EEC4113 Data Communication

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EEC4113Data Communication Multimedia
SystemChapter 6 Media Access Controlof Data
Link Sub-Layerby Muhazam Mustapha, October 2011
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Learning Outcome

• By the end of this chapter, students are expected
  to understand and able to explain the various
  protocols and technologies in MAC sub-layer

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Chapter Content

• MAC Sub-Layer Issues
• ALOHA Protocols
• CSMA Protocols
• Collision-Free Protocols
• Topology
• IEEE 802.3 Ethernet
• IEEE 802.11/15/16 Wireless Ethernet
• IEEE 802.5 Token Ring

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Media Access ControlSub-Layer
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Media Access Control

• Media Access Control is a sub-layer of data link
  layer in OSIs 7 layer model
• Provides access to the shared networking medium
  in LAN or MAN
• The currently most popular technology that
  provides MAC is the Ethernet technology
• Others are FDDI (Fiber Distributed Data
  Interface), ARCNET and Token ring

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Ethernet

• A family of frame-based technology defining
  standards for wiring and signaling
• Standardized in IEEE 802.3 document
• Combination of twisted wire pair and optical
  fiber
• Characterized by the used of 8P8C connector

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Shared Network Medium

• In shared environment, packets sent by one sender
  will be received by all nodes, but only the
  packet addressee will process it, the rest will
  discard

packet sent out
sender
recipient
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Multiple Access Protocol

• Since the network medium is shared, there is a
  need to resolve competition between the nodes
• Two general schemes
• Static
• Frequency / Time Division Multiplexing(digital
  communication)
• Dynamic
• ALOHA, Carrier Sense Multiple Access(data
  communication)

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Channel Allocation Problem

• In shared medium, a user will first listen to the
  channel for its availability, then sends its
  frame
• COLLISION occurs when more than one user start
  using the medium at the same time
• At collision incidence, both user release the
  medium and wait for random time before re-sending

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ALOHA Protocols
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ALOHA Protocols

• Created in 1970s in the University of Hawaii by
  Norman Abramson
• First ingenious method to resolve channel
  allocation problem
• It was best for wireless communication and the
  concept is still in used by modern protocol like
  Wi-fi

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Pure ALOHA

• Basic ideas
• Anyone is allowed to transmit their data whenever
  they have something to transmit, without checking
  the channel availability first
• After sending, the sender will listen to its own
  frequency to tell whether its frame has been
  destroyed due to collision or not
• This is possible due to feedback property of
  broadcasting channel, or
• The sender will require an acknowledgement

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Pure ALOHA

• Basic ideas
• If there is no feedback, then there is collision
• If collision occurs, the sender will wait for a
  random amount of time, then re-send this called
  backoff

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Pure ALOHA
User
A
B
C
D
E
Time
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Slotted ALOHA

• Time is divided into slots, and users can only
  transmit at start of slot
• Resulting advantage Efficiency is doubled (see
  graph)
• Disadvantages
• Requires synchronization clock
• Still poor at high loads (see graph)

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Pure vs Slotted ALOHA
0.40
Slotted ALOHA S Ge-G
S (throughput per frame time)
0.30
0.20
Pure ALOHA S Ge-2G
0.10
0.5
1.0
1.5
2.0
2.5
3.0
G (attempts per packet time)
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Carrier Sense Multiple Access Protocols
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Carrier Sensing Protocols

• Network communication can be improved greatly if
  the nodes can sense the existence of any
  transmission signal inside the transmission
  medium
• Implemented in Carrier Sense Multiple Access
  (CSMA) and a few of its variations
• Improvement is due to the fact that collisions is
  reduced since hosts will only send data if medium
  is not in use

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CSMA

• A host that needs to transmit data will first
  listen into communication medium and decide
  whether another host is using the medium or not
• The host will only transmit its data if no one is
  using the medium
• After finish sending the data frame, there will
  be an interframe gap of 9.6µs idle before any
  host can take the medium

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Persistent and Non-persistent CSMA

• CSMA is called persistent if
• when sensing that a medium is being used, the
  host waits and will definitely transmit once the
  current transmission ends
• may cause collision if more than one host was
  waiting
• And non-persistent if
• the host waits for a random duration and re-sends
  only if no one using it
• results in less collision

