PPP

PPP is a protocol that is able to:

handle authentication,
compression,
error detection,
monitor link quality,
logically bundles multiple serial connections together to share the load

1. Serial Point to point Overview

There are three important serial communication standards affecting LAN-to-WAN connections:

RS-232 - Most serial ports on personal computers conform to the RS-232C or newer RS-422 and RS-423 standards. Both 9-pin and 25-pin connectors are used. A serial port is a general-purpose interface that can be used for almost any type of device, including modems, mice, and printers. These types of peripheral devices for computers have been replaced by new and faster standards such as USB but many network devices use RJ-45 connectors that conform to the original RS-232 standard.

V.35 - Typically used for modem-to-multiplexer communication, this ITU standard for high-speed, synchronous data exchange combines the bandwidth of several telephone circuits. In the U.S., V.35 is the interface standard used by most routers and DSUs that connect to T1 carriers. V.35 cables are high-speed serial assemblies designed to support higher data rates and connectivity between DTEs and DCEs over digital lines. There is more on DTEs and DCEs later in this section.

HSSI - A High-Speed Serial Interface (HSSI) supports transmission rates up to 52 Mb/s. Engineers use HSSI to connect routers on LANs with WANs over high-speed lines, such as T3 lines. Engineers also use HSSI to provide high-speed connectivity between LANs, using Token Ring or Ethernet. HSSI is a DTE/DCE interface developed by Cisco Systems and T3 plus Networking to address the need for high-speed communication over WAN links.

Point-to-point links are usually more expensive than shared services.

Multiplexing refers to a scheme that allows multiple logical signals to share a single physical channel. Two common types of multiplexing are time-division multiplexing (TDM) and statistical time-division multiplexing (STDM).

TDM

TDM divides the bandwidth of a single link into separate time slots. TDM transmits two or more channels (data stream) over the same link by allocating a different time slot for the transmission of each channel. In effect, the channels take turns using the link.

STDM

STDM uses a variable time slot length allowing channels to compete for any free slot space. It employs a buffer memory that temporarily stores the data during periods of peak traffic. STDM does not waste high-speed line time with inactive channels using this scheme. STDM requires each transmission to carry identification information or a channel identifier.

2. HDLC Encapsulation

The following are short descriptions of each type of WAN protocol:

HDLC - The default encapsulation type on point-to-point connections, dedicated links, and circuit-switched connections when the link uses two Cisco devices. HDLC is now the basis for synchronous PPP used by many servers to connect to a WAN, most commonly the Internet.

PPP - Provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. PPP works with several network layer protocols, such as IPv4 and IPv6. PPP uses the HDLC encapsulation protocol, but also has built-in security mechanisms such as PAP and CHAP.

Serial Line Internet Protocol (SLIP) - A standard protocol for point-to-point serial connections using TCP/IP. SLIP has been largely displaced by PPP.

X.25/Link Access Procedure, Balanced (LAPB) - An ITU-T standard that defines how connections between a DTE and DCE are maintained for remote terminal access and computer communications in public data networks. X.25 specifies LAPB, a data link layer protocol. X.25 is a predecessor to Frame Relay.

Frame Relay - An industry standard, switched, data link layer protocol that handles multiple virtual circuits. Frame Relay is a next generation protocol after X.25. Frame Relay eliminates some of the time-consuming processes (such as error correction and flow control) employed in X.25.

ATM - The international standard for cell relay in which devices send multiple service types, such as voice, video, or data, in fixed-length (53-byte) cells. Fixed-length cells allow processing to occur in hardware; thereby, reducing transit delays. ATM takes advantage of high-speed transmission media such as E3, SONET, and T3.

The current standard for HDLC is ISO 13239.

HDLC defines three types of frames, each with a different control field format.

Flag

The flag field initiates and terminates error checking. The frame always starts and ends with an 8-bit flag field. The bit pattern is 01111110. Because there is a likelihood that this pattern occurs in the actual data, the sending HDLC system always inserts a 0 bit after every five consecutive 1s in the data field, so in practice the flag sequence can only occur at the frame ends. The receiving system strips out the inserted bits. When frames are transmitted consecutively, the end flag of the first frame is used as the start flag of the next frame.

Address

The address field contains the HDLC address of the secondary station. This address can contain a specific address, a group address, or a broadcast address. A primary address is either a communication source or a destination, which eliminates the need to include the address of the primary.

Control

The control field uses three different formats, depending on the type of HDLC frame used:

Information (I) Frame - I-frames carry upper layer information and some control information. This frame sends and receives sequence numbers, and the poll final (P/F) bit performs flow and error control. The send sequence number refers to the number of the frame to be sent next. The receive sequence number provides the number of the frame to be received next. Both sender and receiver maintain send and receive sequence numbers. A primary station uses the P/F bit to tell the secondary whether it requires an immediate response. A secondary station uses the P/F bit to tell the primary whether the current frame is the last in its current response.

Supervisory (S) Frame - S-frames provide control information. An S-frame can request and suspend transmission, report on status, and acknowledge receipt of I-frames. S-frames do not have an information field.

Unnumbered (U) Frame - U-frames support control purposes and are not sequenced. Depending on the function of the U-frame, its control field is 1 or 2 bytes. Some U-frames have an information field.

Protocol

Only used in Cisco HDLC. This field specifies the protocol type encapsulated within the frame (e.g. 0x0800 for IP).

Data

The data field contains a Path Information Unit (PIU) or Exchange Identification (XID) information.

Frame Check Sequence (FCS)

The FCS precedes the ending flag delimiter and is usually a Cyclic Redundancy Check (CRC) calculation remainder. The CRC calculation is redone in the receiver. If the result differs from the value in the original frame, an error is assumed.

3. PPP Operation

PPP contains three main components:

HDLC-like framing for transporting multiprotocol packets over point-to-point links.

Extensible Link Control Protocol (LCP) for establishing, configuring, and testing 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 (IPv4) Control Protocol, IPv6 Control Protocol, AppleTalk Control Protocol, Novell IPX Control Protocol, Cisco Systems Control Protocol, SNA Control Protocol, and Compression Control Protocol.

There are many advantages to using PPP including the fact that it is not proprietary. PPP includes many features not available in HDLC:

The link quality management feature, as shown in the figure, monitors the quality of the link. If too many errors are detected, PPP takes the link down.
PPP supports PAP and CHAP authentication.

3.1 PPP Layer Architecture: Physical Layer