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Chapter 2

Protocol Attributes

Unlike Token Ring/IEEE 802.5, both frames also have Frame control

fields. The Frame control field tells the station the size of the address field
and the type of data that is being carried by the frame.

In data frames, the Frame control field is followed by the destination

field and source address field, which are six bytes long. Continuing with
the data frame, the actual Data field follows. This is the information that will
be processed by upper-layer protocols. The Data field is followed by the
Frame check sequence

, which calculates the CRC for the received frame.

Again, both frame types contain an End delimiter field. For data

frames, this field is followed by the Frame status field. Token frames end
with the end delimiter. As with Token Ring/IEEE 802.5, the frame status
indicates whether the frame was copied or stored by any stations on the ring.

Point-to-Point Protocol (PPP)

Point-to-point Protocol (PPP) is used to transfer data over serial point-to-
point links. It accomplishes this by using a Layer 2 serial encapsulation called
High-level Data Link Control (HDLC). HDLC is used for frame encapsula-
tion on synchronous serial lines. It uses a Link Control Protocol (LCP) to
manage the serial connection. Network Control Protocols (NCPs) are used
to allow PPP to use other protocols from Layer 3, thus enabling PPP to assign
IP addresses dynamically.

PPP uses the same frame structure as HDLC. Figure 2.31 gives you a pic-

ture of what the frame looks like. As always, we move from right to left.

F I G U R E 2 . 3 1

PPP packet structure

First, we have the Flag field, which uses one byte to specify the beginning

or ending of a frame. Then there is another byte that is used in the Address
field to hold a broadcast address of 11111111.

The Address field is followed by the one-byte Control field. The

Control

field requests a transmission of user data. The two-byte Protocol

field follows the Control field. This field indicates the encapsulated data's
protocol.

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