Introducing PPP


What is PPP?
Recall that HDLC is the default serial encapsulation method when you connect two Cisco routers. With an added protocol type field, the Cisco version of HDLC is proprietary. Thus, Cisco HDLC can only work with other Cisco devices. However, when you need to connect to a non-Cisco router, you should use PPP encapsulation.
PPP encapsulation has been carefully designed to retain compatibility with most commonly used supporting hardware. PPP encapsulates data frames for transmission over Layer 2 physical links. PPP establishes a direct connection using serial cables, phone lines, trunk lines, cellular telephones, specialized radio links, or fiber-optic links. There are many advantages to using PPP, including the fact that it is not proprietary. Moreover, it includes many features not available in HDLC:
The link quality management feature monitors the quality of the link. If too many errors are detected, PPP takes the link down.
PPP supports PAP and CHAP authentication. This feature is explained and practiced in a later section.
PPP contains three main components:
HDLC protocol for encapsulating datagrams over point-to-point links.
Extensible Link Control Protocol (LCP) to establish, configure, and test the data link connection.
Family of Network Control Protocols (NCPs) for establishing and configuring different network layer protocols. PPP allows the simultaneous use of multiple network layer protocols. Some of the more common NCPs are Internet Protocol Control Protocol, Appletalk Control Protocol, Novell IPX Control Protocol, Cisco Systems Control Protocol, SNA Control Protocol, and Compression Control Protocol.
PPP Architecture
A layered architecture is a logical model, design, or blueprint that aids in communication between interconnecting layers. The figure maps the layered architecture of PPP against the Open System Interconnection (OSI) model. PPP and OSI share the same physical layer, but PPP distributes the functions of LCP and NCP differently.
At the physical layer, you can configure PPP on a range of interfaces, including:
Asynchronous serial
Synchronous serial
HSSI
ISDN
PPP operates across any DTE/DCE interface (RS-232-C, RS-422, RS-423, or V.35). The only absolute requirement imposed by PPP is a duplex circuit, either dedicated or switched, that can operate in either an asynchronous or synchronous bit-serial mode, transparent to PPP link layer frames. PPP does not impose any restrictions regarding transmission rate other than those imposed by the particular DTE/DCE interface in use.
Most of the work done by PPP is at the data link and network layers by the LCP and NCPs. The LCP sets up the PPP connection and its parameters, the NCPs handle higher layer protocol configurations, and the LCP terminates the PPP connection.
PPP Architecture - Link Control Protocol Layer
The LCP is the real working part of PPP. The LCP sits on top of the physical layer and has a role in establishing, configuring, and testing the data-link connection. The LCP establishes the point-to-point link. The LCP also negotiates and sets up control options on the WAN data link, which are handled by the NCPs.
The LCP provides automatic configuration of the interfaces at each end, including:
Handling varying limits on packet size
Detecting common misconfiguration errors
Terminating the link
Determining when a link is functioning properly or when it is failing
PPP also uses the LCP to agree automatically on encapsulation formats (authentication, compression, error detection) as soon as the link is established.
PPP Architecture - Network Control Protocol Layer
Point-to-point links tend to worsen many problems with the current family of network protocols. For instance, assignment and management of IP addresses, which is a problem even in LAN environments, is especially difficult over circuit-switched point-to-point links (such as dialup modem servers). PPP addresses these issues using NCPs.
PPP permits multiple network layer protocols to operate on the same communications link. For every network layer protocol used, PPP uses a separate NCP. For example, IP uses the IP Control Protocol (IPCP), and IPX uses the Novell IPX Control Protocol (IPXCP).

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