1 Announcements Prelim and submission for 3 rd CS 415 project Homework 4 graded

The Link Layer

3 What is purpose of this layer? Physically encode bits on the wire Link = pipe to send information –E.g. point to point or broadcast Can be built out of: –Twisted pair, coaxial cable, optical fiber, radio waves, etc Links should only be able to send data –Could corrupt, lose, reorder, duplicate, (fail in other ways)

4 How to connect routers/machines? WAN/Router Connections –Commercial: T1 (1.5 Mbps), T3 (44 Mbps) OC1 (51 Mbps), OC3 (155 Mbps) ISDN (64 Kbps) Frame Relay (1-100 Mbps, usually 1.5 Mbps) ATM (some Gbps) –To your home: DSL Cable Local Area: –Ethernet: IEEE (10 Mbps, 100 Mbps, 1 Gbps) –Wireless: IEEE b/g/a (11 Mbps, 22 Mbps, 54 Mbps)

5 Link level Issues Encoding: map bits to analog signals Framing: Group bits into frames (packets) Arbitration: multiple senders, one resource Addressing: multiple receivers, one wire

6 Encoding Map 1s and 0s to electric signals Simple scheme: Non-Return to Zero (NRZ) –0 = low voltage, 1 = high voltage Problems: –How to tell an error? When jammed? When is bus idle? –When to sample? Clock recovery is difficult. Idea: Recover clock using encoding transitions

7 Manchester Encoding Used by Ethernet Idea: Map 0 to low-to-high transition, 1 to high-to-low Plusses: can detect dead-link, can recover clock Bad: reduce bandwidth, i.e. bit rate = ½ baud rate –If wire can do X transition per second?

8 Framing Why send packets? –Error control How do you know when to stop reading? –Sentinel approach: send start and end sequence –For example, if sentinel is – –What if sentinel appears in the data? map sentinel to something else, receiver maps it back –Bit stuffing

9 Example: HDLC Same sentinel for begin and end: packet format: Bit stuffing –Sender: If 5 1s then insert a 0 –Receiver: if 5 1s followed by a 0, remove 0 Else read next bit Packet size now depends on the contents header data CRC

10 Arbitration One medium, multiple senders –What did we do for CPU, memory, readers/writers? –New Problem: No centralized control Approaches –TDMA: Time Division Multiple Access Divide time into slots, round robin among senders If you exceed the capacity  do not admit more (busy signal) –FDMA: Frequency Division Multiple Access (AMPS) Divide spectrum into channels, give each sender a channel If no more channels available, give a busy signal –Good for continuous streams: fixed delay, constant data rate –Bad for bursty Internet traffic: idle slots

11 Ethernet Developed in 1976, Metcalfe and Boggs at Xerox Uses CSMA/CD: –Carrier Sense Multiple Access with Collision Detection Easy way to connect LANs Metcalfe’s Ethernet sketch

12 CSMA/CD Carrier Sense: –Listen before you speak Multiple Access: –Multiple hosts can access the network Collision Detection: –Can make out if someone else started speaking Older Ethernet Frame

13 CSMA Wait until carrier free

14 CSMA/CA Garbled signals If the sender detects a collision, it will stop and then retry! What is the problem?

15 CSMA/CD Packet? Sense Carrier Discard Packet Send Detect Collision Jam channel b=CalcBackoff(); wait(b); attempts++; No Yes attempts < 16 attempts == 16

16 Ethernet’s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits; Exponential Backoff: Goal: adapt retransmission attempts to estimated current load –heavy load: random wait will be longer first collision: choose K from {0,1}; delay is K x 512 bit transmission times after second collision: choose K from {0,1,2,3}… after ten or more collisions, choose K from {0,1,2,3,4,…,1023}

17 Packet Size If packets are too small, the collision goes unnoticed  Limit packet size  Limit network diameter  Use CRC to check frame integrity  truncated packets are filtered out

18 Ethernet Problems What if there is a malicious user? –Might not use exponential backoff –Might listen promiscuously to packets Integrating Fast and Gigabit Ethernet

19 Addressing & ARP ARP is used to discover physical addresses ARP = Address Resolution Protocol “What is the physical address of the host named ” “I’m at 1a:34:2c:9a:de:cc”

20 Addressing & RARP RARP is used to discover virtual addresses RARP = Reverse Address Resolution Protocol “I just got here. My physical address is 1a:34:2c:9a:de:cc. What’s my name ?” RARP Server ??? “Your name is ”

21 Repeaters and Bridges Both connect LAN segments Usually do not originate data Repeaters (Hubs): physical layer devices –forward packets on all LAN segments –Useful for increasing range –Increases contention Bridges: link layer devices –Forward packets only if meant on that segment –Isolates congestion –More expensive

22 Backbone Bridge