XRT7250 datasheet(217/463 Pages) EXAR

217 / 463 page

XRT7250

DS3/E3 FRAMER IC

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REV. 1.1.1

198

The Source Terminal Equipment (located at one edge

of the wide-area network) will typically receive its DS3

payload data from some Customer Premise Equip-

ment (CPE). As the Source Terminal Equipment re-

ceives this data from the CPE, it will compute the

even-parity value over all bits within a given outbound

DS3 frame. The Terminal Equipment will then insert

this even parity value into both of the P-bit fields and

both of the CP-bits fields, within the very next out-

bound DS3 frame.

Hence, both the P-bit values and CP-bit values will

originate at the Source Terminal Equipment.

Processing at the Mid-Network Terminal Equip-

ment

The Mid-Network Terminal Equipment has the task of

doing the following.

• Receiving a DS3 data stream, via the Receive

WAN Interface Line Card.

• Transmitting this same DS3 data stream (out to

another Remote Terminal Equipment) via the

Transmit WAN Interface Line Card.

Figure 83 presents an illustration of the basic archi-

tecture of the Mid-Network Terminal Equipment.

Operation of the Receive WAN Interface Line Card

The Receive WAN Interface line card receives a DS3

data stream from some remote Terminal Equipment.

As the Receive WAN Interface card does this, it will

also do the following:

1. Compute and verify the “P-Bits” of each inbound

DS3 frame.

2. Compute and verify the “CP-Bits” of each

inbound DS3 frame.

3. Output both the payload and overhead bits to the

system back-plane.

Operation of the Transmit WAN Interface Line

Card

The Transmit WAN Interface Line Card receives the

outbound DS3 data stream from the system back-

plane. As the Transmit WAN Interface Line Card re-

ceives this data it will also do the following.

1. Extract out the “CP-bit” values, from the Receive

WAN Interface line card (via the system back-

plane) and insert these values into the CP-bit

fields, within the outbound DS3 data stream, via

the Transmit Overhead Data Input Interface block

of the XRT7250 Framer IC.

FIGURE 83. ILLUSTRATION OF THE PRESUMED CONFIGURATION OF THE MID-NETWORK TERMINAL EQUIPMENT

The Receiving

DS3 Line Card

The Receiving

DS3 Line Card

The Transmitting

DS3 Line Card

The Transmitting

DS3 Line Card

System Back-plane

DS3 Traffic

from “Source”

Terminal

Equipment

DS3 Traffic to

“Sink” Terminal

Equipment

The Mid-Network Terminal Equipment

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