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Fractional T1 Services
In a traditional T1 network, the customer leases a T1 link between two locations.
The entire T1 bandwidth of 24 DS0s is transmitted from one customer location
to another. In a Fractional T1 (FT1) network, a customer can lease a subset of the
24 DS0s on a T1 link. Thus, for an application requiring 8 DS0s (512 kbps), the
customer might lease DS0s 1 through 8 of a T1 link. Because customers contract
only for the bandwidth needed, FT1 can provide substantial cost savings.
FT1 networks also allow the switching of DS0s within the network, giving
customers greater flexibility and economy when configuring multipoint net-
works (i.e., networks serving three or more customer locations). Multiplexed
communication channels originating at one customer location may be switched
to two different destinations within the network, as shown in Figure 1-6.
Switching is performed by a DACS (Digital Access Cross-Connect Switch) at
each network office.
FT1 is used with DTEs operating at less than the full T1 rate. Thus, a DSU is
required to convert the lower-rate signals to the T1 rate and format. A DSU may
serve a single DTE, or it may multiplex the signals from two or more DTEs. In
addition, a DSU may provide “drop-and-insert” capability for T1 DTEs. This
feature allows data from another DTE to be inserted into unused DS0s in the
output of a T1 PBX or channel bank.
Figure 1-6. Multipoint Networking Using Fractional T1
T1
T1
T1
T1
20 DS0s
Used
12 DS0s
Used
16 DS0s
Used
DSU/CSU
Site A
DSU/CSU
Site B
DSU/CSU
Site C
PBX
DTE 1
8 DS0s
DTE 3
8 DS0s
DTE 5
4 DS0s
DTE 2
12 DS0s
DTE 4
4 DS0s
DTE 6
12 DS0s
Fractional T1 Network
CO 1
(with DCS)
CO 2
(with DCS)
CO 3
(with DCS)
T1 linking CO 2 to CO 3 includes
4 time slots from Site B/Site C link.
T1 linking CO 1 to CO 3 includes
12 time slots from Site A/Site C link.
T1 linking CO 1 to CO 2 includes
8 time slots from Site A/Site B link.
ACST-0351-005
Chapter 1
June 1996
Introduction
1-15