Dedicated-circuit networks (Data Communications and Networking)

There are three main problems with POTS and ISDN circuit-switched networks. First, each connection goes through the regular telephone network on a different circuit. These circuits may vary in quality, meaning that although one connection will be fairly clear, the next call may be noisy. Second, the data transmission rates on these circuits are usually low. Generally speaking, transmission rates range from 28.8 Kbps to 56 Kbps for dialed POTS circuits to 128 Kbps to 1.5 Mbps for ISDN circuits. Third, you usually pay per use for circuit-switched services. One alternative is to establish a dedicated-circuit network, in which the user leases circuits from the common carrier for his or her exclusive use 24 hours per day, 7 days per week.

Basic Architecture

With a dedicated-circuit network, you lease circuits from common carriers. All connections are point to point, from one building in one city to another building in the same or a different city. The carrier installs the circuit connections at the two end points of the circuit and makes the connection between them. The circuits still run through the common carrier’s cloud, but the network behaves as if you have your own physical circuits running from one point to another (Figure 9.2).

Once again, the user leases the desired circuit from the common carrier (specifying the physical end points of the circuit) and installs the equipment needed to connect computers and devices (e.g., routers or switches) to the circuit. This equipment may include multiplexers or a channel service unit (CSU) and/or a data service unit (DSU); a CSU/DSU is the WAN equivalent of a NIC in a LAN.

Unlike circuit-switched services that typically use a pay-per-use model, dedicated circuits are billed at a flat fee per month, and the user has unlimited use of the circuit. Once you sign a contract, making changes can be expensive because it means rewiring the buildings and signing a new contract with the carrier. Therefore, dedicated circuits require more care in network design than do switched circuits, both in terms of locations and the amount of capacity you purchase.

Dedicated-circuit services. CSU = channel service unit; DSU = data service unit; MUX = multiplexer

Figure 9.2 Dedicated-circuit services. CSU = channel service unit; DSU = data service unit; MUX = multiplexer

There are three basic architectures used in dedicated-circuit networks: ring, star, and mesh. In practice, most networks use a combination of architectures. For example, a distributed star architecture has a series of star networks that are connected by a mesh or ring architecture.

Ring Architecture A ring architecture connects all computers in a closed loop with each computer linked to the next (Figure 9.3). The circuits are full-duplex or half-duplex circuits, meaning that messages flow in both directions around the ring. Computers in the ring may send data in one direction or the other, depending on which direction is the shortest to the destination.

One disadvantage of the ring topology is that messages can take a long time to travel from the sender to the receiver. Messages usually travel through several computers and circuits before they reach their destination, so traffic delays can build up very quickly if one circuit or computer becomes overloaded. A long delay in any one circuit or computer can have significant impacts on the entire network.

In general, the failure of any one circuit or computer in a ring network means that the network can continue to function. Messages are simply routed away from the failed circuit or computer in the opposite direction around the ring. However, if the network is operating close to its capacity, this will dramatically increase transmission times because the traffic on the remaining part of the network may come close to doubling (because all traffic originally routed in the direction of the failed link will now be routed in the opposite direction through the longest way around the ring).

Ring-based design

Figure 9.3 Ring-based design

Star Architecture A star architecture connects all computers to one central computer that routes messages to the appropriate computer (Figure 9.4). The star topology is easy to manage because the central computer receives and routes all messages in the network. It can also be faster than the ring network because any message needs to travel through at most two circuits to reach its destination, whereas messages may have to travel through far more circuits in the ring network. However, the star topology is the most susceptible to traffic problems because the central computer must process all messages on the network. The central computer must have sufficient capacity to handle traffic peaks, or it may become overloaded and network performance will suffer.

In general, the failure of any one circuit or computer affects only the one computer on that circuit. However, if the central computer fails, the entire network fails because all traffic must flow through it. It is critical that the central computer be extremely reliable.

Star-based design

Figure 9.4 Star-based design

Mesh Architecture In a full-mesh architecture, every computer is connected to every other computer (Figure 9.5a). Full-mesh networks are seldom used because of the extremely high cost. Partial-mesh architecture (usually called just mesh architecture),in which many, but not all, computers are connected, is far more common (Figure 9.5b). Most WANs use partial-mesh topologies.

