Transports are the physical lines installed at the company or consumer premises to provide all sorts of network access. Many folks think of T1 or T3 transport lines when they hear the phrase dedicated transport, and with good reason. T1 and T3 lines are the most popular dedicated network transports in the country.
Dedication pays off
VoIP is making dedicated transports even more popular. If you're going to run VoIP on your company's network, dedicated transports give you the best-quality VoIP. Dedicated transports also allow you to connect all the data applications that your company uses at all your locations.
With a dedicated transport, your network can support massive volumes of on-net VoIP calls. Huge networks with multiple locations and hundreds or thousands of callers are best supported with dedicated transports. Dedicated transports enable a quality of service that meets or exceeds the quality found in traditional circuit-switched PSTN telephone calls.
Bandwidth and speed
Bandwidth and speed are the darling twins of data networks. When you compare speeds between dedicated and switched networks, you'll find that a dedicated network transport generally provides far more throughput.
 | Throughput is the total amount of data that can be passed over a transport line in a given amount of time. Throughput is directly related to bandwidth and is often used synonymously with data speed. |
Two factors affect both bandwidth and speed when it comes to dedicated lines:
- Routing on a dedicated line is directly between two points, passing through few routers and switches. Data passing through a switched network, such as the public switched telephone network (PSTN), will go through many routers and switches. The more switching points involved, the less throughput because each switching point adds overhead to track the data.
- Exclusivity refers to the fact that a dedicated transport permits only a single customer's data on the line. In a switched network, data is aggregated and shared with others, reducing the bandwidth available to any single customer. Aggregation also involves resource contention, which can increase delay and signal degradation.
Dedicated networks are often called private networks or private line networks.
Types of dedication
For the corporate sector, dedicated network transports come in two major flavors: the digital service (or digital signal) carrier services infrastructure (CSI) and the optical carrier CSI.
The DS transports
The original digital service (DS) series of standards had five levels of dedicated lines. Each DS standard provided a set number of 64-Kbps channels, or DS0s. For example, a DS1 (T1) transport includes 24 channels, and a DS3 (T3) includes 672 channels.
| In the old days, when DS transports were new and costly, the DS0 was leased as a single 64-Kbps transport line. Today, hardly anyone leases a DS0 transport. If you're implementing VoIP on your company's network, don't consider using just a single DS0 channel. Not only does it not provide enough bandwidth, but it also isn't cost effective. |
T1 line
The T in the T1 version of transport represents terrestrial, or over land. The tariffs controlled by the government for setting the pricing of DS transports are based on the total terrestrial mileage between point A and point B.
The T1 transport continues to be the most popular transport on the market, and prices continue to drop. A big reason the T1 is so popular is that it permits network configuration folks to divide the total available bandwidth into smaller individual channels. This makes the T1 particularly suitable for VoIP networks that run computer data, telephony voice, and even videoconferencing over the same network transport. T1, however, does not provide adequate bandwidth for large multilocation networks with hundreds of users in each location.
T3 line
If you need more bandwidth than what you can obtain from a T1 or a group of T1s, consider the T3 transport. The T3 transport provides a total aggregate bandwidth of 45 Mbps. This breaks down to about 672 DS0 channels. A growing company can also consider upgrading to T3 transports or some mix of T1 and T3 lines. (The latter is more commonplace for larger companies, which use T1 lines for smaller locations and T3 for the larger locations.)
The OC transports
The advent of fiber-optic cabling in the 1980s changed the way that DS lines were installed. By the 1990s, most dedicated transport lines were going in as fiber-optic cables or being implemented through existing fiber lines. The terminating equipment would then be programmed to deliver the equivalent of however many DS0 channels were needed (1, 24, or 672).
 | Fiber-optic transports are defined according to the OC (optical carrier) CSI standards. The four most common transports are OC-3 (155 Mbps), OC-12 (622 Mbps), OC-48 (2.5 Gbps), and OC-192 (10 Gbps). These types of dedicated transports are used by only the largest corporations and the carriers themselves. The most popular OC standard is OC-3. |
Not surprisingly, most small- and medium-sized companies rarely have the need for even an OC-3. But carrier companies put in a minimum of an OC-3 whenever they install carrier services for multi-tenant buildings and the largest customers. Their rationale is to put in enough fiber bandwidth to cover future customer needs. When those needs arise, the carrier then simply programs its equipment to deliver the necessary bandwidth.
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