Announcements
• Prelim and submission for 3rd CS 415 project
• Homework 4 graded
1
The Link Layer
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)
3
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)
4
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
5
Encoding
• Map 1s and 0s to electric signals
• Simple scheme: Non-Return to Zero (NRZ)
– 0 = low voltage, 1 = high voltage
1
0
1
1
0
• 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
6
Manchester Encoding
• Used by Ethernet
• Idea: Map 0 to low-to-high transition, 1 to high-to-low
0
1
1
0
• 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?
7
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
8
Example: HDLC
• Same sentinel for begin and end: 0111 1110
• packet format:
0111 1110
header
data
CRC
0111 1110
• Bit stuffing
– Sender: If 5 1s then insert a 0
0111 1110
0111 1101 0
– Receiver: if 5 1s followed by a 0, remove 0
0111 1101 0
0111 1110
• Else read next bit
• Packet size now depends on the contents
9
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
10
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
11
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
12
CSMA
Wait
until
carrier
free
13
CSMA/CA
Garbled signals
If the sender detects a collision, it will stop and then retry!
What is the problem?
14
CSMA/CD
Packet?
No
Sense
Carrier
Send
Detect
Collision
Yes
Discard
Packet
attempts < 16
Jam channel
b=CalcBackoff();
wait(b);
attempts++;
attempts == 16
15
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}
16
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
17
Ethernet Problems
• What if there is a malicious user?
– Might not use exponential backoff
– Might listen promiscuously to packets
• Integrating Fast and Gigabit Ethernet
18
Addressing & ARP
128.84.96.89
128.84.96.90
128.84.96.91
“What is the physical
address of the host
named 128.84.96.89”
“I’m at 1a:34:2c:9a:de:cc”
• ARP is used to discover physical addresses
• ARP = Address Resolution Protocol
19
Addressing & RARP
???
128.84.96.90
RARP Server
128.84.96.91
“I just got here. My
physical address is
1a:34:2c:9a:de:cc.
What’s my name ?”
“Your name is
128.84.96.89”
• RARP is used to discover virtual addresses
• RARP = Reverse Address Resolution Protocol
20
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
21
Backbone Bridge
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