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CSMA/CD (Collision Detection)

• The system will be having 3 states transmission,
  contention and idle
• Transmission state is the state where one host
  sends data.
• After that host finishes, more than one of other
  hosts might be sending at the same time a
  collision

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CSMA/CD (Collision Detection)

• On sensing a collision, all hosts involve would
  release the medium and they send a jamming signal
  to tell others that there is collision happened
• so that everyone releases the medium
• Then they will wait for a random duration and
  re-try
• The above two steps is the contention state

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CSMA/CD (Collision Detection)

• Once one of the competing host gains control the
  system is in transmission state again
• Idle state is just the state that no one is using
  the medium

collisions
transmission
transmission
transmission
transmission
contention
idle
contention
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CSMA/CA (Collision Avoidance)

• CSMA/CD is a persistence variation of CSMA it
  handles collision when it happens
• CSMA/CA is a non-persistence variation CSMA
• CSMA/CA avoids collision by
• not sending jamming signal
• instead, just wait for a random duration then
  re-sends if no one is using

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Collision-Free Protocol
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Collision-Free Protocols

• In collision free protocols, instead of sensing
  the medium, the hosts will tell if they want to
  transmit
• There is a special frame called contention frame
  whose content is contributed by all hosts

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Collision-Free Protocols

• Contention frame is slotted and the hosts will
  take turns at a very precise timing to write
  information into the frame
• A host sets a binary 1 at bit location reserved
  for it in contention frame if it wants to use the
  medium

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Collision-Free Protocols

• Once all hosts write the binary bits according to
  its intention, the actual transmission will be
  granted to the requesting hosts in sequence.
• Once all transmissions finish, the hosts will
  then re-fill the contention frame
• This protocol is called basic bit-map protocol

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Collision-Free Protocols
8 contention slots
8 contention slots
8 contention slots
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
1
1
1
1
1
1
1
7
3
5
1
2
frames
frames
frames
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Ethernet Topology
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Topology
Bus
Linear Bus 2 ends
Distributed Bus more than 2 ends
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Topology
Star
Ring
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Topology
Mesh
Tree
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Bus Topology

• Use of multipoint medium
• All stations attach directly to transmission
  medium (bus) through appropriate hardware
  interfacing known as tap

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Bus Topology

• A transmission from any station propagates the
  length of the medium in both directions can be
  received by all other stations
• At each end of the bus is a terminator, which
  absorbs any signal, removing it from the bus

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Tree Topology

• Use of multipoint medium
• Transmission medium is a branching cable with no
  closed loops
• Tree layout begins at a point known as the headend

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Tree Topology

• One or more cables start at the headend, and each
  of these may have branches
• The branches in turn may have additional branches
  to allow quite complex layouts
• A transmission from any station propagates
  throughout the medium can be received by all
  other stations

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Ring Topology

• Repeaters joined by point-to-point links in
  closed loop
• Receive data on one link and retransmit on
  another
• Links are unidirectional
• Stations attached to repeaters

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Ring Topology

• Data in frames
• Circulate past all stations
• Destination recognizes address and copies frame
• Frame circulates back to source where it is
  removed
• Medium access control determines when station can
  insert frame

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Star Topology

• Each station connected directly to central node
• Usually via two point-to-point links
• Two alternatives operation of central node
• Broadcast Physical star, logical bus
• Frame-switching device Only one station can
  transmit at a time

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Star Topology

• Broadcast
• A transmission of a frame from one station to the
  central node is retransmitted on all of the
  outgoing links
• Central node is referred as hub
• Frame-switching device
• Incoming frame is buffered in the node
  retransmitted on an outgoing link to the
  destination station

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IEEE 802.3 Standard ofEthernet
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IEEE 802.3 Standard

• Defines Ethernet as CSMA/CD protocol on bus or
  ring topology
• Also defines the minimum frame length
• Also defines the cabling hardware
• Frame format

7
1
6
6
0-46
0-1500
4
Bytes
S O F
Destination address
Source address
Preamble
Length
Data
Pad
Checksum
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Frame Fields

• Preamble 7 bytes of alternating 1-s and 0-s for
  synchronization
• Start of Frame (SOF) Sequence of 10101011
• Destination Address 6 bytes of MAC address
• Source Address 6 bytes of address
• Length Total size of data and pad

