CS 352 Internet Technology Media Access Protocols

1
CS 352Internet TechnologyMedia Access Protocols

• Richard Martin
• Dept. of Computer Science
• Rutgers University

2
Medium Access Sublayer
3
What is Media
Any resource which requires the parties exclusive
access in order to communicate
Shared Wire
Radio
Ring
4
Medium Access Sub layer
Network Layer
Medium Access Sublayer
Data Link Layer
Physical Layer
5
Medium Access Sublayer (contd)

• Medium access (MAC) sub-layer is not important on
  point-to-point links
• The MAC sublayer is only used in broadcast or
  shared channel networks
• Examples Satellite, Ethernet, Cellular

6
Contents

• Error Detection and Correction
• Fixed Assignment Protocols
• Demand Assignment Protocols
• Contention Access Protocols
• IEEE 802 LANs
• Token Ring protocols

7
Error Detection and Correction

• Used on many link types
• Can be used anytime you dont trust the media
• Caches and memories
• On disks (e.g. Google)
• Major Algorithmic Strategies
• Parity
• 1 and 2 dimensional
• Hamming codes (Interleaved Parity Codes)
• Checksums
• Cyclic Redundancy Codes (CRC)

8
Parity Codes
Two Dimensional Bit Parity Detect and correct
single bit errors
Single Bit Parity Detect single bit errors
0
0
9
Hamming Codes

• Want to be able to correct error with less
  overhead than 2D parity
• Hamming codes correct all single bit errors with
  only log(M) extra bits and detect double bit
  errors
• Uses an interleaved parity scheme

10
Calculating a Hamming Code

• Procedure
• Place message bits in their non-power-of-two
  Hamming positions
• Build a table listing the binary representation
  each each of the message bit positions
• Calculate the check bits

11
Hamming Code Example
1 0 1 1
Message to be sent
1 0 1 1 1 2 3 4 5 6
7 20 21 22
Position
2n check bits
12
Hamming Code Example
1 0 1 1
Message to be sent
1 0 1 1 1 2 3 4 5 6
7 20 21 22
Position
2n check bits
Calculate check bits
3 21 20 0 1 1 5 22 20
1 0 1 6 22 21 1 1 0 7
22 21 20 1 1 1
13
Hamming Code Example
1 0 1 1
Message to be sent
1 0 1 1 1 2 3 4 5 6
7 20 21 22
1
Starting with the 20 position Look at positions
with 1s in them Count the number of 1s in
the corresponding message bits If even, place a
1 in the 20 check bit, i.e., use odd
parity Otherwise, place a 0
Position
2n check bits
Calculate check bits
3 21 20 0 1 1 5 22 20
1 0 1 6 22 21 1 1 0 7
22 21 20 1 1 1
14
Hamming Code Example
1 0 1 1
Message to be sent
1 0 1 1 1 2 3 4 5 6
7 20 21 22
1
0
Repeat with the 21 position Look at positions
those positions with 1s in them Count the
number of 1s in the corresponding message
bits If even, place a 1 in the 21 check
bit Otherwise, place a 0
Position
2n check bits
Calculate check bits
3 21 20 0 1 1 5 22 20
1 0 1 6 22 21 1 1 0 7
22 21 20 1 1 1
15
Hamming Code Example
1 0 1 1
Message to be sent
1 0 1 1 1 2 3 4 5 6
7 20 21 22
1
0
1
Repeat with the 22 position Look at positions
those positions with 1s in them Count the
number of 1s in the corresponding message
bits If even, place a 1 in the 22 check
bit Otherwise, place a 0
Position
2n check bits
Calculate check bits
3 21 20 0 1 1 5 22 20
1 0 1 6 22 21 1 1 0 7
22 21 20 1 1 1
16
Hamming Code Example

• Original message 1011
• Sent message 1011011
• Now, how do we check for a single-bit error in
  the sent message using the Hamming code?

