In 1964, the carriers began channelizing and aggregating analog inbound tele- phone calls onto digital, high-bandwidth transports. The digital service (DS) carrier service infrastructure was born. The type of wiring used for DS trans- ports was copper, like the PSTN transport lines in the PSTN. The DS transport lines, however, were of a thicker gauge and were capable of sustaining higher bandwidth capacities. The carrier often referred to DS type transport lines as “high-cap” T1 lines to distinguish them from other types of copper transport lines in the PSTN. Today most T1 transport lines are provided using fiber- optic lines.
Because of higher bandwidth capacities, transports under the DS CSI are ter- minated differently than lines in the PSTN CSI. This is in part what led to the designation of DS lines as being dedicated. Unlike PSTN transport lines, which potentially could be terminated and switched out all over the CSI prior to con- necting to their destination point, DS transport lines were installed to provide a direct connection between source and target destination points.
DS transports do not share any switching points with other customers in
the carrier service infrastructure. For this reason, after a DS transport line is installed, it is said to be “nailed up” for that customer’s use only. This direct, nailed-up connection enables not only higher bandwidth, but also much greater throughput and no contention from other parts of the CSI.
Contention occurs when a number of users try to use a limited number of resources at the same time. In a public network, all users vie for a limited amount of bandwidth. That’s why you get the “We’re sorry, but all circuits are busy now” message periodically. A dedicated DS line has no contention because it has no other users — it is your line and your line only.
VoIP transports can be dedicated
When Ethernet LANs were standardized in the late 1980s, a huge demand emerged from big multilocation companies wanting to connect their LANs in a wide area network (WAN). Many had hundreds or thousands of locations, each of which was running its own Ethernet LAN. Back then, networks had strictly a computer data context (VoIP had not yet been discovered).
In response to customer demand, the telecommunications industry provided frame-relay transport services, one of the most important transport services to come out of the DS CSI. Frame relay takes the frames on the LAN side des- tined for another location on the WAN and packetizes them for transport over the WAN. Today, 86 percent of corporate America continues to use frame- relay for computer data service.
A frame is just another word for a data packet. Technically, a frame is a data packet on a local network. Only when the frame is encapsulated for transfer over nonlocal networks (see Chapter 1) does a frame become correctly referred to as a packet.
Frame relay is losing ground to DSL because DSL is now available at commer- cial bandwidth levels. But frame relay still appears to be the transport service of choice when it comes to interconnecting large, multilocation, data-only LANs. Usually all sites are connected with a T1 or T3 transport line, but
under frame relay they do not always operate with the line’s full capacity. Thus, their bandwidth is purposefully throttled by the carrier to provide frame-relay service through what would otherwise be a very large “pipe.”
The carrier charges a monthly access fee for the transport line itself. In addi- tion, charges are paid monthly for the transport usage in a frame-relay net- work. This charge is for port speed, which is based on the number of channels (versus the entire transport line’s capacity) that the customer uses. Therefore, it is not uncommon to see a lot of fractional T1 (a T1 line that uses only a frac- tion of the total twenty-four channels) services in a frame-relay network. The good news is that any frame-relay network can be updated cost-effectively to support a dedicated VoIP network because the T1 or T3 transport lines are already in place.
The DS CSI’s two most popular transports are the T1 line, which has 24 DS0 channels, and the T3 line, which has an aggregate capacity of 672 DS0 channels. (DS0 channels are 64 Kbps channels, as described in Chapter 7.) Because DS transports are dedicated and channelizable, the T1 and T3 trans- ports work well with VoIP. On a dedicated transport, specific channels on the DS line can be allocated to VoIP calls when needed and returned to the DS transport’s channel pool when not needed. As a result, DS transports can be used not only for VoIP but also for integrated computer data and videoconferencing.
Other VoIP transports
Many companies are finding the T1 line an effective transport for supporting VoIP. The cost of a T1 line has dropped significantly in the past five years. It is still priced based on total mileage covered, but with the emerging fiber glut, many T1 lines can be leased to companies from the carrier’s excess fiber- transport lines. When this occurs, the T1 line is said to be carved (multi- plexed) out of the much higher bandwidth fiber-optic transport line. A
fiber-optic line has enough capacity for thousands of DS0 channels.
As mentioned, a T1 line provides twenty-four DS0 channels. If a fiber-optic transport line is already in place, it’s just a matter of the carrier programming their equipment to deliver the twenty-four channels to the customer. The LEC delivers a single, huge bandwidth pipe, in this case an OC-3. The OC-3 is then subdivided as needed to provide various types of other bandwidth lines.
The LEC often installs a larger transport line and then throttles back what is delivered through the line because the labor costs are about the same for any dedicated line. The LEC’s logic is reasonable: Pull (install) the most effective high-bandwidth transport possible. In this way, they position themselves to support the current and future bandwidth needs of all the companies in the building. The LEC expends labor costs once in return for many future band- width requests.
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