1. The Medium Access Sublayer
Chapter 4
The Medium Access Sublayer

2. The Medium Access Layer
5.1 Channel Allocation problem
- Static and dynamic channel allocation in LANs & MANs
5.2 Multiple Access Protocols
- ALOHA, CSMA, CSMA/CD, Collision-free protocols, Limited-contention protocols, Wireless LAN protocols
5.3 Ethernet
- Cabling, MAC sublayer protocol, Backoff algorithm, Performance, Gigabit Ethernet, Logical Link Control
5.4 Wireless LANs
protocol stack, physical layer, MAC sublayer protocol, frame structure

3. 5.5 Broadband Wireless
- Comparison of with , protocol stack, frame structure
5.6 Bluetooth
- Bluetooth architecture, Application, Protocol stack, Frame structure
5.7 Data Link Layer Switching
- Bridges from 802.x to 802.y, Local internetworking, Spanning tree bridges, Remote bridges

4. What is MAC Network assumption: Broadcast channel
One channel, many stations
Competition, interference among stations.
MAC: Medium Access Control
Also known as Multiple-Access Control
The protocol used to determine who goes next on a shared physical media
Classification of MAC protocols
Channel allocation (centralized)
Contention based protocols (distributed)
Contention – free protocols (distributed)

5. Medium Access Sublayer
Key issue for broadcast network
who can use the channel when there is competition for it
Medium Access Control:
a sublayer of data link layer that controls the access of nodes to the medium.
Broadcast channels are also referred as multiaccess channels or random access channels
Allocation of a single broadcast channel among competing users:
Static
Dynamic

6. Static Channel Allocation
5.1 The Channel Allocation problem
Static Channel Allocation
FDMA
The whole spectrum is divided into sub-frequency.
TDMA
Each user has its own time slot.
CDMA
Simultaneous transmission, Orthogonal code
Analogy:

7. The M/M/1 Queue Average number of customers
Applying Little’s Theorem (L*=T*), we have
Similarly, the average waiting time and number of customers in the queue is given by
E[ ]
= L L
T*:delay time in system

8. Example: Slowing Down
M/M/1 system: slow down the arrival and service rates by the same factor m
Utilization factors are the same ⇒stationary distributions the same, average number in the system the same
Delay in the slower system is m times higher
Average number in queue is the same, but in the 1st system the customers move out faster

9. Example: Statistical MUX-ing vs. TDM or FDM
m identical Poisson streams with rate λ/m; link with capacity 1; packet lengths iid, exponential with mean 1/μ
Alternative: split the link to m channels with capacity 1/m each, and dedicate one channel to each traffic stream
Delay in each “queue” becomes m times higher
Statistical multiplexing vs. TDM or FDM
iid: Independent and Identically Distributed

10. The Channel Allocation problem
5.1.1 Static channel Allocation in LANs and WANs
Frequency Division Multiplexing (FDM)

12. Both FDM and TDM are not good when traffic is bursty!
We will now explore dynamic methods

13. 5.1.2 Dynamic Channel Allocation in LANs and WANs
Five Assumptions
Station Model. The model consists of N independent stations, each generates the frame with probability in an interval Once a frame is generated, the station is blocked.
Single Channel Assumption. A single channel is available for all communication.
Collision Assumption. If two frames are transmitted simultaneously, they are destroyed and must be retransmitted again later. There are no other errors.

14. 4a. Continuous Time. Frame transmission can begin at any instant.
4b. Slotted Time. Time is divided into slots. Frame transmission always begin at the start of a slot.
5a. Carrier Sense. Stations can tell if the channel is in use before trying to use it. (ex. LANs)
5b. No Carrier Sense. Stations cannot sense the channel before trying to use it. (ex. satellite network due to long propagation delay)

15. 5.2 Multiple Access Protocols
ALOHA
Carrier Sense Multiple Access Protocols
Collision-Free Protocols
Limited-Contention Protocols
Wavelength Division Multiple Access Protocols
Wireless LAN Protocols

16. ALOHA
Users send whenever they want to send. If it fails, wait random time and resend it.
Independent stations
Single channel assumption
Collision occurs
Types of ALOHA
Pure ALOHA: stations transmit at any time (Continuous time)
Slotted ALOHA: Transmission can only occur at certain time instances
carrier sense vs no carrier senses

17. In pure ALOHA, frames are transmitted at completely arbitrary times.
Users transmit whenever they have data to be sent.
The colliding frames are destroyed.
The sender waits a random amount of time and sends it again.
1970s from University of Hawaii
Pure ALOHA (infinite population)
In pure ALOHA, frames are transmitted at completely arbitrary times.

