563489578934

Sec. 8–3 Digital Subscriber Lines (DSL) 581
DSL
160 kb/s
(2B1Q line code)
T1 or fiberoptic
lines
To CO
Remote
terminal
U interface
(2 wire)
To POTS
subscriber
ASL
DSL
DSL
U interface
(2 wire)
T interface (4 wire)
NT 1 NT 2
Receive bus
192 kb
(bipolar line code)
S interface (4 wire)
Transmit bus
192 kb/s
(bipolar line code)
Terminal
adaptor
(TA)
Video
Terminal
(TE 1)
R interface
Multiple
RS-232
lines to
other
equipment
Owned by
telephone company
Figure 8–6
Owned by
subscriber
Telephone
Terminal
(TE 2)
100 Ω 100 Ω
S bus terminator resistors
N-ISDN system with subscriber equipment attached.
For N-ISDN service, the line must be no longer than 18 kft (3.4 miles) for the 160-kbs
N-ISDN aggregate data rate. (If the subscriber is located within 18 kft of the CO, an RT
is not necessary.) The data rate available to the N-ISDN subscriber is 144 kbs, consisting
of data partitioned into two B channels of 64 kbs each and one D channel of 16 kbs.
In addition to the 2B +D data, the telephone company adds 12 kbs for framingtiming
plus 4 kbs for overhead to support network operations. This gives an overall data rate of
160 kbs on the DSL in both the transmit and receive directions (simultaneously) for fullduplex
operation. The DSL is terminated on the subscriber’s end at the U interface as
shown on Fig. 8–6.
N-ISDN service to the subscriber via two-wire twisted-pair DSL up to 18 kft from
an RT is made possible by the use of multilevel signaling. Referring to Fig. 5–33 with
R = 160 kbs, a four-level (i.e., = 2 bits) 80-kbaud line code has a null bandwidth of
only 80 kHz instead of the 160-kHz null bandwidth for a binary ( = 1) line code.
The 80-kHz bandwidth is supported by 26-gauge twisted-pair cable if it is less than about
18 kft in length. The particular four-level signal used is the 2B1Q line code (for two binary
digits encoded into one quadrenary symbol) shown in Fig. 8–7. Note that the 2B1Q line
code is a differential symbol code. Thus, if (due to wiring error) the twisted pair is “turned
over” so that the tip wire is connected to the ring terminal and the ring to the tip so that the
2BIQ signal polarity is inverted, the decoded binary data will still have the correct polarity
(i.e., not complemented).