Unit installation

Appendix C Operating Environment Installation and Operation Manual
C-2 E1 Environment FCD-E1
TimeSlot 0
Timeslot 0 is used for two main purposes:
• Delineation of frame boundaries. For this purpose, in every second frame
timeslot 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), timeslot 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 is always set to 1, a fact used by the frame alignment algorithm.
 Bit 3 is used as a remote alarm indication (RAI), to notify the equipment at
the other end that the local equipment lost frame alignment, or does not
receive an input signal.
 The other bits, identified as S
a4
through S
a8
, are designated national bits,
and are actually available to the users, provided agreement is reached as to
their use. RAD equipment with SNMP agents can use the S
a4
-S
a8
bits for
inband management traffic. The total data rate that can be carried by a
national bit, including the S
a4
bit, is 4 kbps.
Multiframes
To increase the information carrying capacity without wasting bandwidth, the
frames are organized in larger patterns, called multiframes. Two types of
multiframes are generally used:
• 256N, which consists of 2 frames (one odd frame and one even frame). The
256N multiframe is generally used when timeslot 16 is available to the user. In
this mode, the maximum number of timeslots available for payload is 31
(maximum payload data rate of 1984 kbps). For systems that use the common-
channel signaling (CCS) method, the CCS information is often transmitted in
timeslot 16.
• 256S, which consists of 16 frames. The 256S multiframe is generally used
when timeslot 16 serves for the transmission of end-to-end signaling using
channel-associated signaling (CAS). CAS is typically used on links that transfer
voice channels. In this mode, the maximum number of timeslots available for
payload is 30 (maximum data rate of 1920 kbps).
The 256S multiframe requires a special multiframe alignment sequence (MAS),
which is carried in timeslot 16 (see Figure C-1), together with the Y bit, which
indicates loss of multiframe alignment. As shown in Figure C-1, four signaling
bits, designated A, B, C, and D, are available for each channel, thereby
enabling end-to-end transmission of four signaling states. Each frame in the
multiframe carries the signaling information of two channels.