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Frame Fields

• Data Packet from upper layer
• Pad Series of 0-s to make up a minimum total
  size of 46 bytes of data and pad so that the
  min frame size is 64 bits
• Checksum 32 bit CRC

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MAC Address

• Identifying each individual network card uniquely
• 46 bits address in 48 bits string
• Binary 0 in MSB indicates ordinary address
• Binary 1 in MSB indicates the 46 bits address is
  a group address (for multicast)

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MAC Address

• If all address bit are 1-s then it is a broadcast
  (all nodes are getting the message)
• If two MSB are 0-s then the 46 bits address is a
  combination of source and destination MAC address

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MAC Address

• Examples of possible MAC addresses include
• 00-0C-F1-56-98-AD
• 00-11-F5-4B-20-56
• The first three bytes of this address identify
  the manufacture of this network device
• 00-0C-F1 for Intel
• Assigned by the IEEE and the database is
  available online at IEEE OUI and Company_id
  Assignments website

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Need for Frame Minimum Size

• In CSMA/CD, if there is a collision, the first
  node to detect it will send a jamming signal
• We need to calculate the maximum delay after a
  node sends a message until the first jamming
  signal is heard by all nodes
• Then from there we can calculate what is the
  minimum frame size so that NO nodes will finish
  transmitting before it hears the jamming signal

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Need for Frame Minimum Size

• The diagram below shows that there is a maximum
  of 2td delay before the first jamming signal is
  heard by every node(td propagation delay)

A
B
A
B
Frame sent at t 0s
At t td s, the frame almost reach the receiver
collision
A
B
A
B
At t td s, suddenly the receiver sends out frame
Jamming signal finishes propagating at t 2td s
Jamming signal sent out
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Need for Frame Minimum Size

• (compare this calculation to link utilization
  calculation)
• Hence max delay is a function of bit rate, max
  distance allowed and velocity of propagation
• Given
• Ethernet bit rate 10 Mbps (802.3 Standard for
  10Base5 and 10Base-T)
• Max distance 500m (802.3 Standard)
• Velocity of propagation 2 108 ms-1

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Need for Frame Minimum Size

• (compare this calculation to link utilization
  calculation)
• Hence
• td 2.5µs, hence 2td 5µs
• Bit duration 0.1µs
• No. bits traveling in 2td time 50
• Adding some gap for error, the best min frame
  size chosen is 64 bits
• 802.3 Std sets 512 bits as min, because it allows
  max distance of 2.5km with 4 passive repeaters

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Ethernet Physical Standard

• 10Base5 10 Mbps, Baseband transmission, 500m
  cable length
• 10Base2 10 Mbps, Baseband transmission, 200m
  cable length
• 10Base-T 10 Mbps, Baseband transmission, 500m
  UTP cable
• 100Base-TX 100 Mbps, Baseband transmission, 200m
  UTP cable

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Ethernet Physical Standard

• Wiring
• Unshielded Twisted Pair (UTP)
• Bundle of eight wires (only uses four)
• Terminates in RJ-45 connector

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Ethernet Physical Standard

• Hubs (10Base-T)
• A kind of passive repeater
• Used to connects nodes in bus topology
• Max length of UTP 100m
• Max no. hubs in series 4
• Hence, max distance between farthest nodes 500m

100m
10Base-T hubs
100m
100m
500m, 4 hubs
100m
100m
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Repeaters

• Regenerates signal
• Used to extend the network coverage
• Hubs are repeaters
• There will be a limit to the length of the
  farthest node due to physical signal limitation

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Bridges

• Used to join LANs
• Results in local internet
• May filter the data traffic

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Switches

• More intelligent kind of bridges
• Must be arranged in hierarchical arrangement
  only one path from one switch to another
• Due to its intelligent close to a small node,
  there is no limit in number of switches in a LAN
  as opposed to hubs

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Hubs vs Bridges vs Switches

• Hub
• Has many ports
• Redistributes data to all nodes
• It depends on the receiver to process the data
• Almost no intelligence
• Used to extend connection within standard limit

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Hubs vs Bridges vs Switches

• Bridge
• Only two ports
• Transfers data from one end to the other only if
  the receiver address is at the other end
• Have intelligence to interpret MAC addresses
• Used to join two separate LANs