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Using Hamming Codes to Correct Single-Bit Errors
1 0 1 1 0 0 1
Received message
1 0 1 1 0 0 1 1 2 3 4 5 6
7 20 21 22
Position
2n check bits
Calculate check bits
3 21 20 0 1 1 5 22 20
1 0 1 6 22 21 1 1 0 7
22 21 20 1 1 1
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Using Hamming Codes to Correct Single-Bit Errors
1 0 1 1 0 0 1
Received message
1 0 1 1 0 0 1 1 2 3 4 5 6
7 20 21 22
Starting with the 20 position Look at positions
with 1s in them Count the number of 1s in
both the corresponding message bits and the 20
check bit and compute the parity. If even
parity, there is an error in one of the four bits
that were checked.
Position
2n check bits
Calculate check bits
3 21 20 0 1 1 5 22 20
1 0 1 6 22 21 1 1 0 7
22 21 20 1 1 1
Odd parity No error in bits 1, 3, 5, 7
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Using Hamming Codes to Correct Single-Bit Errors
1 0 1 1 0 0 1
Received message
1 0 1 1 0 0 1 1 2 3 4 5 6
7 20 21 22
Repeat with the 21 position Look at positions
with 1s in them Count the number of 1s in
both the corresponding message bits and the 21
check bit and compute the parity. If even
parity, there is an error in one of the four bits
that were checked.
Position
2n check bits
Calculate check bits
3 21 20 0 1 1 5 22 20
1 0 1 6 22 21 1 1 0 7
22 21 20 1 1 1
Even parity ERROR in bit 2, 3, 6 or 7!
20
Using Hamming Codes to Correct Single-Bit Errors
1 0 1 1 0 0 1
Received message
1 0 1 1 0 0 1 1 2 3 4 5 6
7 20 21 22
Repeat with the 22 position Look at positions
with 1s in them Count the number of 1s in
both the corresponding message bits and the 22
check bit and compute the parity. If even
parity, there is an error in one of the four bits
that were checked.
Position
2n check bits
Calculate check bits
3 21 20 0 1 1 5 22 20
1 0 1 6 22 21 1 1 0 7
22 21 20 1 1 1
Even parity ERROR in bit 4, 5, 6 or 7!
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Finding the errors location
1 0 1 1 0 0 1 1 2 3 4 5 6
7
Position
22
Finding the errors location
1 0 1 1 0 0 1 1 2 3 4 5 6
7
Position
No error in bits 1, 3, 5, 7
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Finding the errors location
erroneous bit, change to 1
1 0 1 1 0 0 1 1 2 3 4 5 6
7
Position
Error must be in bit 6 because bits 3, 5, 7 are
correct, and all the remaining information agrees
on bit 6
ERROR in bit 2, 3, 6 or 7
ERROR in bit 4, 5, 6 or 7
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Finding the errors locationAn Easier
Alternative to the Last Slide
3 21 20 0 1 1 5 22 20
1 0 1 6 22 21 1 1 0 7
22 21 20 1 1 1
E
E
NE
6
1
1
0
E error in column NE no error in column
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Hamming Codes

• Hamming codes can be used to locate and correct a
  single-bit error
• If more than one bit is in error, then a Hamming
  code cannot correct it
• Hamming codes, like parity bits, are only useful
  on short messages

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CRC Polynomial Codes

• Can detect errors on large chunks of data
• Has low overhead
• More robust than parity bit
• Requires the use of a code polynomial
• Example x2 1

27
Cyclic Redundancy Check

• CRC Example of a polynomial code
• Procedure
• 1. Let r be the degree of the code polynomial.
  Append r zero bits to the end of the transmitted
  bit string. Call the entire bit string S(x)
• 2. Divide S(x) by the code polynomial using
  modulo 2 division.
• 3. Subtract the remainder from S(x) using modulo
  2 subtraction.
• The result is the checksummed message

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Generating a CRCExample
Message 1011 1 x x3 0 x x2 1 x x 1
x3 x 1 Code Polynomial x2 1
(101)
Step 1 Compute S(x) r 2 S(x) 101100
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Generating a CRCExample (contd)
Step 2 Modulo 2 divide
Remainder
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Generating a CRCExample (contd)
Step 3 Modulo 2 subtract the remainder from S(x)
101100
- 01
101101
Checksummed Message
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Decoding a CRC

• Procedure
• 1. Let n be the length of the checksummed message
  in bits
• 2. Divide the checksummed message by the code
  polynomial using modulo 2 division. If the
  remainer is zero, there is no error detected.

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Decoding a CRCExample
101101
Checksummed message (n 6)
1011
Original message (if there are
no errors)
Remainder 0 (No error detected)
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Decoding a CRCAnother Example

• When a bit error occurs, there is a large
  probability that it will produce a polynomial
  that is not an even multiple of the code
  polynomial, and thus errors can usually be
  detected.

