EEC4113 Data Communication

1
EEC4113 Data Communication Multimedia
System Chapter 6 Media Access Control of Data
Link Sub-Layer by Muhazam Mustapha, October 2011
2
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

3
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

4
Media Access Control Sub-Layer
CO1
5
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

CO1
6
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

CO1
7
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
CO1
8
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)

CO1
9
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

CO1
10
ALOHA Protocols
CO1
11
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

CO1
12
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

CO1
13
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

CO1
14
Pure ALOHA
User
A
B
C
D
E
Time
CO1
15
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)

CO1
16
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)
CO1
17
Carrier Sense Multiple Access Protocols
CO1
18
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

CO1
19
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

CO1
20
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

CO1
21
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

CO1
22
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

CO1
23
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
CO1
24
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

CO1
25
Collision-Free Protocol
CO1
26
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

CO1
27
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

CO1
28
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

CO1
29
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
CO1
30
Ethernet Topology
CO1
31
Topology
Bus
Linear Bus 2 ends
Distributed Bus more than 2 ends
CO1
32
Topology
Star
Ring
CO1
33
Topology
Mesh
Tree
CO1
34
Bus Topology

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

CO1
35
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

CO1
36
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

CO1
37
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

CO1
38
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

CO1
39
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

CO1
40
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

CO1
41
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

CO1
42
IEEE 802.3 Standard of Ethernet
CO1
43
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
CO1
44
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

CO1
45
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

CO1
46
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)

CO1
47
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

CO1
48
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

CO1
49
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

CO1
50
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
CO1
51
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

CO1
52
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

CO1
53
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

CO1
54
Ethernet Physical Standard

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

CO1
55
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
CO1
56
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

CO1
57
Bridges

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

CO1
58
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

CO1
59
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

CO1
60
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

CO1
61
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

CO1
62
Hubs vs Bridges vs Switches
CO1
63
IEEE 802.11/15/16 Standards of Wireless Ethernet
CO1
64
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

CO1
65
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)

CO1
66
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)

CO1
67
IEEE 802.11g

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

CO1
68
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

CO1
69
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

CO1
70
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

CO1
71
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

CO1
72
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

CO1
73
Token Based Protocol
CO1
74
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

CO1
75
IEEE 802.5 Token Ring
CO1
76
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

CO1
77
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

CO1
78
FDDI
CO1
79
FDDI Interface in High Speed LANs
CO1