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

2 Message, Segment, Packet, and Frame HTTP TCP IP Ethernet interface HTTP TCP IP Ethernet interface IP Ethernet interface Ethernet interface SONET interface SONET interface host router HTTP message TCP segment IP packet Ethernet frame SONET 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 Adaptors Communicating Link layer implemented in adaptor (network interface card) –Ethernet card, card, PCMCIA cardPCMCIA 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 sending node frame receiving node packet frame adapter link layer protocol

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 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

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

12 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 * ) Bits NRZ Clock Manchester *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