參數(shù)資料
型號(hào)： T7264
英文描述： T7264 U-Interface 2B1Q Transceiver
中文描述： T7264 U型接口2B1Q收發(fā)器
文件頁(yè)數(shù)： 44/54頁(yè)
文件大?。?/td> 876K
代理商： T7264
44
Lucent Technologies Inc.
Data Sheet
April 1998
T7264 U-Interface 2B1Q Transceiver
Appendix A. Questions and Answers
(continued)
Q23
: Can the range of the T7264 on the U-interface be
specified in terms of loss What is the range for
over straight 24 AWG wire
A23
: ANSI Standard T1.601, Section 5.1, states that
transceivers meeting the U-interface standard are
intended to operate over cables up to the limits of
18 kft (5.5 km) 1300
resistance design. Resis-
tance design rules specify that a loop (of single-
or mixed-gauge cable; e.g., 22 AWG, 24 AWG,
and 26 AWG) should have a maximum dc resis-
tance of 1300
, a maximum working length of
18 kft, and a maximum total bridged tap length of
6 kft.
The standard states that, in terms of loss, this is
equivalent to a maximum insertion loss of 42 dB
@ 40 kHz. Lucent has found that, for assessing
the condition of actual loops in the field in a
2B1Q system, specifying insertion loss as
33.4 dB @ 20 kHz more closely models ANSI
circuit operation. This is equivalent to a straight
26 AWG cable with 1300
dc resistance
(~15.6 kft).
The above goals are for actual loops in the out-
side loop plant. These loops may be subjected to
noise and jitter. In addition, as mentioned above,
there may be bridge taps at various points on the
loop. The T1.601 standard defines 15 loops, plus
the null, or 0 length loop, which are intended to
represent a generic cross section of the actual
loop plant.
A 2B1Q system must perform over all these loops
in the presence of impairments with an error rate
of <1e–7. Loop #1 (18 kft, where 16.5 kft is
26 AWG cable and 1.5 kft is 24 AWG cable) is the
longest, and so has the most loss (37.6 dB @
20 kHz and 47.5 dB @ 40 kHz). Note that this is
more loss than discussed in the preceding para-
graph. The difference is based on test require-
ments vs. field deployment. The test
requirements are somewhat more stringent than
the field goal in order to provide some margin
against severe impairments, complex bridged
taps, etc.
If a transceiver can operate over Loop #1 error-
free, it should have adequate range to meet all
the other loops specified in T1.601. Loop #1 has
no bridged taps, so passing Loop #1 does not
guarantee that a transceiver will successfully
start up on every loop. Also, due to the complex
nature of 2B1Q transceiver start-up algorithms,
there may be shorter loops which could cause
start-up problems if the transceiver algorithm is
not robust. The T7264 has been tested on all of
the ANSI loops per the T1.601 standard and
passes them all successfully. Two loops com-
monly used in the lab to evaluate the perfor-
mance of the T7264 silicon are as follows:
The T7264 is able to start up and operate error-
free on both of these loops. Neither of these
loops is specified in the ANSI standard, but both
are useful for evaluation purposes. The first loop
is used because it is simple to construct and easy
to emulate using a lumped parameter cable
model, and it is very similar to ANSI Loop #1, but
slightly worse. Thus, if a transceiver can start up
on this loop and operate error-free, its range will
be adequate to meet the longest ANSI loop.
The second loop is used because, due to its diffi-
cult bridge tap structure and its length, it stresses
the transceiver start-up algorithms more than any
of the ANSI-defined loops. Thus, if a transceiver
can start up on this loop, it should be able to meet
any of the ANSI-defined loops which have bridge
taps. Also, on a straight 26 AWG loop, the T7264
can successfully start up at lengths up to 21 kft.
This fact, combined with reliable start-up on the
15 kft 2BT loop above, illustrates that the T7264
provides ample start-up sensitivity, loop range,
and robustness on all ANSI loops.
Another parameter of interest is pulse height loss
(PHL). PHL can be defined as the loss in dB of
the peak of a 2B1Q pulse relative to a 0 length
loop. For an 18 kft 26 AWG loop, the PHL is
about 36 dB, which is 2 dB worse than on ANSI
Loop #1. A signal-to-noise ratio (SNR) measure-
ment can be performed on the received signal
after all the signal processing is complete (i.e., at
the input to the slicer in the decision feedback
equalizer). This is a measure of the ratio of the
recovered 2B1Q pulse height vs. the noise
remaining on the signal. The SNR must be
greater than 22 dB in order to operate with a bit
error rate of <1e–7.
Loop
Config-
uration
18 kft
26 AWG
15 kft
26 AWG
Bridge Taps
(BT)
Loss @
20 kHz
(dB)
38.7
Loss @
40 kHz
(dB)
49.5
None
2 at near end,
each 3 kft,
22 AWG
37.1
46.5
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