Encoding: Section 2.2 (Section 2.1 read on your own)

1. Point-to-Point Links: Encoding Encoding: Section 2.2 (Section 2.1 read on your own)

2. IP IP IP Ethernet interface Message, Segment, Packet, and Frame host host HTTPmessage HTTP HTTP TCP segment TCP TCP router router IP packet IP packet IPpacket IP Ethernet interface SONET interface Ethernet interface Ethernet interface SONET interface Ethernetframe SONET frame Ethernet frame

3. Link Layer Protocol for Each Hop • IP packet transferred over multiple hops • Each hop has a link layer protocol • May be different on different hops • Analogy: trip from Amman to Salzburg (Austria) • Car: Home to Airport (QAIA) • Plane: QAIA to Vienna • Train: Vienna to Salzburg

4. packet link layer protocol frame frame adapter receiving node adapter sending node Adaptors Communicating • Link layer implemented in adaptor (network interface card) • Ethernet card, 802.11 card, PCMCIA card • Sending side: • Encapsulates packet in a frame • Adds error checking bits, flow control, etc. • Encodes bits into signals • Receiving side • Decodes signals into bits • Looks for errors, flow control, etc. • Extracts packet and passes to receiving node

5. Link-Layer Services • Encoding • Framing • Using Media Access Control (MAC) addresses, rather than IP addresses • Error detection • Errors caused by signal attenuation, noise. • Receiver detecting presence of errors • Error correction • Receiver correcting errors without retransmission • Flow control • Pacing between adjacent sending and receiving nodes

6. Encoding • Goal: • Connect nodes in such a way that bits can be transmitted in signals from one node to another • Idea: The physical medium is used to propagate signals • Source node encodes the bits into a signal • Modulate electromagnetic/light waves • Vary voltage, frequency, power • Receiving node decodes the signal back into bits

7. Signals and bits Signals travel between signaling components; bits flow between adaptors.

8. Voltage Encoding • Common binary voltage encodings • Non-return to zero (NRZ) • NRZ inverted (NRZI) • Manchester (used by IEEE 802.3—10 Mbps Ethernet) • 4B/5B

9. 0 0 1 1 0 0 1 1 0 0 0 1 1 1 1 1 0 0 Non-return to zero (NRZ) Encoding • Simplify some electrical engineering details • Assume two discrete signals, high and low • E.g., could correspond to two different voltages • Simple approach • High for a 1, low for a 0

10. Problem With Simple Approach • Long strings of 0s or 1s introduce problems • No transitions from low-to-high, or high-to-low • Baseline Wander problem • Receiver keeps average of signal it has received • Uses the average to distinguish between high and low • Long flat strings make receiver sensitive to small change • Clock Drift problem • Transitions necessary for clock synchronization (recovery) at receiver • Receiver uses transitions to drive its own clock • Long flat strings do not produce any transitions to synchronize sender/receiver clocks

11. Bits 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0 NRZ NRZI Non-Return to Zero Inverted (NRZI) • Signal to Data • Transition  1 • Maintain  0 • Comments • Strings of 0’s still a problem

12. Bits 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0 NRZ Clock Manchester Manchester Encoding • Signal to Data • XOR NRZ data with clock • High to low transition  1 • Low to high transition  0 • Comments • Solves clock recovery problem • Only 50% efficient (bit rate = 1/2 baud rate* ) *baud rate= rate at which the signal changes

13. 4B/5B • Signal to Data • Encode every 4 consecutive bits as a 5 bit symbol • Symbols • At most 1 leading 0 • At most 2 trailing 0s • Never more than 3 consecutive 0s • Transmit with NRZI • Comments • 80% efficient

14. Table 2.4 4B/5B encoding • 4-Bit Data Symbol 5-Bit Code • 0000 11110 • 0001 01001 • 0010 10100 • 0011 10101 • 0100 01010 • 0101 01011 • 0110 01110 • 0111 01111 • 1000 10010 • 1001 10011 • 1010 10110 • 1011 10111 • 1100 11010 • 1101 11011 • 1110 11100 • 1111 11101