18. Vulnerable period for the shaded frame.
Pure ALOHA (2)
Vulnerable period for the shaded frame.

22. Poisson pmf (1)
Suppose we are observing the arrival of jobs to a large computation center for the time interval (0, t]
Assume that for each small interval of time Dt, the probability of a new job arrival is lDt, where l is the average arrival rate..
If Dt is sufficiently small, the probability of two or more jobs arriving in the interval of duration Dt may be neglected.
Divide (0, t] into n subintervals of length t/n, and suppose the arrival of a job in any given interval is independent of the arrival of a job in any other interval.
n very large => the n intervals constitutes a sequence of Bernoulli trials with the probability of success p = lt / n
Bernoulli trials: P(X = 0) = p, with P(X = 1) = 1p

23. Poisson pmf (2)
The Probability of k arrivals in a total of n intervals each with a duration t/n is approximately given by
As n -> infinity =>
Let t be a frame time => lt = G

25. Pure ALOHA (2) efficiency: 18.4 % for channel utilization at best
Assume that infinite population of users generates new frames according to a Poisson distribution with mean N frames per frame time, where 0 < N < 1.
Assume that the probability of k transmission attempts per frame time, old and new combined, is also poisson, with mean G per frame time.
P0: probability that a frame does not suffer a collision
throughput S = G * P0 (Offered load times transmission succeeding prob.)
vulnerable interval: t0 ~ t0+2t (See Fig. 4-2)
probability that k frames are generated during a given frame time is given by the Poisson distribution
Probability of no other traffic during the vulnerable period,
P0 = e -2G , 2G: mean of two frame time.
S = Ge -2G (See Fig. 4-3) 
Pure ALOHA (2) 

26. Throughput versus offered traffic for ALOHA systems.
0.368
0.184
Throughput versus offered traffic for ALOHA systems.

27. Slotted ALOHA

30. 5.2.2 Carrier Sense Multiple Access Protocols
100% persistent to wait the channel

31. Zero persistent to wait the channel

33. Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access protocols.

35. CSMA/CD Abort transmission as soon as they detect a collision Fig. 4-5
saves time and bandwidth
waits a random time and tries again
Fig. 4-5
minimum time to detect collision: the signal propagates from one station to the other
worst case: 2t (t : propagation time between two farthest stations)
model the contention interval as a slotted ALOHA system with slot = 2t
special signal encoding: to detect a collision of two 0-volt signals
No MAC sublayer protocol guarantees reliable delivery. Packets may be lost due to
collision
lack of buffer space
missed interrupt

36. CSMA with Collision Detection
CSMA/CD can be in one of three states: contention, transmission, or idle.

37. 5.2.3 Collision-Free Protocols
Collision is serious (affects performance) as.
large t: long cable (increase collision probability)
short frames (undetectable collision): high bandwidth (propagation dominate the delivering time.
Bit-Map Protocol (See Fig. 4-6)
A cycle consists of a contention period and a data transmission period.
Contention period contains N slots, one bit for a station
a station inserts 1 at its slot when has data. After the contention period, stations transmit data in the sequence in the contention period.
problem: overhead is 1 bit per station

38. The basic bit-map protocol.
5.2.3 Collision-Free Protocols
The basic bit-map protocol.
4-6.

39. Collision-Free Protocols (2)
Binary Countdown
Give priority to higher address by OR bit-by-bit addresses of the stations waiting for transmission.
virtual station number: to change priority
The binary countdown protocol. A dash indicates silence.

40. ln N.

41. 4.2.4 Limited-contention Protocols

42. Limited-Contention Protocols

46. Success prob. decreases as ready station number increases.
4.8
4.8.
4.8.

47. Adaptive Tree Walk Protocol (Method for testing soldiers for syphilis)
Depth first search
Light load
level 0; 8/20 below it.
Collision #
Heavy load
level 3
Fig. 5-9 The tree for eight stations.

50. WDMA Protocols (1) Channel allocation schemes:
divide the channel into subchannels using FDM, TDM, or both, and dynamically allocate them as needed
commonly used in fiber optic LANs: different conversations to use different wavelengths (frequencies) at the same time

51. WDMA (2)
Each station is assigned two channels : control channel (narrow) and data channel (broad)
m slots in control channel and n+1 slots in data channel.
Support three classes of traffic
constant data rate connection-oriented traffic
variable data rate connection-oriented traffic
datagram traffic: UDP packets

52. WDMA Protocols (3) Each station has Communication from A to B
fixed-wavelength receiver for its own control channel
tuneable transmitter to other station’s control channel
fixed-wavelength transmitter to output data frames
tuneable receiver: selecting a data transmitter to listen to
Communication from A to B
A inserts a Connection Request in a free slot on B’s control channel.
If B accepts, A sends its data on its data channel.

53. Wavelength Division Multiple Access Protocols
Fig Wavelength division multiple access.

54. 4.2.6 Wireless LAN Protocols

55. 5-11.

56. 4-11
5-11

57. 4-12.
4-12
4-12
4-12,

58. 5-12.

