Understanding the network.pdf - Back to Home

NOTE
The "DS" abbreviation is used by both the ANSI and ITU-T DSH definitions. The
"I-DS" abbreviation is only used in Table 5.1 to make distinctions between the two
standards.
T1 Bandwidth Provisioning
T1 circuits are provided to customers as Type III (4-wire interface with PSTN
signaling) and Type IV (4-wire interface without signaling). Pricing is based on
distance, not usage. This makes it advantageous to the user to make the most
efficient use of the available bandwidth (if applicable) through the use of a DMUX.
Due to the cost of T1s, for many years the LECs offered dedicated fractional T1
service, which was bandwidth based on 64K increments. Due to the large drop in T1
pricing and the availability ISDN and Frame Relay, fractional T1 service in many
areas is being phased out by LECs.
T1 facilities are used for all types of primary rate digital transmission services (ISDN,
Frame Relay, and so on). The T-carrier transport system operates on OSI-RM Layer
1. The T1 circuit's bandwidth is partitioned into 24 DS0 channels, or 24 ´ 64Kbps
channels, for a user data transfer rate of 1,536 Kbps. To provide in-band control and
management data, an additional 8Kbps were added to provide transport for
signaling, making the total aggregate DS1 data rate 1.544Mbps. In the case of ISDN
and Frame Relay service, which are transport services provided by telephone
carriers, there are OSI Layer 2 and 3 operations involved. These services are
implemented by using different signaling and multiplexing facilities. The digital
transport system, however, is still T-carrier.
T1 Framing
T-carrier uses PCM and TDM to transport digital signals. The T1 is made up of 24
(DS0) 64Kbps channels or time-slots and transmits at 1.544 clock intervals per
second. Each time-slot/channel gets a turn to transmit its data (650ns per interval).
The incoming analog signal on a time-slot/channel is sampled 8,000 times per
second. The sample is then converted into an 8-bit word (pulse code). This creates
a digital stream of 8 bits ´ 8,000 samples/second = 64Kbps, the DS0 data rate.
In order for the remote end to use the data sent across the time-slots, it must know
how the bits are organized. Framing provides the organization needed for the
bitstream to be understood in a meaningful way. T1 framing information is provided
through the use of the 8,000 additional signaling bits added to the T1 datastream.