Leased Lines

A dedicated leased line typically connects two sites via a point-to-point link.This dedicated leased line is completely dedicated to those two sites.Unlike a packet-switched connection, a dedicated leased line connection doesn't need to be shared among several companies.

T1, E1, T3, and E3 circuits are common WAN technologies for dedicated leased lines.Multiple conversations are simultaneously carried in different 64-kbps channels by using multiplexing technology.DS0 signifies a single 64-kbps channel.

Each of these circuits arrives at your location and terminates on a device called a channel service unit/data service unit (CSU/DSU).On dedicated leased lines, the Layer 2 protocol used is PPP.RJ-48C, which looks similar to an RJ-45 (Ethernet) connector, is a common connector used to connect to a CSU/DSU.A dedicated leased line is shown in Figure 3-1.


Figure 3-1, Dedicated Leased Line A

As the name implies, T1 circuits were originally designed for use in telephony networks, with the intention of carrying a single voice conversation over one channel (i.e., a DS0).DS1s are Digital Signal 0s (DS0s) that are configured into a T1 circuit.This circuit has a bandwidth of 1.544 Mbps.

The T1 circuit is popular in North America and Japan.

E1 is a circuit that has 32 channels, as opposed to 24 on a T1 circuit.In fact, only 30 of those 32 channels can transmit data (or audio or video).One of these 32 channels is reserved for framing and synchronization, and the seventeenth channel is reserved for signaling (set up, maintain, and tear down a call).

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

A T1 circuit groups frames into an SF or an ESF, but an E1 circuit does not.Multiframes are instead created by grouping 16 frames together in an E1 circuit.

Circuits like the E1 are popular outside of North America and Japan.

An increased capacity T3 circuit falls in the same family of T-carrier standards as a T1.T1 circuits combine 24 DS0s to offer 1.544 Mbps, while T3 circuits combine 672 DS0s to offer 1.525 Mbps, which is known as Digital Signal 3.There are 44.7 Mbps of bandwidth on a T3 circuit.

The capacity of an E3 circuit makes it significantly more than an E1 circuit, whose available bandwidth is only 2.048 Mbps. Likewise, a T3 circuit offers significantly more bandwidth than a T1 circuit.There is a misconception that an E1's bandwidth is greater than a T1's bandwidth.In reality, though, a T3 has a much higher bandwidth (44,7 Mbps) than an E3 (34,4 Mbps).

Even though analog modems are no longer popular, they allowed phone lines to be brought into a business or home and terminate on analog modems, which provided data connections for devices like PCs.A single data conversation could be conducted per modem.

Generally, digital circuits (such as T1, E1, T3, or E3 circuits) have multiple data interactions multiplexed onto a single physical connection.CSU/DSU, as opposed to a modem with an analog signal, are needed.To distinguish between DS0s, the digital modem must distinguish between data arriving on various DS0s.

The CSU/DSU circuit can terminate an incoming digital circuit from a service provider and send the properly formatted bits to the router.It is determined when one bit stops and another one starts by clocking (often provided by the service provider).Thus, a circuit that originates from a service provider and terminates on a CPU/DPU is a synchronous one (synchronization is accomplished by clocking).