Page 123 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Computer Networks/Networks

Notes Flag: It is consisted of a single byte that indicates the beginning or end of a frame.
Address: Address comprises of a single byte that contains the binary sequence. PPP does
not assign individual station addresses.

Control: It makes up of a single byte that contains the binary sequence, which calls for
transmission of user data. It is a connectionless link service similar to that of Logical Link
Control (LLC).
Protocol: It is consisted of two bytes that identify the protocol encapsulated in the
information field of the frame.

Data: Data may range from zero or more bytes containing the datagram for the protocol
specified in the protocol field. The default maximum length of the information field is
1,500 bytes. Closing flag sequence indicates the end of the information field with 2 bytes
for the Frame Check Sequence (FCS) field.

Frame Check Sequence (FCS): It has 2 bytes. In some case, it may also use 4-byte FCS for
improved error detection but with prior agreement.

Figure 8.5: PPP Frame

1 byte 1 byte 1 byte 2 byte Variable 2 or 4 byte

Flag Address Control Protocol Data FCS

PPP Link-Control Protocol (PPP LCP)

The protocols that differentiate PPP from HDLC are the Link Control Protocol (LCP) and the
Network Control Protocol (NCP). LCP establishes, configures, maintains, and terminates point-
to-point links. As we know by now that PPP LCPLCP provides a method of establishing,
configuring, maintaining, and terminating the point-to-point connection for which it uses four
steps. These are link establishment and configuration negotiation. A configuration-
acknowledgment frame completes this step. Thereafter the quality of link is tested as to whether
the link quality is sufficient to bring up network layer protocols. This phase is optional. After
testing the link quality, network layer protocols can be configured separately by the appropriate
NCP and can be brought up and taken down at any time. If LCP closes the link, it informs the
network layer protocols so that they can take appropriate action. Finally, link termination takes
place and LCP can terminate the link at any time. This usually is done at the request of a user but
can happen because of a physical event, such as the loss of carrier or the expiration of an
idle-period timer.

Did u know? Three classes of LCP frames exist. Link-establishment frames are used to
establish and configure a link. Link-termination frames are used to terminate a link, and
link-maintenance frames are used to manage and debug a link.

Point-to-Point Protocol Network Control Protocols (PPP NCP)

The Network Control Protocol (NCP) phase in the PPP link connection process is used for
establishing and configuring different network-layer protocols such as IP, IPX or AppleTalk.
Once NCP has reached, PPP will carry the corresponding network-layer protocol packets. During
this phase, link traffic consists of any possible combination of LCP, NCP, and network-layer

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