Remainder 1 (Error detected)
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Choosing a CRC polynomial

• The longer the polynomial, the smaller the
  probability of undetected error
• Common standard polynomials
• (1) CRC-12 x12 x11 x3 x2 x1 1
• (2) CRC-16 x16 x15 x2 1
• (3) CRC-CCITT x16 x12 x5 1

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Access Protocols

• Who gets to use the channel next?
• Fixed/Static assignment
• Dynamic assignment
• Randomized/Probabilistic
• Turn-Based

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Fixed Assignment Protocols

• Static and predetermined allocation of channel
  access independent of user activity
• Idle users may be assigned to the channel, in
  which case channel capacity is wasted
• Examples TDMA, FDMA

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Channel Partitioning MAC protocols TDMA

• TDMA time division multiple access
• access to channel in "rounds"
• each station gets fixed length slot (length pkt
  trans time) in each round
• unused slots go idle
• example 6-station LAN, 1,3,4 have pkt, slots
  2,5,6 idle

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Channel Partitioning MAC protocols FDMA

• FDMA frequency division multiple access
• channel spectrum divided into frequency bands
• each station assigned fixed frequency band
• unused transmission time in frequency bands go
  idle
• example 6-station LAN, 1,3,4 have pkt, frequency
  bands 2,5,6 idle

time
frequency bands
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Demand Assignment Protocols

• Allocate channel capacity to hosts on a demand
  basis (i.e., only to active users)
• Requires methods for measuring the demand for the
  channel
• Polling
• Reservation schemes

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Polling

• A central controller interrogates each host and
  allocates channel capacity to those who need it
• Good for systems with
• Short propagation delay
• Small polling messages
• Non-bursty traffic

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Reservation Schemes

• Hosts independently reserve the channel for
  period of time
• Reservations are usually piggybacked on data
  messages passing along the channel
• Good for systems with
• short propagation delay
• no central controller node
• non-bursty traffic

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Reservation Protocols (contd)

• Reservation protocol examples
• Bit-Map Protocol
• Binary Countdown Protocol

43
Bit-Map Protocol

• Contention and data transmission periods
  alternate
• The contention period is divided into slots, with
  1 bit-wide slots for each host in the network.
• If a host wants to transmit a packet, it sets its
  contention slot equal to 1. Otherwise, it sets
  it to 0.
• The slots pass all hosts in sequence, so every
  host is aware of who will transmit

1

1
1
1

1



1



2
4
5
6
0
4
01234567
01234567
data frame
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Bit-Map Protocol (contd)

• But what if there are a large number of hosts in
  the network?
• The contention period will have to grow to
  include them all
• With a large number of hosts, the contention
  period may be very long, leading to inefficiency

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Binary Countdown Protocol

• During contention period,
• each host broadcasts its binary address one bit
  at a time, starting with the most significant bit
• bits transmitted simultaneously are boolean ORd
  together
• Arbitration rule
• If a host sent a zero bit but the boolean OR
  results in a one bit, the host gives up and stops
  sending address bits
• Whichever host remains after the entire address
  has been broadcast gets access to the medium

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Binary Countdown (contd)
Bit Time
Host Addresses
0 1 2 3
0 0 1 0
0 - - -
0 1 0 1
0 - - -
1 0 0 1
1 0 0 -
1 0 1 0
1 0 1 0
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Binary Countdown Fairness

• Stations with the highest addresses will always
  win.
• This is good if you want to implement priority,
  but bad if you want to give all hosts fair access
  to the channel
• Used in CAN-bus networks (cars)
• Solution
• Change the address of a host after a successful
  transmission

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Binary Countdown Permuting Addresses

• After host A successful transmission, all hosts
  with addresses less than host A add one to their
  address, improving their priority
• Host A changes its address to zero, giving it the
  lowest priority

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Contention Access Protocols

• Single channel shared by a large number of hosts
• No coordination between hosts
• Control is completely distributed
• Outcome is probabilistic
• Examples ALOHA, CSMA, CSMA/CD

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Contention Access (contd)

• Advantages
• Short delay for bursty traffic
• Simple (due to distributed control)
• Flexible to fluctuations in the number of hosts
• Fairness

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Contention Access (contd)

• Disadvantages
• Can not be certain who will acquire the
  media/channel
• Low channel efficiency with a large number of
  hosts
• Not good for continuous traffic (e.g., voice)
• Cannot support priority traffic
• High variance in transmission delays

