(Ethernet) connector. Figure 3-1 shows a dedicated leased line.

Figure 3-1

Figure 3-1 A Dedicated Leased Line

T1—T1 circuits were originally used in telephony networks, with the intent of one voice conversation being carried in a single channel (that is, a single DS0). A T1 circuit is composed of 24 DS0s, which is called a Digital Signal 1 (DS1). The bandwidth of a T1 circuit is 1.544 Mbps.

T1 circuits are popular in North America and Japan.

E1—An E1 circuit contains 32 channels, in contrast to the 24 channels on a T1 circuit. Only 30 of those 32 channels, however, can transmit data (or voice or video). Specifically, the first of those 32 channels is reserved for framing and synchronization, and the seventeenth channel is used for signaling (that is, setting up, maintaining, and tearing down a call).

Because an E1 circuit has more DS0s than a T1, it has a higher bandwidth capacity. Specifically, an E1 has a bandwidth capacity of 2.048 Mbps.

Unlike a T1 circuit, an E1 circuit does not group frames together in an SF or ESF. Instead, an E1 circuit groups 16 frames together in a multiframe.

E1 circuits are popular outside North America and Japan.

T3—In the same T-carrier family of standards as a T1, a T3 circuit offers an increased bandwidth capacity. Although a T1 circuit combines 24 DS0s into a single physical connection to offer 1.544 Mbps of bandwidth, a T3 circuit combines 672 DS0s into a single physical connection, which is called a Digital Signal 3 (DS3). A T3 circuit has a bandwidth capacity of 44.7 Mbps.

E3—Just as a T3 circuit provides more bandwidth than a T1 circuit, an E3 circuit’s available bandwidth of 34.4 Mbps is significantly more than the 2.048 Mbps of bandwidth offered by an E1 circuit. A common misconception is that the bandwidth of an E3 is greater than the bandwidth of a T3 because an E1’s bandwidth is greater than a T1’s bandwidth. However, that is not the case—a T3 has a greater bandwidth (that is, 44.7 Mbps) than an E3 (that is, 34.4 Mbps).

CSU/DSU—Although far less popular than they once were, analog modems allowed a phone line to come into a home or business and terminate on analog modems, which provided data connections for devices such as PCs. These analog modems supported a single data conversation per modem.

However, digital circuits (for example, T1, E1, T3, or E3 circuits) usually have multiple data conversations multiplexed together on a single physical connection. Therefore CSU/DSU, a digital modem, is needed, as opposed to an analog modem. This digital modem must be able to distinguish between data arriving on various DS0s.

A CSU/DSU circuit can terminate an incoming digital circuit from a service provider and send properly formatted bits to a router. A CSU/DSU uses clocking (often provided by the service provider) to determine when one bit stops and another starts. Therefore, the circuit coming from a service provider and terminating on a CSU/DSU is a synchronous circuit (in which the synchronization is made possible by clocking).

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