RAD Data comm HCD-E1 - Manual (Page 35)

HCD-E1 Installation & Operation Manual Chapter 1 - Introduction
01/01/01 08:07 Functional Description 1-17
Time Slot 0 Time Slot 16 Time Slots 1-15, 17-31
FAS MAS
a. Even Frames (0,2,4-14)
b. ODD Frames (1,3,5-15)
a. Frame 0
b. Frames 1-15
Channel Data
1 0 0 1 1 0 1 1
I 1 A N N N N N
0 0 0 0 X Y X X
A B C D A B C D 1 2 3 4 5 6 7 8
32 Time Slots/Frame
8 Bits/
Time Slot
16 Frames/Multiframe
TS
0
TS
1
TS
2
TS
3
TS
4
TS
5
TS
6
TS
7
TS
8
TS
9
TS
10
TS
11
TS
12
TS
13
TS
14
TS
15 TS
16 TS
17 TS
18 TS
19 TS
20 TS
21 TS
22 TS
23 TS
24
TS
25
TS
26
TS
27
TS
28
TS
29
TS
30
TS
31
FR
0
FR
1
FR
2
FR
3
FR
4
FR
5
FR
6
FR
7
FR
8
FR
9
FR
10
FR
11
FR
12
FR
13
FR
14
FR
15
Notes
ABCD
X
Y
MAS I
N
A
FAS International Bit
National Bit
Alarm Indication Signal (Loss of Frame Alignment - Red Alarm)
Frame Alignment Signal, Occupies alternate
(but not necessarily even) frames ABCD Signaling Bits
Extra Bit
Loss of Multiframe Alignment
Multiframe Alignment Signal
Figure 1-9 E1 (CEPT) Frame Format
The 256 bits consist of 32 time slots of eight bits each, that carry the data
payload. The frame repetition rate is 8,000 per second, and therefore the
data rate supported by each time slot is 64 kbps. The number of time slots
available for user data is maximum 31, because time slot 0 is always used for
frame synchronization.
Time Slot 0
Time slot 0 is used for two main purposes:
•
Delineation of frame boundaries. For this purpose, in every second frame
time slot 0 carries a fixed pattern, called Frame Alignment Signal (FAS).
Frames carrying the FAS are defined as even frames, as they are assigned
number 0, 2, 4, etc. when larger structures (multiframes) are used.
The receiving equipment searches for this fixed pattern in the data stream
using a special algorithm, a process called frame synchronization. Once this
process is successfully completed, the equipment can identify each bit in the
received frames.
•
Transmission of housekeeping information. In every frame without FAS
(odd frames), time slot 0 carries housekeeping information. This
information includes:
− Bit 1 - this bit is called the international (I) bit. Its main use is for error
detection using the optional CRC-4 function.
− Bit 2 - this bit is always set to 1, a fact used by the frame alignment
algorithm.