Byte Oriented Protocols, Point-to-point protocol (PPP)

Byte-Oriented Framing

• Oldest approach, viewing frames as collections of bytes.
• Examples: BISYNC by IBM, DDCMP in DECNET, PPP.

Length Field Approach

• Include frame byte count in header (DDCMP approach).
• Risk: Transmission error corrupting count field; framing error.

Sentinel-based Approach/Byte Stuffing

• Use special characters (SYN, STX, ETX) to indicate frame boundaries.
• Challenge: Special characters in data; overcome by character stuffing or escape sequences similar to C.

PPP Frame Format

• Used for IP packet transmission over point-to-point links.
• Start-of-text character (Flag field: 01111110).
• Negotiable field sizes, CRC used for checksum.

LCP Protocol and Negotiation

• Link Control Protocol (LCP) negotiates field sizes in PPP.
• Controls encapsulated in PPP frames.
• Involvement in link establishment between peers.

Bit Oriented Protocols (HDLC)

Bit-Oriented Framing

• Views frames as a bit stream, not concerned with byte boundaries.
• Examples: SDLC developed by IBM, standardized as HDLC by ISO.

HDLC Frame Format

• Denotes frame start and end with bit sequence 01111110.
• Uses sentinel approach, similar to byte-oriented protocols.
• Bit stuffing employed to handle the sequence within the frame.

Bit Stuffing in HDLC

• Inserts 0 after five consecutive 1s during transmission.
• Receiver removes stuffed 0 based on the next bit.
• Distinguishes between end-of-frame marker and errors.

Frame Size Dependency

• Frame size depends on payload data; not all frames can be the same size.
• Challenges with ensuring consistent frame size discussed in the next subsection.

Clock-Based Protocols (SONET)

Clock-Based Framing in SONET

• Exemplified by Synchronous Optical Network (SONET) standard.
• Addresses framing, encoding, and multiplexing for data over optical fiber.

SONET Frame Structure

• SONET frame has special information for start and end.
• No bit stuffing used; frame length independent of data.
• Special bit pattern in STS-1 frame helps receiver locate frame boundaries.

Overhead and Payload

• SONET complexity due to overhead bytes and network-level considerations.
• Payload bytes scrambled for clock recovery.
• SONET supports multiplexing of low-speed links.

Multiplexing in SONET

• SONET links run at rates ranging from STS-1 to STS-768.
• A single SONET frame can contain subframes for multiple lower-rate channels.
• STS-N frame consists of N interleaved STS-1 frames.

Concatenation in SONET

• STS-N signal used to multiplex N STS-1 frames; payload may be concatenated.
• Denoted as STS-Nc for concatenated links.
• Simplifies clock synchronization across carriers' networks.