Layer 2

1
Layer 2
2
Datalink / Media Access

• This layer is responsible for allowing the
  devices to access the media.
• Creates connections between devices
• If more than one device on the physical link
• Requires some kind of address
• MAC Address, Ethernet
• DLCI, Frame Relay
• VPI/VCI, ATM
• LABEL, MPLS

3
Simplest case

• Point to Point link
• HDLC High level data link control
• The physical might be T1, FiberChannel, SONET
• Note the layer 2 data technologies CO-EXIST on
  Layer 1 technologies
• Ethernet often runs over a T1 line.
• PPP over RS232
• Etc.

4
Bit oriented (Peterson)

• HDLC, High-Level Data Link Control
• Standardized version of SDLC (IBM)
• Also uses the 0x7E control sequence to delineate
  beginning and end of frames
• If 0x7E appears in the body of the message,
  special provisions must be made via an escape
  sequence

5
Byte Oriented

• PPP, Point to Point Protocol
• Follow on from SLIP
• Simple method of placing IP in Layer2
• Byte oriented
• Protocol field determines whats in the payload.
• Byte oriented means the frame frame contains an
  integer number of bytes.
• LCP/NCP are sister protocols used for setting up
  PPP session
• Negotiate Frame size, IP address, etc.

6
PPP Frame Format (TCP/IP, Stevens)
7
Typical layer 2 requirements

• CRC Cylical Redundancy Check
• Checks to make sure there are no errors in the
  frame.
• May include FEC Forward Error Correction
• Can detect and correct effort
• Flag telling which Layer 3 protocol should
  process the frame
• Ethernet can carry several protocols
  simultaneously (IP, IPX, NetBEUI, etc)
• Sequence numbers
• So frames can be ordered and missing ones resent

8
CRC vs. FEC

• Simple parity case is similar to (CRC)
• 00,01,10,11 possible patterns to be sent
• 000, 011,101,110 actually sent (bit 3 is parity)
• If receiver sees 111, it knows theres a problem
• Error correction codes are more complicated
• When problem is seen, error can be used to
  determine proper sequence that was sent
• Will be discussed later this course

9
From Peterson and Davie If ACK not received in
time, frame is retransmitted.
10
Flow control

• Need to make sure packets are getting to where
  they are being sent.
• General concepts
• If message gets there, send another message
• If they are getting lost, try again
• If trying again and again doesnt solve the
  problem
• Give up and notify upper protocol layer
• Continuing to dump duplicate messages degrades
  the network performance dramatically.
• Most protocols time-out

11
Stop and wait flow control
Light in vacuum 300 m/?s Light in fiber
200 m/?s Electricity 250 m/?s
12
Window based flow control
Ntframe
U
2tproptframe
tframe
Data
N
tprop
2?1

1 if Ngt2?1
Ack
13
Sharing a Medium
14
Pure ALOHA

• In pure ALOHA, frames are transmitted at
  completely arbitrary times.

15
Pure ALOHA (2)

• Vulnerable period for the shaded frame.

16
Pure ALOHA (3)

• Throughput versus offered traffic for ALOHA
  systems.

Best case 18 utilization pure Aloha, 36.8 for
slotted Aloha. Slotted requires a clock source,
only transmit at frame boundaries
17
Protocols that listen before transmitting

• Station listens to network, if busy
• Wait till net available then transmit if
  collision then,
• Back off for some time then send again if channel
  available.
• If the stations always retransmits when the
  network becomes available
• CSMA 1 persistant
• If the station gradually becomes less aggressive
  about siezing the network when its busy
• CSMA nonpersistent
• If the station attempts to retransmit with some
  probability p less than 1
• CSMA p-persistent

18
Persistent and Nonpersistent CSMA

• Comparison of the channel utilization versus load
  for various random access protocols.

19
CSMA with Collision Detection

• CSMA/CD can be in one of three states
  contention, transmission, or idle.

