Page 122 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Unit 8: Data Link Protocols

IP protocol, providing error detection on the link itself, allowing the hosts to negotiate options Notes
like IP address, maximum datagram size at start-up time, host authorization, etc.

8.5.1 PPP Components

The point-to-point protocol has the following components for transmitting datagrams over
serial point-to-point links:
Encapsulating Datagrams: PPP uses the High-Level Data Link Control (HDLC) protocol as a
mechanism for encapsulating datagrams over point-to-point links. The HDLC protocol defines
the boundaries around the individual PPP frames and provides a 16-bit checksum. A PPP frame
adds a protocol field to the basic HDLC frame to identify the type of packet carried by the frame
so that it could enable to hold packets from protocols other than IP, such as Novell’s IPX or
Appletalk.

Implementing LCP: An extensible link control LCP is used to set up, configure and test the data-
link connection. The LCP is implemented on top of HDLC to negotiate options pertaining to the
data link.
Implementing NCP: A family of network control protocols (NCPs) are used for setting up and
configuring different network-layer protocols like IP and AppleTalk, which are routed across
the data link. They are configured dynamically using a corresponding NCP. Before transmitting
IP datagrams across the link, both the hosts running PPP are required to negotiate the IP address
being used by each of them. The control protocol used for such negotiation is known as the
Internet protocol control protocol (IPCP).
In order to communicate over a point-to-point link, the PPP transmits LCP frames to configure
the data-link so that a connection over a point-to-point link is set up. When the link is set up, the
optional facilities are also negotiated which are essential for the LCP. Thereafter, the originating
PPP transmits NCP frames to select and configure one or more network layer protocols. This
leads to the transmission of packets from each network-layer protocol over the link. The link
remains configured unless a LCP or NCP frames requests to close the link. Sometimes some
external event also closes the link. In brief, communication over a point-to-point link is given as
below:

LCP frames from originating PPP first configure and test the data link for establishing a
connection.
Once the link is established, the originating PPP sends NCP frames for negotiating optional
facilities and configuring one or more network layer protocols according to LCP.
When each of the chosen network layer protocols has been configured, packets from each
network layer protocol can be sent over the link. The link will remain configured for
communications until explicit LCP or NCP frames close the link, or until some external
event occurs.

!
Caution PPP is capable of operating across any DTE/DCE interface like RS-232-C, RS-422,
etc. However, the PPP requires duplex circuit for operation. They may be either dedicated
or switched in asynchronous or synchronous mode.

8.5.2 PPP Frame

There are six fields making up the PPP frame. These are given below and also diagrammatically
represented in Figure 8.5.

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