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Configuring PPP Services

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308639-14.00 Rev 00

Datagram Encapsulation

Before transmitting data across the link, PPP encapsulates data in a frame similar

to a high-level data link control (HDLC) frame (Figure 2-3

Figure 2-3. PPP-Encapsulated Frame

Flag fields mark the beginning and end of each frame. Peers on synchronous lines

exchange flags when there are no frames to transmit. The parts of the PPP frame

function as follows:

• The flag field marks the beginning and end of a frame. Peers on synchronous

lines exchange flags continuously when there are no frames to transmit.

• The address field indicates which device originated the frame.

• The control field shows the frame type (information or administrative).

• The protocol field indicates the operative network layer protocol.

• The data field contains the data that one link sends to another. The default

maximum length is 1594 bytes; LCP negotiations determine the actual length.

• The frame check sequence (FCS) field shows the sequence order of the frame;

router hardware computes the FCS. A 16- or 32-bit cyclic redundancy check

(CRC) is at the end of each frame.

PPP Dial Services Support

Nortel Networks dial services offer access to switched networks through dial-up

line connections (also called switched lines) that are active only when you choose

to use them. In contrast, a leased line is always available. If you transmit limited

amounts of data, or if your data transmission is intermittent, dial services may let

you run your network more effectively and economically.

PPP0003A

PPP frame

Address Control Protocol

Data

FCS

1 byte 1 byte 2 bytes Variable 2 or 4

bytes

1 2 ... 33 34 35 36 37 38 39 40 41 42 43 ... 151 152

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Avaya Configuring PPP Services Manual de usuario Pagina 38