20
Collision-Free Protocols

• The basic bit-map protocol.
• Station asserts the bit in its slot if it wants
  to transmit.
• Stations then transmit in turn.
• Reservation based, no chance of collision

21
Collision-Free Protocols (2)

• Binary countdown protocol
• More efficient than bit-map protocol
• Bits in the stations addresses determine when
  they access the channel
• Reduces overhead compared to bit-map

22
Limited-Contention Protocols

• Acquisition probability for a symmetric
  contention channel.

23
Ethernet overview
24
Channel Capacity (C)

• Bandwidth, Bit Rate, SNR, and BER related
• Channel Capacity defines relationship C
  Maximum reliable bit rate C WLog2(1 SNR)
  bps

Bandwidth sets the maximum Baud rate
25
Channel Capacity (C)

• Bandwidth, Bit Rate, SNR, and BER related
• Channel Capacity defines relationship C
  Maximum reliable bit rate C WLog2(1 SNR)
  bps

Bandwidth sets the maximum Baud rate
SNR sets the maximum number of different symbols
(the "M" in M-ary)
26
Normalized Propagation Delay

• NPD End-to-End Propagation Delay
  Average time to inject a Packet
• NPD gt 1 implies High Speed Network1 or more
  packets can simultaneously be in transit
• NPD lt 1 implies Slow Speed NetworkPacket front
  end hits far side before back end transmitted

High SpeedLow Speed
Transmitter
Receiver
27
Types of Traffic...

• Computer Data Bursty Highly sensitive to
  errors Not as time sensitive as voice or video
  
• Interactive Voice/Video Fixed Rate (if not
  compressed) Not sensitive to errors Fixed or
  Variable Rate (if compressed) Sensitive to
  errors Time Sensitive

28
IEEE 802.3 Ethernet

• Based on late 1970s technology
• Covers OSI Layers 1 2
• 10 Mbps Line Speed
• Logical Bus
• Designed to move Computer Data

29
Serial Bit Stream NRZ Coding
Logic One
Logic Zero
0
0
Coax Cable
T
volts
Called Non Return to Zero because voltage never
dwells on zero volts.
1
0
time
-1
T
30
Ethernet Uses Manchester Coding
Logic One
Logic Zero
0
0
All symbols have a transition in the middle.
volts
1
0
time
-1
T
31
Ethernet Uses Manchester Coding
volts
1
0
time
-1
T
High Pass Filters Emphasize Change
32
High Pass Filter Output
1
0
time
-1
33
Rectify (Absolute Value)
1
0
time
T
-1
Result always has pulses T seconds apart. Useful
for receiver synchronization.
34
What is CSMA/CD?

• Polite Conversation
• One node active at a time
• No deliberate interruptions
• Collisions sometimes occur after a break

35
802.3 Back-Off Algorithm

• choose random number1st Collision 0, 12nd
  Collision 0, 1, 2, 33rd Collision 0, 1, ...,
  6, 74th Collision 0, 1, ..., 14, 1510th
  Collision 0, 1, ..., 1022, 102315th
  Collision 0, 1, ..., 1022, 102316th
  Collision Punt
• Wait (Random Number.0000512) seconds

36
802.3 Flow Chart
No
Packet to Send?
Drop Packet. Notify Higher Layer
Yes
Yes
No
Set Collision Counter 0
16th Collision?
Back-Off
Yes
Traffic on Network?
Bump Collision Counter by 1
No
Yes
Send Packet
Collision?
Jam
No
37
Major Drawbacks of
CSMA/CD...

• Worst case waiting time equals infinity(No
  guaranteed Bandwidth)
• No Priorities These make Ethernet the worst LAN
  protocol to use for Multimedia Traffic

38
802.3 Packet Format
Bytes 7 1 6
6 2
46-1500 4
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Preamble
Logic One
Logic Zero
Series of pulses generated at receiver T
seconds apart in middle of each symbol.
0
0
volts
1
0
time
-1
T
40
Transmitting a File

• Broken into smaller packets
• Initial packets from Layer 5 Open Logical
  Connection
• Packets from Layer 7 Data Contains Layer 7
  traffic Data Contains Layer 3-5 info
• Packets from Layer 4 Acknowledgements
• Final packets from Layer 5 Close Logical
  Connection

