XRT7250 datasheet(148/463 Pages) EXAR

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DS3/E3 FRAMER IC

XRT7250

REV. 1.1.1

129

two framing formats is in the use of the C-bits. In the

M13 Format, the C-bits reflect the status of stuff-op-

portunities that either were or were not used while

multiplexing the 7 DS2 signals into this DS3 signal. If

two of the three stuff bits, within a F-frame, are "1",

then the associated stuff bit, Si (not shown in

Figure 41), is interpreted as being a stuff bit. In the

C-bit Parity framing format, the C bits take on different

roles, as presented in Table 17.

Definition of the DS3 Frame Overhead Bits

In general, the DS3 Frame Overhead Bits serve the

following three purposes:

1. Support Frame Synchronization between the

Local and Remote DS3 Terminals

2. Provide parity bits in order to facilitate perfor-

mance monitoring and error detection.

3. Support the transmission of Alarms, Status, and

Data Link information to the Remote DS3 Termi-

nal.

The Overhead bits supporting each of these purpos-

es are further defined below.

4.1.1

Frame Synchronization Bits (Applies to

both M13 and C-bit Parity Framing Formats)

Each DS3 Frame (M-frame) contains a total of 31 bits

that support frame synchronization. Each DS3 M-

frame contains three M-bits. According to Figure 40

and Figure 41, these M-bits are the first bits in F-

frames 5, 6 and 7. These three bits appear in each

M-frame with the repeating pattern of "010". This fact

is also presented in Figure 40 and Figure 41, which

contains bit-fields that are designated as: M0, M1,

and M0 (where M0 = "0", and M1 = "1").

Each F-frame contains four F-bits, which also aid in

synchronization between the Local and the remote

DS3 terminals. Therefore, each DS3 M-frame con-

sists of a total of 28 F-bits. These F-bits exhibit a re-

peating pattern of "1001" within each F-frame. This

fact is also presented in Figure 40 and Figure 41,

which contains bit-fields that are designated as: F1,

F0, F0, and F1 (where F0 = "0", and F1 = "1").

Each of these bit-fields will be used by the Receive

DS3 Framer block, within the remote terminal equip-

ment, to perform Frame Acquisition and Frame Main-

tenance functions.

NOTE: For more information on how the Receive DS3

Framer uses these bit-fields, please see Section 3.3.2.

4.1.2

Performance Monitoring/Error Detection

Bits (Parity)

The DS3 Frame uses numerous bit fields to support

performance monitoring of the transmission link be-

tween the Local Transmitting Terminal and the Re-

mote Receiving Terminal. The DS3 frame can con-

tain two types of parity bits, depending upon the fram-

ing format chosen. P-bits are available in both the

M13 and C-bit Parity Formats. However, the C-bit

Parity format also includes additional CP-Parity bits.

P-Bits (Applies to M13 and C-Bit Parity Frame For-

mats)

Each DS3 M-frame consists of two (2) P-bits. These

two P-bits carry the parity information of the previous

DS3 frame for performance monitoring. These two P-

bits must be identical, within a given DS3 frame. The

Transmit Section will compute the even parity over all

4704 payload bits within a given DS3 frame, and in-

sert the resulting parity information in the P-bit fields

of the very next DS3 frame. The two P-bits are set to

"1" if the payload of the previous DS3 frame consists

of an odd number of "ones" in the frame. Conversely,

TABLE 17: C-BIT FUNCTIONS FOR THE C-BIT PARITY DS3 FRAME FORMAT

C - BIT

FUNCTION OF C-BITS WHILE IN THE C-BIT PARITY FRAMING FORMAT

C11

AIC (C-Bit Parity Mode)

C12

NA (Reserved for Network Application)

C13

FEAC (Far End Alarm & Control)

C21, C22, C23

(UDL) User Data Link (undefined for DS3 Frame)

C31,C32, C33

CP (Path) Parity Bits

C41, C42, C43

FEBE (Far End Block Error) Indicators

C51, C52, C53

(DL) Path Maintenance Data Link

C61, C62, C63,

C71, C72, C73

(UDL) User Data Link (undefined for DS3 Frame)

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