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Contention Access Methods

• Pure ALOHA
• Slotted ALOHA
• CSMA
• 1-Persistent CSMA
• Non-Persistent CSMA
• P-Persistent CSMA
• CSMA/CD

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Slotted ALOHA

• Assumptions
• all frames same size
• time is divided into equal size slots, time to
  transmit 1 frame
• nodes start to transmit frames only at beginning
  of slots
• nodes are synchronized
• if 2 or more nodes transmit in slot, all nodes
  detect collision

• Operation
• when node obtains fresh frame, it transmits in
  next slot
• no collision, node can send new frame in next
  slot
• if collision, node retransmits frame in each
  subsequent slot with prob. p until success

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Slotted ALOHA

• Pros
• single active node can continuously transmit at
  full rate of channel
• highly decentralized only slots in nodes need to
  be in sync
• simple

• Cons
• collisions, wasting slots
• idle slots
• nodes may be able to detect collision in less
  than time to transmit packet
• clock synchronization

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Slotted Aloha efficiency

• For max efficiency with N nodes, find p that
  maximizes Np(1-p)N-1
• For many nodes, take limit of Np(1-p)N-1 as N
  goes to infinity, gives 1/e .37

Efficiency is the long-run fraction of
successful slots when there are many nodes, each
with many frames to send

• Suppose N nodes with many frames to send, each
  transmits in slot with probability p
• prob that node 1 has success in a slot
  p(1-p)N-1
• prob that any node has a success Np(1-p)N-1

At best channel used for useful transmissions
37 of time!
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Pure (unslotted) ALOHA

• unslotted Aloha simpler, no synchronization
• when frame first arrives
• transmit immediately
• collision probability increases
• frame sent at t0 collides with other frames sent
  in t0-1,t01

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Pure Aloha efficiency

• P(success by given node) P(node transmits) .
• P(no
  other node transmits in p0-1,p0 .
• P(no
  other node transmits in p0-1,p0
• p .
  (1-p)N-1 . (1-p)N-1
• p .
  (1-p)2(N-1)
• choosing optimum
  p and then letting n -gt infty ...
• 
  1/(2e) .18

Even worse !
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Carrier Sense Multiple Access (CSMA)

• We could achieve better throughput if we could
  listen to the channel before transmitting a
  packet
• This way, we would stop avoidable collisions.
• To do this, we need Carrier Sense Multiple
  Access, or CSMA, protocols

59
Assumptions with CSMA Networks

• 1. Constant length packets
• 2. No errors, except those caused by collisions
• 3. No capture effect
• 4. Each host can sense the transmissions of all
  other hosts
• 5. The propagation delay is small compared to the
  transmission time

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CSMA collisions
spatial layout of nodes
collisions can still occur propagation delay
means two nodes may not hear each others
transmission
collision entire packet transmission time wasted
note role of distance propagation delay in
determining collision probability
61
CSMA/CD (Collision Detection)

• CSMA/CD carrier sensing, deferral as in CSMA
• collisions detected within short time
• colliding transmissions aborted, reducing channel
  wastage
• collision detection
• easy in wired LANs measure signal strengths,
  compare transmitted, received signals
• difficult in wireless LANs receiver shut off
  while transmitting
• human analogy the polite conversationalist

62
CSMA (contd)

• There are several types of CSMA protocols
• 1-Persistent CSMA
• Non-Persistent CSMA
• P-Persistent CSMA

63
1-Persistent CSMA

• Sense the channel.
• If busy, keep listening to the channel and
  transmit immediately when the channel becomes
  idle.
• If idle, transmit a packet immediately.
• If collision occurs,
• Wait a random amount of time and start over again.

64
1-Persistent CSMA (contd)

• The protocol is called 1-persistent because the
  host transmits with a probability of 1 whenever
  it finds the channel idle.