41
10Base5 10Base2 (Obsolete)
Coax Cable
PC
PC
Printer
Logical Physical Bus All nodes monitor
traffic 3 Nodes share 10 Mbps
42
10BaseT Shared Hub
PC
PC
Twisted Pair
Hub
PC
PC
Logical Bus Physical Star Shared hub (OSI
Level 1) copies input bits to all outputs. All
nodes monitor traffic. 4 nodes share 10 Mbps.
43
10BaseT Switched Hub
PC
PC
Switched Hub
PC
PC
Logical Bus Physical Star Switched Hub (OSI
Level 1 2) copies packet to proper output. Only
the destination monitors traffic.
44
10BaseT Switched Hub
PC
PC
Switched Hub
PC
PC
Logical Bus Physical Star This example system
can move up to 20 Mbps
45
10BaseT Switched Hub
PC
PC
Switched Hub
PC
PC
Logical Bus Physical Star Each node shares 10
Mbps with the Switched Hub.
46
10BaseT Switched Hub
PC
PC
Switched Hub
reception is screwed up
PC
PC
Using Half Duplex 10BaseT, a collision occurs if
PC Switched Hub simultaneously transmit.
47
IEEE 802.3u 100 Mbps Fast Ethernet

• Preserves CSMA/CD
• Preserves Packet Format
• Maximum End-to-End Lengths (a.k.a. Collision
  Domain) reduced to keep Normalized Propagation
  Delay low
• Sales are pretty good

48
Full Duplex System
PC
PC
Switched Hub
PC
PC
Most 1 Gbps ( many 100 Mbps) systems are Full
Duplex. NICs are designed to simultaneously
transmit receive. Line no longer shared. No
Collisions. No need for CSMA/CD.
49
1995

• Two 100 Mbps Ethernets introduced
• Version A
• CSMA/CD MAC, Ethernet Frames
• Version B
• Demand Priority MAC, Ethernet Frames
• IEEE said Version A is Ethernet
• IEEE 802.3u Fast Ethernet
• IEEE said Version B is not Ethernet
• IEEE 802.12 100VG-AnyLAN
• 802.12 is currently Dead

50
IEEE 802.3z 1 Gbps Ethernet

• Uses an extended version of CSMA/CD, including
  Frame Bursting
• Best performance uses full duplex connections
  switched hubs
• CSMA/CD included so it can be called Ethernet
• Collision Domain same as Fast Ethernet
• Preserves Packet Format
• Good Sales
• More on GigE later

51
IEEE 802.1p Priority Tags

• 8 priorities
• MAC protocols remain unchanged
• Used by 802.1p enabled switches
• Allows interactive voice or video to receive
  preferential treatment on an Ethernet LAN

52
IEEE 802.5 Token Ring

• Based on early 1980s technology
• Covers OSI Layers 1 2
• 4 or 16 Mbps Line Speed
• Logical Ring
• A Token is passed around the ringNode must
  have the Token to transmit
• Guaranteed Bandwidth
• Has Priorities

53
802.5 Token Format
Starting Delimiter Token/Frame starts
here Access Control Indicates whether Token or
Frame, Priority Ending Delimiter End of
Token/Frame
54
802.5 Packet Format
Bytes 1 1 1 6
6
Destination Address
Source Address
SD
AC
FC
Modified Tokena.k.a Starting Frame Delimiter
gt0 4 1 1
CRC
Data
ED
FS
Frame Control Ring Status Frame Status Receiver
indicates whether received OK
55
IEEE 802.5 Token Ring

• Technically Superior to Ethernet
• 2nd most widely used LAN protocol
• Similar evolution to Ethernet
• Logical Physical Ring
• Logical Ring, Shared Physical Star
• Logical Ring, Switched Physical Star
• 100 Mbps products available since 98
• Sales sharply declining. Heading for LAN
  graveyard.

56
Layer 2 Switching

• Why??
• Bridges
• Spanning Tree Algorithm