546
NETWORK MANAGEMENT
errored seconds, and severely errored seconds. One can "look" at each link and examine
its performance in 15-minute windows for a 24-hour period. The Newbridge equipment
can also monitor selected other data services such as frame relay, which, in our example
above, rides on E1 aggregates. Such equipment can be a most important element in the
network management suite.
The E1 digital network hierarchy also provides capability of in-service monitoring and
test. We remember from Chapter 8 that E1 has 32 channels or time slots: 30 are used for
the payload and 2 channels or time slots serve as support channels. The first of these is
channel (or time slot) 0, and the second is channel (time slot) 16. This latter is used for
signaling. Time slot (channel) 0 is used from synchronization and framing. Figure 21.2
shows the E1 multiframe structure.
The sequence of bits in the frame alignment (TS0) signal of successive frames is
illustrated in Figure 21.2. In frames not containing the frame alignment signal, the first
bit is used to transmit the CRC multiframe signal (001011) that defines the start of the
sub-multiframe (SMF). Alternate frames contain the frame alignment word (0011011)
preceded by one of the CRC-4 bits. The CRC-4 remainder is calculated on all the 2048
bits of the previous sub-multiframe (SMF), and the 4-bit word is sent as C1, C2, C3, C4
of the current SMF. Note that the CRC-4 bits of the previous SMF are set to zero before
the calculation is made.
At the receive end, the CRC remainder is recalculated for each SMF, and the result is
compared with the CRC-4 bits received in the next SMF. If they differ, then the checked
SMF is in error. What this is telling us is that a block of 2048 bits had one or more errors.
One thousand CRC-4 block error checks are made every second. It should be noted that
this in-service error detection scheme does not indicate BER unless one assumes a certain
error distribution (random or burst errors), to predict the average errors per block. Rather,
it provides a block error measurement.
This is very useful for estimating percentage of errored seconds (%ES) that is usually
considered the best indication of quality for data transmission--itself a block or frame
transmission process. CRC-4 error checking is fairly reliable with the ability of detecting
94% of errored blocks even under poor BER conditions (see ITU-T Rec. G.706, Ref. 5).
Sub-multiframe
(SMF)
Frame number
Multiframe
Bits 1 to 8 of the frame in timeslot 0
2
1
3
5
7
8
6
4
I
II
C
1
0
0
1
1
0
1
1
1
A
0
S
a4
S
a5
S
a6
S
a7
S
a8
0
0
1
1
0
1
1
C
2
1
A
S
a4
S
a5
S
a6
S
a7
S
a8
0
0
0
1
1
0
1
1
C
3
1
A
S
a4
S
a5
S
a6
S
a7
S
a8
1
0
0
1
1
0
1
1
C
4
1
A
S
a4
S
a5
S
a6
S
a7
S
a8
0
C
1
0
0
1
1
0
1
1
1
A
1
S
a4
S
a5
S
a6
S
a7
S
a8
0
0
1
1
0
1
1
C
2
1
A
S
a4
S
a5
S
a6
S
a7
S
a8
1
0
0
1
1
0
1
1
C
3
1
A
S
a4
S
a5
S
a6
S
a7
S
a8
E
0
0
1
1
0
1
1
C
4
1
A
S
a4
S
a5
S
a6
S
a7
S
a8
E
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Figure 21.2
E1 multiframe, CRC-4 structure. Notes: E
= CRC-4 error indication bits; S
a4
to S
a8
are
spare bits. These bits may be used for a network management (maintenance) link. C
1
to C
4
are CRC-4
bits. A stands for remote alarm indication. (From Table 4b/G.704, p. 81, CCITT Rec. G.704, Fascicle III.4,
IXth Plenary Assembly, Melbourne, 1988.)