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The Effect of Propagation Delayon CSMA
packet
A
B
carrier sense idle Transmit a packet Collision
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Propagation Delay and CSMA

• Contention (vulnerable) period in Pure ALOHA
• two packet transmission times
• Contention period in Slotted ALOHA
• one packet transmission time
• Contention period in CSMA
• up to 2 x end-to-end propagation delay

Performance of CSMA gt Performance of Slotted
ALOHA gt Performance of Pure ALOHA
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1-Persistent CSMA (contd)

• Even if prop. delay is zero, there will be
  collisions
• Example
• If stations B and C become ready in the middle of
  As transmission, B and C will wait until the end
  of As transmission and then both will begin
  transmitted simultaneously, resulting in a
  collision.
• If B and C were not so greedy, there would be
  fewer collisions

68
Non-Persistent CSMA

• Sense the channel.
• If busy, wait a random amount of time and sense
  the channel again
• If idle, transmit a packet immediately
• If collision occurs
• wait a random amount of time and start all over
  again

69
Tradeoff between 1- and Non-Persistent CSMA

• If B and C become ready in the middle of As
  transmission,
• 1-Persistent B and C collide
• Non-Persistent B and C probably do not collide
• If only B becomes ready in the middle of As
  transmission,
• 1-Persistent B succeeds as soon as A ends
• Non-Persistent B may have to wait

70
P-Persistent CSMA

• Optimal strategy use P-Persistent CSMA
• Assume channels are slotted
• One slot contention period (i.e., one round
  trip propagation delay)

71
P-Persistent CSMA (contd)

• 1. Sense the channel
• If channel is idle, transmit a packet with
  probability p
• if a packet was transmitted, go to step 2
• if a packet was not transmitted, wait one slot
  and go to step 1
• If channel is busy, wait one slot and go to step
  1.
• 2. Detect collisions
• If a collision occurs, wait a random amount of
  time and go to step 1

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P-Persistent CSMA (contd)

• Consider p-persistent CSMA with p0.5
• When a host senses an idle channel, it will only
  send a packet with 50 probability
• If it does not send, it tries again in the next
  slot.

73
Comparison of CSMA and ALOHA Protocols
(Number of Channel Contenders)
74
CSMA/CD

• In CSMA protocols
• If two stations begin transmitting at the same
  time, each will transmit its complete packet,
  thus wasting the channel for an entire packet
  time
• In CSMA/CD protocols
• The transmission is terminated immediately upon
  the detection of a collision
• CD Collision Detect

75
CSMA/CD collision detection
76
CSMA/CD

• Sense the channel
• If idle, transmit immediately
• If busy, wait until the channel becomes idle
• Collision detection
• Abort a transmission immediately if a collision
  is detected
• Try again later after waiting a random amount of
  time

77
CSMA/CD (contd)

• Carrier sense
• reduces the number of collisions
• Collision detection
• reduces the effect of collisions, making the
  channel ready to use sooner

78
Collision detection time

• How long does it take to realize there has been a
  collision?
• Worst case 2 x end-to-end prop. delay

packet
A
B
79
IEEE 802 LANs

• LAN Local Area Network
• What is a local area network?
• A LAN is a network that resides in a
  geographically restricted area
• LANs usually span a building or a campus

80
Characteristics of LANs

• Short propagation delays
• Small number of users
• Single shared medium (usually)
• Inexpensive

81
Common LANs

• Bus-based LANs
• Ethernet ()
• Token Bus ()
• Ring-based LANs
• Token Ring ()
• Switch-based LANs
• Switched Ethernet
• ATM LANs

() IEEE 802 LANs
82
IEEE 802 Standards

• 802.1 Introduction
• 802.2 Logical Link Control (LLC)
• 802.3 CSMA/CD (Ethernet)
• 802.4 Token Bus
• 802.5 Token Ring
• 802.6 DQDB
• 802.11 CSMA/CA (Wireless LAN)

83
IEEE 802 Standards (contd)

• 802 standards define
• Physical layer protocol
• Data link layer protocol
• Medium Access (MAC) Sublayer
• Logical Link Control (LLC) Sublayer

84
OSI Layers and IEEE 802
IEEE 802 LAN standards
OSI layers
Higher Layers
Higher Layers
802.2 Logical Link Control 802.3 802.4 802.5 Medi
um Access Control
Data Link Layer
CSMA/CD Token-passing Token-passing bus
bus ring
Physical Layer
85
IEEE 802 LANs (contd)

• Ethernet
• Token Ring

86
Ethernet (CSMA/CD)

• IEEE 802.3 defines Ethernet
• Layers specified by 802.3
• Ethernet Physical Layer
• Ethernet Medium Access (MAC) Sublayer

87
Ethernet (contd)

• Possible Topologies
• 1. Bus
• 2. Branching non-rooted tree for large Ethernets

88
Minimal Bus Configuration
Coaxial Cable
Transceiver
Terminator
Transceiver Cable
Host
89
Typical Large-Scale Configuration
Repeater
Host
Ethernet segment
90
Ethernet Physical Layer

• Transceiver
• Transceiver Cable
• 4 Twisted Pairs
• 15 Pin Connectors
• Channel Logic
• Manchester Phase Encoding
• 64-bit preamble for synchronization

91
Ethernet Physical Configuration(for thick
coaxial cable)

• Segments of 500 meters maximum
• Maximum total cable length of 1500 meters between
  any two transceivers
• Maximum of 2 repeaters in any path
• Maximum of 100 transceivers per segment
• Transceivers placed only at 2.5 meter marks on
  cable

92
Manchester Encoding
1 0 1 1 0 0
Data stream Encoded bit pattern

• 1 bit high/low voltage signal
• 0 bit low/high voltage signal

93
Manchester Encoding
1 1 1 1 1 1
Data stream Encoded bit pattern

• How to determine data from stuck-at fault?
• Continuous differential

94
Ethernet Synchronization

• 64-bit frame preamble used to synchronize
  reception
• 7 bytes of 10101010 followed by a byte containing
  10101011
• Manchester encoded, the preamble appears like a
  sine wave

95
Ethernet Cabling Options

• 10Base5 Thick Coax
• 10Base2 Thin Coax (cheapernet)
• 10Base-T Twisted Pair
• 10Base-F Fiber optic
• Each cabling option carries with it a different
  set of physical layer constraints (e.g., max.
  segment size, nodes/segment, etc.)

96
Ethernet MAC Layer

• Data encapsulation
• Frame Format
• Addressing
• Error Detection
• Link Management
• CSMA/CD
• Backoff Algorithm

97
MAC Layer Ethernet Frame Format
Multicast bit
Destination (6 bytes)
Source (6 bytes)
Length (2 bytes)
Data (46-1500 bytes)
Pad
Frame Check Seq. (4 bytes)
98
Ethernet MAC Frame Address Field

• Destination and Source Addresses
• 6 bytes each
• Two types of destination addresses
• Physical address Unique for each user
• Multicast address Group of users
• First bit of address determines which type of
  address is being used
• 0 physical address
• 1 multicast address

99
Ethernet MAC FrameOther Fields

• Length Field
• 2 bytes in length
• determines length of data payload
• Data Field between 0 and 1500 bytes
• Pad Filled when Length lt 46
• Frame Check Sequence Field
• 4 bytes
• Cyclic Redundancy Check (CRC-32)

100
CSMA/CD

• Recall
• CSMA/CD is a carrier sense protocol.
• If channel is idle, transmit immediately
• If busy, wait until the channel becomes idle
• CSMA/CD can detect collections.
• Abort transmission immediately if there is a
  collision
• Try again later according to a backoff algorithm

101
CSMA/CD (contd)

• Carrier sense reduces the number of collisions
• Collision detection reduces the impact of
  collisions

102
CSMA/CD and Ethernet

• Ethernet
• Short end-to-end propagation delay
• Broadcast channel
• Ethernet access protocol
• 1-Persistent CSMA/CD
• with Binary Exponential Backoff Algorithm

103
Ethernet Backoff AlgorithmBinary Exponential
Backoff

• If collision,
• Choose one slot randomly from 2k slots, where k
  is the number of collisions the frame has
  suffered.
• One contention slot length 2 x end-to-end
  propagation delay

This algorithm can adapt to changes in network
load.
104
Binary Exponential Backoff (contd)
slot length 2 x end-to-end delay 50 ms
A
B
t0ms Assume A and B collide (kA kB
1) A, B choose randomly from 21 slots
0,1 Assume A chooses 1, B chooses
1 t100ms A and B collide (kA kB 2) A, B
choose randomly from 22 slots 0,3 Assume A
chooses 2, B chooses 0 t150ms B transmits
successfully t250ms A transmits successfully
105
Binary Exponential Backoff (contd)

• In Ethernet,
• Binary exponential backoff will allow a maximum
  of 15 retransmission attempts
• If 16 backoffs occur, the transmission of the
  frame is considered a failure.

