Title: EEC4113 Data Communication
1EEC4113Data Communication Multimedia
SystemChapter 6 Media Access Controlof Data
Link Sub-Layerby Muhazam Mustapha, October 2011
2Learning 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
3Chapter 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
4Media Access ControlSub-Layer
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5Media 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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6Ethernet
- 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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7Shared 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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8Multiple 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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9Channel 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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10ALOHA Protocols
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11ALOHA 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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12Pure 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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13Pure 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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14Pure ALOHA
User
A
B
C
D
E
Time
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15Slotted 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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16Pure 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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17Carrier Sense Multiple Access Protocols
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18Carrier 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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19CSMA
- 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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20Persistent 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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21CSMA/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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22CSMA/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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23CSMA/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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24CSMA/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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25Collision-Free Protocol
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26Collision-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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27Collision-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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28Collision-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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29Collision-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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30Ethernet Topology
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31Topology
Bus
Linear Bus 2 ends
Distributed Bus more than 2 ends
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32Topology
Star
Ring
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33Topology
Mesh
Tree
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34Bus Topology
- Use of multipoint medium
- All stations attach directly to transmission
medium (bus) through appropriate hardware
interfacing known as tap
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35Bus 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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36Tree 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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37Tree 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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38Ring 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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39Ring 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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40Star 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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41Star 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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42IEEE 802.3 Standard ofEthernet
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43IEEE 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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44Frame 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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45Frame 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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46MAC 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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47MAC 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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48MAC 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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49Need 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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50Need 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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51Need 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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52Need 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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53Ethernet 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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54Ethernet Physical Standard
- Wiring
- Unshielded Twisted Pair (UTP)
- Bundle of eight wires (only uses four)
- Terminates in RJ-45 connector
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55Ethernet 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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56Repeaters
- 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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57Bridges
- Used to join LANs
- Results in local internet
- May filter the data traffic
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58Switches
- 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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59Hubs 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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60Hubs 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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61Hubs 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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62Hubs vs Bridges vs Switches
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63IEEE 802.11/15/16Standards ofWireless Ethernet
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64IEEE 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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65IEEE 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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66IEEE 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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67IEEE 802.11g
- Frequency 2.4 GHz
- Maximum Speed 54 Mbps
- Range about 38 meters (varies)
- Encoding Scheme OFDM
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68IEEE 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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69IEEE 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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70IEEE 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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71IEEE 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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72IEEE 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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73Token Based Protocol
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74IEEE 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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75IEEE 802.5 Token Ring
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76IEEE 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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77FDDI
- 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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78FDDI
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79FDDI Interface in High Speed LANs
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