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2-WIRE SHORT-RANGE DSL LINE DRIVER (HDSL) WITH 10BASE-T
3. PPP Operational Background
PPP is a protocol used for multiplexed transport over a point-to-point link. PPP
operates on all full-duplex media. It’s a symmetric peer-to-peer protocol, which can
be broken into three main components:
1. A standard method to encapsulate datagrams over serial links.
2. A Link Control Protocol (LCP) to establish, configure, and test the data-link
connection.
3. A family of Network Control Protocols (NCPs) to establish and configure
different network layer protocols.
In order to establish communications over a point-to-point link, each end of the
PPP link must first announce its capabilities and agree on the parameters of the
link’s operation. This exchange is facilitated through LCP Configure-Request
packets.
Once the link has been established and optional facilities have been negotiated,
PPP will attempt to establish a network protocol. PPP will use Network Control
Protocol (NCP) to choose and configure one or more network layer protocols.
Once each of the network layer protocols have been configured, datagrams from
the established network layer protocol can be sent over the link. The link will
remain configured for these communications until explicit LCP or NCP packets
close the link down, or until some external event occurs.
The PPP Bridging Control Protocol (BCP), defined in RFC 1638, configures and
enables/disables the bridge protocol on both ends of the point-to-point link. BCP
uses the same packet exchange mechanism as the Link Control Protocol (LCP).
BCP is a Network Control Protocol of PPP; bridge packets may not be exchanged
until PPP has reached the network layer protocol phase.
Applications
In situations where a routed network requires connectivity to a remote Ethernet
network, the interface on a router can be configured as a PPP IP half-bridge. The
serial line to the remote bridge functions as a virtual Ethernet interface, effectively
extending the router’s serial port connection to the remote network. The bridge
device sends bridge packets (BPDUs) to the router’s serial interface. The router
will receive the layer three address information and will forward these packets
based on its IP address.