106
4.1.4 Ethernet Performance
107
Ethernet Features and Advantages

• 1. Passive interface No active element
• 2. Broadcast All users can listen
• 3. Distributed control Each user makes own
  decision

Simple Reliable Easy to reconfigure
108
Ethernet Disadvantages

• Lack of priority levels
• Cannot perform real-time communication
• Security issues

109
Hubs, Bridges, Switches, Routers

• Hub
• Behaves like Ethernet
• Bridge
• Supports multiple collision domains
• A collision domain is a segment
• Switch
• collision domains operate at pairwise level
• Router operates on level-3 packets

110
Why Ethernet Switching?

• LANs may grow very large
• The switch has a very fast backplane
• It can forward frames very quickly to the
  appropriate subnet
• Cheaper than upgrading all host interfaces to use
  a faster network

111
Ethernet Switching

• Connect many Ethernet through an Ethernet
  switch
• Each Ethernet is a segment
• Make one large, logical segment

to segment 1
to segment 4
to segment 2
to segment 3
112
Collision Domains
D
switch
A
B
E
A,B,C
D,E,F
F
C
Host
Z
Each segment runs a standard CMSA protocol
G
H
Ethernet Hub
113
Layer-2 routing tables
D
switch
A
B
E
A,B,C
D,E,F
F
C
Host
Z
Switch must forward packets from A,B,C to the
other segment Switch builds a large table For
each packet, look up in table and maybe forward
the packet
G
H
Ethernet Hub
114
Learning MAC addresses
D
switch
A
B
E
A,B,C
D,E,F
F
C
Host
Per-port routing table
Z
G
Switch adds hosts to routing table when it sees
a packet with a given source address
H
Ethernet segment
115
Spanning Trees

• Want to allow multiple bridges and switches to
  connect together
• What If there is a cycle in the graph of switches
  connected together?
• Cant have packets circulate forever!
• Must break the cycle by restricting routes

116
Spanning Trees
D
switches
A
B
E
1
2
F
C
Host
J
Z
G
H
k
3
117
Spanning Trees
D
switches
A
B
E
1
2
F
C
Host
J
Z
G
H
k
3
no cycles in the graph of switches
118
Spanning Tree Protocol

• Each switch periodically sends a configuration
  message out of every port. A message contains
  (ID of sender, ID of root, distance from sender
  to root).
• Initially, every switch claims to be root and
  sends a distance field of 0.
• A switch keeps sending the same message
  (periodically) until it hears a better message.
• Better means
• A root with a smaller ID
• A root with equal ID, but with shorter distance
• The root ID and distance are the same as we
  already have, but the sending bridge has a
  smaller ID.
• When a switch hears a better configuration
  message, it stops generating its own messages,
  and just forwards ones that it receives (adding 1
  to the distance).
• If the switch realizes that it is not the
  designated bridge for a segment, it stops sending
  configuration messages to that segment.
• Eventually
• Only the root switch generates configuration
  messages,
• Other switches send configuration messages to
  segments for which they are the designated switch

119
Bridges vs. Switches

• A bridge connects networks at layer 2
• Work to make a single logical layer 2 address
• Some bridges can connect different 802.X network
  types
• E.g. Ethernet to Token ring

120
Token Ring

• IEEE 802.5 Standard
• Layers specified by 802.5
• Token Ring Physical Layer
• Token Ring MAC Sublayer

121
Token Ring (contd)

• Token Ring, unlike Ethernet, requires an active
  interface

Host
Ring interface
122
Token Ring Physical Layer

• Ring Interfaces
• Listen and Transmit Modes
• Channel Logic
• Differential Manchester Encoding

123
Token Ring Interface Modes
Listen Mode
Transmit Mode
one-bit delay
To station
From station
To station
From station
124
Differential Manchester Encoding
1 0 0 1 1

• Transitions take place at midpoint of interval
• 1 bit the initial half of the bit interval
  carries the same polarity as the second half of
  the previous interval
• 0 bit a transition takes place at both the
  beginning and the middle of the bit interval

125
Token Ring MAC Sublayer

• Token passing protocol
• Frame format
• Token format

126
Token Passing Protocol

• A token (8 bit pattern) circulates around the
  ring
• Token state
• Busy 11111111
• Idle 11111110

127
Token Passing Protocol (contd)

• General Procedure
• Sending host waits for and captures an idle token
• Sending host changes the token to a frame and
  circulates it
• Receiving host accepts the frame and continues to
  circulate it
• Sending host receives its frame, removes it from
  the ring, and generates an idle token which it
  then circulates on the ring