The effects of the loss of computers or circuits in a mesh network depend entirely on the circuits available in the network. If there are many possible routes through the network, the loss of one or even several circuits or computers may have few effects beyond the specific computers involved. However, if there are only a few circuits in the network, the loss of even one circuit or computer may seriously impair the network.

In general, mesh networks combine the performance benefits of both ring networks and star networks. Mesh networks usually provide relatively short routes through the network (compared with ring networks) and provide many possible routes through the network to prevent any one circuit or computer from becoming overloaded when there is a lot of traffic (compared with star networks in which all traffic goes through one computer).

The drawback is that mesh networks use decentralized routing so that each computer in the network performs its own routing. This requires more processing by each computer in the network than in star or ring networks. Also, the transmission of network status information (e.g., how busy each computer is) "wastes" network capacity.

Mesh design

Figure 9.5 Mesh design

There are two types of dedicated-circuit services in common use today: T carrier services and synchronous optical network (SONET) services. Both T carrier and SONET have their own data link protocols, which are beyond the focus of this topic.

T Carrier Services

T carrier circuits are the most commonly used form of dedicated-circuit services in North America today. As with all dedicated-circuit services, you lease a dedicated circuit from one building in one city to another building in the same or different city. Costs are a fixed amount per month, regardless of how much or how little traffic flows through the circuit. There are several types of T carrier circuits (Figure 9.6).

A T1 circuit (also called a DS1 circuit) provides a data rate of 1.544 Mbps. T1 circuits can be used to transmit data but often are used to transmit both data and voice. In this case, inverse TDM provides 24 64-Kbps circuits.2 Digitized voice using PCM requires a 64-Kbps circuit,so a T1 circuit enables 24 simultaneous voice channels. Most common carriers make extensive use of PCM internally and transmit most of their voice telephone calls in digital format using PCM, so you will see many digital services offering combinations of the standard PCM 64-Kbps circuit.

A T2 circuit, which transmits data at a rate of 6.312 Mbps, is an inverse multiplexed bundle of four T1 circuits. A T3 circuit allows transmission at a rate of 44.736 Mbps although most articles refer to this rate as 45 megabits per second. This is equal to the capacity of 28 T1 circuits. T3 circuits are becoming popular as the transmission medium for corporate MANs and WANs because of their higher data rates. At low speed, these T3 circuits can be used as 672 different 64-Kbps channels or voice channels. A T4 circuit transmits at 274.176Mbps, which is equal to the capacity of 178T1 circuits.

Fractional T1, sometimes called FT1, offers portions of a 1.544-Mbps T1 circuit for a fraction of its full cost. Many (but not all) common carriers offer sets of 64 Kbps DS-0 channels as FT1 circuits. The most common FT1 services provide 128 Kbps, 256 Kbps, 384 Kbps, 512 Kbps, and 768 Kbps.

Synchronous Optical Network

The synchronous optical network (SONET) is the American standard (ANSI) for high-speed dedicated-circuit services. The ITU-T recently standardized an almost identical service that easily interconnects with SONET under the name synchronous digital hierarchy (SDH).

T Carrier Designation

DS Designation




64 Kbps



1.544 Mbps



6.312 Mbps



44.376 Mbps



274.176 Mbps

Figure 9.6 T carrier services

SONET Designation

SDH Designation



51.84 Mbps



155.52 Mbps



622.08 Mbps



1.244 Gbps



2.488 Gbps



9.953 Gbps



39.813 Gbps

Figure 9.7 SONET (synchronous optical network) and SDH (synchronous digital hierarchy) services. OC = optical carrier (level)

SONET transmission speeds begin at the OC-1 level (optical carrier level 1) of 51.84 Mbps. Each succeeding rate in the SONET fiber hierarchy is defined as a multiple of OC-1, with SONET data rates defined as high as OC-768, or about 40 Gbps. Figure 9.7 presents the commonly used SONET and SDH services. Each level above OC-1 is created by an inverse multiplexer. Notice that the slowest SONET transmission rate (OC-1) of 51.84 Mbps is slightly faster than the T3 rate of 44.376 Mbps.

Next post:

Previous post: