Leased Lines

A dedicated leased line is typically a point-to-allude link interconnecting two sites. All the bandwidth on that dedicated leased line is accessible to those sites. This suggests that, unchoose a packet-switched connection, the bandwidth of a dedicated leased line connection does not must be common among multiple service provider customers.

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WAN modern technologies typically provided via dedicated leased lines incorporate digital circuits, such as T1, E1, T3, and also E3 circuits. These circuits usage multiplexing innovation to concurrently carry multiple conversations in different 64-kbps networks. A single 64-kbps channel is dubbed a Digital Signal 0 (DS0).

When one of these circuits comes right into your location, it terminates on a maker called a channel company unit/data organization unit (CSU/DSU). Also, be aware that a customary Layer 2 protocol supplied on dedicated leased lines is PPP. A common link kind supplied to sign up with to a CSU/DSU is an RJ-48C, which looks equivalent to an RJ-45(Ethernet) connector. Figure 3-1 mirrors a devoted leased line.


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Figure 3-1 A Dedicated Leased Line


T1—T1 circuits were initially used in telephony netfunctions, with the intent of one voice conversation being brought in a solitary channel (that is, a single DS0). A T1 circuit is created of 24 DS0s, which is dubbed a Digital Signal 1 (DS1). The bandwidth of a T1 circuit is 1.544 Mbps.

T1 circuits are famous in The United States and Canada and Japan.

E1—An E1 circuit includes 32 channels, in comparison to the 24 channels on a T1 circuit. Only 30 of those 32 channels, however, have the right to transmit data (or voice or video). Specifically, the initially of those 32 channels is scheduled for framing and synchronization, and also the seventeenth channel is supplied for signaling (that is, establishing up, preserving, and also tearing down a call).

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

Unprefer a T1 circuit, an E1 circuit does not team frames together in an SF or ESF. Instead, an E1 circuit teams 16 frames together in a multiframe.

E1 circuits are renowned outside The United States and Canada and also Japan.

T3—In the very same T-carrier household of criteria as a T1, a T3 circuit supplies an boosted bandwidth capacity. Although a T1 circuit combines 24 DS0s into a solitary physical connection to offer 1.544 Mbps of bandwidth, a T3 circuit combines 672 DS0s into a solitary physical link, which is called a Digital Signal 3 (DS3). A T3 circuit has actually a bandwidth capacity of 44.7 Mbps.

E3—Just as a T3 circuit offers even more bandwidth than a T1 circuit, an E3 circuit’s accessible bandwidth of 34.4 Mbps is substantially even more than the 2.048 Mbps of bandwidth available by an E1 circuit. A prevalent misconception is that the bandwidth of an E3 is better than the bandwidth of a T3 bereason an E1’s bandwidth is higher than a T1’s bandwidth. However before, that is not the case—a T3 has actually a better bandwidth (that is, 44.7 Mbps) than an E3 (that is, 34.4 Mbps).

CSU/DSU—Although far much less popular than they when were, analog modems permitted a phone line to come right into a house or company and also terminate on analog modems, which gave data relationships for tools such as PCs. These analog modems supported a solitary information conversation per modem.

However before, digital circuits (for instance, T1, E1, T3, or E3 circuits) commonly have actually multiple information conversations multiplexed together on a single physical connection. Therefore CSU/DSU, a digital modem, is essential, as opposed to an analog modem. This digital modem should have the ability to distinguish between information getting here on miscellaneous DS0s.

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A CSU/DSU circuit deserve to terminate an incoming digital circuit from a service provider and sfinish properly formatted bits to a router. A CSU/DSU uses clocking (regularly gave by the business provider) to recognize as soon as one little bit stops and an additional starts. Because of this, the circuit coming from a organization provider and terminating on a CSU/DSU is a synchronous circuit (in which the synchronization is made possible by clocking).