128
Token Ring Frame and Token Formats
1 1 1
Bytes
SD
AC
ED
Token Format
1 1 1 2/6 2/6
unlimited 4 1 1
SC
AC
FC
Destination Address
Source Address
Data
Checksum
ED
FS
Frame Format
129
Token Ring Delimiters
SD
AC
ED
SC
AC
FC
Destination Address
Source Address
Data
Checksum
ED
FS

• SD Starting Delimiter
• ED Ending Delimiter
• They contains invalid differential Manchester
  codes

130
Token Ring Access Control Field
SD
AC
ED
(Note The AC field is also used in frames)
P P P T M R R R

• P Priority bits
• provides up to 8 levels of priority when
  accessing the ring
• T Token bit
• T0 Token
• T1 Frame

131
Token Ring Access Control Field (contd)
SD
AC
ED
P P P T M R R R

• M Monitor Bit
• Prevents tokens and frames from circulating
  indefinitely
• All frames and tokens are issued with M0
• On passing through the monitor station, M is
  set to 1
• All other stations repeat this bit as set
• A token or frame that reaches the monitor station
  with M1 is considered invalid and is purged

132
Token Ring Access Control Fields (contd)
SD
AC
ED
P P P T M R R R

• R Reservation Bits
• Allows stations with high priority data to
  request (in frames and tokens as they are
  repeated) that the next token be issued at the
  requested priority

133
Token Ring Frame Control Field
SC
AC
FC
Destination Address
Source Address
Data
Checksum
ED
FS

• FC Frame Control Field
• Defines the type of frame being sent
• Frames may be either data frames or some type of
  control frame. Example control frames
• Beacon Used to locate breaks in the ring
• Duplicate address test Used to test if two
  stations have the same address

134
Token Ring Address Data Fields
SC
AC
FC
Destination Address
Source Address
Data
Checksum
ED
FS

• Address Fields
• Indicate the source and destination hosts
• Broadcast
• Set all destination address bits to 1s.
• Data
• No fixed limit on length
• Caveat Hosts may only hold the token for a
  limited amount of time (10 msec)

135
Token Ring Checksum and Frame Status
SC
AC
FC
Destination Address
Source Address
Data
Checksum
ED
FS

• Checksum 32-bit CRC
• FS Frame Status
• Contains two bits, A and C
• When the message arrives at the destination, it
  sets A1
• When the destination copies the data in the
  message, it sets C1

136
The Token Ring Monitor Station

• One station on the ring is designated as the
  monitor station
• The monitor station
• marks the M bit in frames and tokens
• removes marked frames and tokens from the ring
• watches for missing tokens and generates new ones
  after a timeout period

137
Using Priority in Token Ring

• If a host wants to send data of priority n, it
  may only grab a token with priority value n or
  lower.
• A host may reserve a token of priority n by
  marking the reservation bits in the AC field of a
  passing token or frame
• Caveat The host may not make the reservation if
  the token or frames AC field already indicates a
  higher priority reservation
• The next token generated will have a priority
  equal to the highest reserved priority

138
Priority Transmission Example
B
A
C
D
Host B has 1 frame of priority 3 to send to
A Host C has 1 frame of priority 2 to send to
A Host D has 1 frame of priority 4 to send to
A Token starts at host A with priority 0 and
circulates clockwise Host C is the monitor
station
139
Example (contd)
Event Token/Frame AC Field A
generates a token P0, M0, T0, R0 B
grabs the token and sets the message destination
to A P3, M0, T1, R0 Frame arrives at C,
and C reserves priority level 2. Monitor bit
set. P3, M1, T1, R2 Frame arrives at D,
and D attempts to reserve priority level 4 P3,
M1, T1, R4 Frame arrives at A, and A copies
it P3, M1, T1, R4 Frame returns to B, so B
removes it, and generates a new token P4, M0,
T0, R0 Token arrives at C, but its priority
is too high. C reserves priority 2. M bit. P4,
M1, T0, R2
140
Example (contd)

• Event Token/Frame AC Field
• Token arrives at D, and D grabs
• it, sending a message to A P4, M0, T1, R2
• Frame arrives at A, and A
• copies it P4, M0, T1, R2
• Frame arrives at B, which does
• nothing to it P4, M0, T1, R2
• Frame arrives at C, which sets the
• monitor bit P4, M1, T1, R2
• Frame returns to D, so D removes
• it and generates a new token with P2 P2, M0,
  T0, R0
• etc Attempt to complete this scenario on your
  own.