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Announcements

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

The Link Layer

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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)

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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 802.3 (10 Mbps, 100 Mbps, 1 Gbps)– Wireless: IEEE 802.11 b/g/a (11 Mbps, 22 Mbps, 54 Mbps)

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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

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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

1 0 1 1 0

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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?

0 1 1 0

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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 11111– 11111 00101001111100 11111 10101001 11111 010011 11111

– What if sentinel appears in the data?• map sentinel to something else, receiver maps it back

– Bit stuffing

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Example: HDLC

• Same sentinel for begin and end: 0111 1110• 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

0111 1110 header data CRC 0111 1110

0111 1110 0111 1101 0

0111 1101 0 0111 1110

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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

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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

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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

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CSMA

Wait until

carrier free

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CSMA/CA

Garbled signals

If the sender detects a collision, it will stop and then retry!

What is the problem?

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CSMA/CD

Packet?

Sense Carrier

Discard Packet

Send Detect Collision

Jam channel b=CalcBackoff();

wait(b);attempts++;

No

Yes

attempts < 16

attempts == 16

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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}

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Packet Size

If packets are too small, the collision goes unnoticedÞ Limit packet sizeÞ Limit network diameterÞ Use CRC to check frame integrityÞ truncated packets are filtered out

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Ethernet Problems

• What if there is a malicious user?– Might not use exponential backoff– Might listen promiscuously to packets

• Integrating Fast and Gigabit Ethernet

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Addressing & ARP

• ARP is used to discover physical addresses• ARP = Address Resolution Protocol

“What is the physical address of the host named 128.84.96.89”

128.84.96.90128.84.96.89

128.84.96.91

“I’m at 1a:34:2c:9a:de:cc”

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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 ?”

128.84.96.90RARP Server

???

128.84.96.91

“Your name is 128.84.96.89”

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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

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Backbone Bridge