1. EEC4113 Data Communication & Multimedia System Chapter 6: Media Access Control of Data Link Sub-Layer by Muhazam Mustapha, August 2010

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 Ethernet
IEEE Wire Ethernet
IEEE Token Ring

4. Media Access Control Sub-Layer

5. Media Access Control
Media Access Control is a sub-layer of data link layer in OSI’s 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

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

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

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)

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

10. ALOHA Protocols

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

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

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

14. Pure ALOHA
User A B C D E
Time

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)

16. Pure vs Slotted ALOHA Slotted ALOHA: S = Ge-G Pure ALOHA: S = Ge-2G
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)

17. Carrier Sense Multiple Access Protocols

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

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

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

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

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 if no one is sending
The above two steps is the contention state

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
contention
idle

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

25. Collision-Free Protocol

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

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

28. Collision-Free Protocols
Once all hosts write the binary bits accordingly 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

29. Collision-Free Protocols
8 contention slots
8 contention slots
8 contention slots 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 1 1 1 3 7 1 1 1 5 1 2
frames
frames
frames

30. Ethernet Topology

31. Topology
Bus
Linear Bus – 2 ends
Distributed Bus – more than 2 ends

32. Topology
Star
Ring

33. Topology
Mesh
Tree

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

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

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

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

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

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

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

41. Star Topology Broadcast Frame-switching device
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

42. IEEE 802.3 Standard of Ethernet

43. IEEE 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
0-1500
0-46 4
Bytes
S O F
Preamble
Destination address
Source address
Length
Data
Pad
Checksum

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

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

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)

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

48. MAC Address Examples of possible MAC addresses include:
00-0C-F 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

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

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 sent out
Jamming signal finishes propagating at t = 2td s

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

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

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

54. Ethernet Physical Standard
Wiring:
Unshielded Twisted Pair (UTP)
Bundle of eight wires (only uses four)
Terminates in RJ-45 connector

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
100m
500m, 4 hubs
100m

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

57. Bridges Used to join LANs Results in local internet
May filter the data traffic

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

59. Hubs vs Bridges vs Switches
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

60. Hubs vs Bridges vs Switches
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

61. Hubs vs Bridges vs Switches
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

62. Hubs vs Bridges vs Switches

63. IEEE 802.11 Standard of Wireless Ethernet

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

65. 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)

66. 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)

67. IEEE 802.11g Frequency = 2.4 GHz Maximum Speed = 54 Mbps
Range = about 38 meters (varies)
Encoding Scheme = OFDM

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

69. Token Based Protocol

70. 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

71. IEEE Token Ring

72. 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

73. 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

74. FDDI

75. FDDI Interface in High Speed LANs