PPP uses a layered architecture. A
layered architecture is a logical model, design, or blueprint that
aids in communication between interconnecting layers. The Open System
Interconnection (OSI) model is the layered architecture used in
networking. PPP provides a method for encapsulating multi-protocol
datagrams over a point-to-point link, and uses the data link layer for
testing the connection. Therefore PPP is made up of two sub-protocols:
- Link Control Protocol – Used for establishing the point-to-point
link.
- Network Control Protocol – Used for configuring the various
network layer protocols.
PPP can be configured on the following types of physical interfaces:
- Asynchronous serial
- Synchronous serial
- High-Speed Serial Interface (HSSI)
- Integrated Services Digital Network (ISDN)
PPP uses Link Control Protocol (LCP) to negotiate and setup
control options on the WAN data link. PPP uses the Network Control
Protocol (NCP) component to encapsulate and negotiate options for
multiple network layer protocols. The LCP sits on top of the physical
layer and is used to establish, configure, and test the data-link
connection.
PPP also uses LCP to automatically agree upon encapsulation format
options such as:
- Authentication – Authentication options require that
the calling side of the link enter information to help ensure
the caller has the network administrator's permission to make
the call. Peer routers exchange authentication messages. Two
authentication choices are Password Authentication Protocol
(PAP) and Challenge Handshake Authentication Protocol (CHAP).
- Compression – Compression options increase the effective throughput
on PPP connections by reducing the amount of data in the frame that
must travel across the link. The protocol decompresses the frame at
its destination. Two compression protocols available in Cisco routers
are Stacker and Predictor.
- Error detection – Error detection mechanisms with PPP enable a
process to identify fault conditions. The Quality and Magic Number
options help ensure a reliable, loop-free data link.
- Multilink – Cisco IOS Release 11.1 and later supports multilink
PPP. This alternative provides load balancing over the router
interfaces that PPP uses.
- PPP Callback – To further enhance security, Cisco IOS Release 11.1
offers callback over PPP. With this LCP option, a Cisco router can act
as a callback client or as a callback server. The client makes the
initial call, requests that it be called back, and terminates its
initial call. The callback router answers the initial call and makes
the return call to the client based on its configuration statements.
LCP will also do the following:
- Handle varying limits on packet size
- Detect common misconfiguration errors
- Terminate the link
- Determine when a link is functioning properly or when it is failing
PPP permits multiple network layer protocols to operate on the same
communications link. For every network layer protocol used, a separate
Network Control Protocol (NCP) is provided. For example, Internet
Protocol (IP) uses the IP Control Protocol (IPCP), and Internetwork
Packet Exchange (IPX) uses the Novell IPX Control Protocol (IPXCP). NCPs
include functional fields containing standardized codes to indicate
the network layer protocol type that PPP encapsulates.
The fields of a PPP frame are as follows:
- Flag – Indicates the beginning or end of a frame and consists of the
binary sequence 01111110.
- Address – Consists of the standard broadcast address, which is the
binary sequence 11111111. PPP does not assign individual station
addresses.
- Control – 1 byte that consists of the binary sequence 00000011,
which calls for transmission of user data in an unsequenced frame. A
connection-less link service similar to that of Logical Link Control
(LLC) Type 1 is provided.
- Protocol – 2 bytes that identify the protocol encapsulated in the
data field of the frame.
- Data – 0 or more bytes that contain the datagram for the protocol
specified in the protocol field. The end of the data field is found by
locating the closing flag sequence and allowing 2 bytes for the frame
check sequence (FCS) field. The default maximum length of the data
field is 1,500 bytes.
- FCS – Normally 16 bits or 2 bytes that refers to the extra
characters added to a frame for error control purposes.
| |
Interactive Media Activity
Drag and Drop: PPP Layered Architecture
When the student has completed this activity, the student
will understand the basic PPP layered architecture.
|
|
|
Web Links