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Hubs vs Bridges vs Switches

• Switch
• More than two ports
• Have intelligence to interpret MAC addresses
• Transfers data from one end to another only if
  the receiver address is at that end
• Extends LAN unlimitedly, but must conform to
  hierarchical (tree) structure
• Router
• Switch that works on IP address instead of MAC
• For internet instead of LAN
• Smart enough to do protocol conversion

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Hubs vs Bridges vs Switches
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IEEE 802.11/15/16Standards ofWireless Ethernet
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IEEE 802.11

• Uses CSMA/CA instead of CSMA/CD
• Could not detect collision due to hidden nodes
  (target nodes beyond signal range)
• Sender listen to the medium (air) to see whether
  it is busy or not
• After the medium is free for a period of DIFS
  (Distributed Inter-Frame Space 128µs), the
  sender sends RTS (request to send) signal to tell
  its intention, and others will make way

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IEEE 802.11a

• Frequency 5 GHz
• Maximum Speed 54 Mbps
• Range about 35 meters (varies)
• Encoding Scheme Orthogonal FDM (closely located
  frequencies but far enough not to interfere each
  other)

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IEEE 802.11b

• Frequency 2.4 GHz
• Maximum Speed 11 Mbps
• Range about 38 meters (varies)
• Encoding Scheme DSSS
• Modulation Technique BPSK(1 Mbps), QPSK(2
  Mbps), CCK(5.5 Mbps,11Mbps)

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IEEE 802.11g

• Frequency 2.4 GHz
• Maximum Speed 54 Mbps
• Range about 38 meters (varies)
• Encoding Scheme OFDM

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IEEE 802.11n

• Frequency 5 GHz, 2.4 GHz
• Modulation OFDM
• Maximum Speed 150 Mbps
• Range about 70 meters (varies)
• Encoding Scheme OFDM
• Addition of MIMO (Multiple Input Multiple Output)
• sender and receiver have 2 antennas to send and
  receive 2 signals (one is modified redundancy) to
  improve performance

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IEEE 802.16 WiMAX

• WiMAX is 802.11/Wi-Fi networks with coverage and
  cellular networks quality of service
• Stands for "Worldwide Interoperability for
  Microwave Access"
• Most of WiMAX physical layer definitions and
  topology follows those of 802.11
• Provider in Malaysia P1 Yes

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IEEE 802.16 WiMAX

• Consists of two standards Fixed Mobile
• Fixed WiMAX (IEEE 802.16d)
• Speed up to 70 Mbps
• Range up to 50 km
• Mobile WiMAX (IEEE 802.16e)
• Speed up to 30 Mbps
• Range up to 15 km

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IEEE 802.15 Bluetooth

• Open proprietary standard created by Ericsson
• Not a direct descendent if 802.11
• Designed for communication between electronics
  devices as alternative to cabled RS-232
• Consisting of 1 master devices and up to 8 slaves
• Logo

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IEEE 802.15 Bluetooth

• Frequency 2.4-2.8 GHz
• Speed 1 Mbps
• Range 10 meters
• Encoding Scheme FHSS with 79 channels at 1600
  hops per second
• Most common uses Mobile phone headset, wireless
  mouse keyboard

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Token Based Protocol
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IEEE 802.5 Token Ring

• The network is arranged in ring topology
• There is a special frame to be passed around the
  nodes named TOKEN
• Whoever is having the token can transmit data
  into transmission medium, otherwise it passes the
  token to the next node

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IEEE 802.5 Token Ring
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IEEE 802.4 Token Bus

• The network is arranged in bus topology
• Just as token ring, there is a special frame
  TOKEN used
• Whoever is having the token can transmit data
  into transmission medium, otherwise it passes the
  token to the next node
• The use of this type of protocol is shown by the
  presence of coaxial cable connector on the
  network card instead of 8P8C

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FDDI

• Fiber Distributed Data Interface
• Data rate 100Mbps
• Used as a backbone
• With multi-mode fiber any given ring segment can
  be up to 200 km in length
• A total of 500 stations can be connected with a
  maximum separation of 2 km
• Two complete rings to overcome failures

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FDDI
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FDDI Interface in High Speed LANs
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