Data link layer framing

Framing is the data link layer function of splitting the bit stream from the network layer into discrete units called frames, so the receiver can identify where each frame starts and ends and check it for errors. Each frame has a header, data and trailer.

1. Character (byte) count

A field in the header tells the number of characters in the frame.

| 5 | a b c d | 6 | e f g h i | 4 | j k l |

Problem: if the count is corrupted, the receiver loses synchronization and cannot find the next frame. Rarely used.

2. Flag bytes with byte stuffing

Each frame starts and ends with a special FLAG byte (for example 01111110). If the flag pattern (or escape byte) appears in the data, the sender inserts an ESC byte before it. The receiver removes the ESC.

 FLAG | A FLAG B | FLAG   -->   FLAG | A ESC FLAG B | FLAG

Used in PPP. Weakness: tied to 8-bit characters.

3. Flag bits with bit stuffing

Each frame starts and ends with the flag 01111110. Whenever the sender sees five consecutive 1s in the data, it inserts a 0. The receiver deletes a 0 that follows five 1s. This allows frames of any bit length. Used in HDLC.

 data    : 0111111 0 1111110
 stuffed : 01111101 0 11111010

4. Physical layer coding violations

Uses signal patterns not used for data, such as high-high or low-low in Manchester encoding, to mark frame boundaries. Used in LANs; wastes no bits, but only works where redundancy exists in the encoding.

In practice many protocols combine character count and another method for safety.