1. Chapter 7 Packets, Frames, and Error Detection
Packet
small block of data eg. Ethernet limits to 1500 bytes of data per data packet ( does not include 18 bytes frame header and 8 byte preamble)
Why packets?
minimize retransmission
fair access to shared medium via TDM (fig 7.2)
minimize delay

2. Frames
Frame format
definition of a packet used with specific type of network eg Ethernet frame vs. Token Ring frame
Frames begin with a header followed by data and a trailer (fig 7.3)

3. Data stuffing
Inserting extra bits or bytes in data in order to distinguish between data being sent and control information such as frame delimiters (fig 7.4) (fig 7.5)
byte stuffing used in character-oriented hardware eg. PPP
bit stuffing used in bit-oriented hardware eg frame relay

4. Transmission errors
Interference can cause random data to appear or transmitted data to be lost or changed.
Need for error detection

5. Error Detection Techniques
Parity Check
involves use of a parity bit for each character being sent
Even/odd parity
allows detection of a change in one bit
cannot detect transmission errors that change an even number of bits.
Checksum
16-bit checksum generated by adding 16-bit data and carry bits together (fig 7.6)
can fail to detect transmission errors (fig 7.7)
Cyclic redundancy checks
Detects more errors than checksum such as burst errors.
Uses exclusive or (fig 7.8) and shift registers (fig 7.9) (fig 7.10)