60. 4.3 Ethernet Ethernet Cabling Manchester Encoding
The Ethernet MAC Sublayer Protocol
The Binary Exponential Backoff Algorithm
Ethernet Performance
Switched Ethernet
Fast Ethernet
Gigabit Ethernet
IEEE 802.2: Logical Link Control
Retrospective on Ethernets

61. The most common kinds of Ethernet cabling.

62. Ethernet Cabling (2) Three kinds of Ethernet cabling.
(a) 10Base5, (b) 10Base2, (c) 10Base-T.
Three kinds of Ethernet cabling.
(a) 10Base5, (b) 10Base2, (c) 10Base-T

63. Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.
Ethernet Cabling (3)
Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.

64. Manchester Coding Encoding (See Fig. 4-16)
Binary encoding: can not be used
Manchester encoding
Binary 1: start at voltage high and down to low at the middle part of bit time
Binary 0: start at voltage low and down to high at the middle part of bit time
synchronous in the middle
requires double bandwidth
used by all baseband systems: v
Differential Manchester encoding
Binary 1: don’t do transition at the start of the bit time
Binary 0: do transition at the start of the bit time
better noise immunity
more complex

65. Ethernet Cabling (4)
(a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding.

66. 802.3 MAC Sublayer Protocol Frame format (See Fig. 4-17)
high order bit of destination address
0: ordinary address
1: group address for multicasting
broadcasting: all 1 bits
Valid frame must be 64 bytes long
from destination address to checksum
pad field
all frames must take > 2t to send (See Fig. 4-18)
To prevent a station from completing the transmission of a short frame before making sure the frame is collided or not
Checksum

67. Ethernet MAC Sublayer Protocol
48 bits 7 1
SOF: Start of frame
Frame formats. (a) DIX Ethernet, (b) IEEE
DIX (DEC, Intel, Xerox)

68. Ethernet MAC Sublayer Protocol (2)

69. Binary Exponential Backoff
After i’th collisions, random number 0 ~ 2i - 1  1023
time slot = 2t (51.2 us)
after 16 collisions: failure, recovery by higher layers
Acknowledgement
CSMA/CD provides no acknowledgements
destination verifies checksum (for fear of noise)
ACK frame (not include in the protocol)
the 1st contention slot following successful transmission (proposed by Toloro)

70. 4.3 Ethernet Performance Assumptions
Heavy and constant load, that is, stations always ready to transmit
Each station transmits during a contention slot with probability
The probability A that some station acquires the channel in that slot is
A is maximized when ,
with as

71. The probability that the contention interval has exactly slots in
it is
so the mean number of slots per contention is given by
Since each slot has a duration , the mean contention interval
Assuming optimal , i.e. as
If the mean frame takes seconds to transmit then
channel efficiency =
where B: bandwidth
F: frame length
L: cable length
C: propagation speed

72. Efficiency of Ethernet at 10 Mbps with 512-bit slot times.
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times.

73. Switched 802.3 LANs System Collision domain
high speed backplane: over 1 Gbps
plug-in line cards
several connectors
100/10 BaseT single host
hub (Fig. 5-20)
traffic: on the same card or via backplane
Collision domain
each card: all ports on the same card are wired together to form a local on-card LAN
each port: each port is buffered, all ports receive or transmit in parallel

74. A simple example of switched Ethernet.
100/
A simple example of switched Ethernet.

75. The original fast Ethernet cabling.
The reasons for fast Ethernet
1. The need to be backward compatible with existing Ethernet LANs
2. The fear that a new protocol might have unforeseen problems
3. The desire to get the job done before the technology changed
All fast Ethernet systems use hubs and switches
100 Base-T4 uses 8B/6T coding and 100 Base-TX uses 4B/5B coding
The original fast Ethernet cabling.

76. Gigabit Ethernet
• All configurations of gigabit Ethernet are point-to-point
• Gigabit Ethernet supports full-duplex mode (with switch) and half-duplex-mode (with hub)
• CSMA/CD protocol is required for half- duplex mode operation (maximum distance is 25 meters)
• When carrier extension (512 bytes frame) and frame bursting are used
the distance can be 200meters
(a) A two-station Ethernet. (b) A multistation Ethernet.

77. Gigabit Ethernet cabling.
•Gigabit Ethernet supports both copper and fiber cabling
•Two wavelengths are permitted = 0.85μm and 1.3μm
•Three fiber core diameters are permitted = 10, 50, and 62.5μm
Gigabit Ethernet cabling.

78. IEEE 802.2: LLC Logical Link Control Three service options of LLC
all 802 LANs and MAN: best-effort datagram services
on top of all 802 LANs and MAN: Fig. 4.33
a single format and interface to the Network Layer
Three service options of LLC
unreliable datagram service
acknowledged datagram service
reliable connection-oriented service

79. IEEE 802.2: Logical Link Control
(a) Position of LLC. (b) Protocol formats.

80. Retrospective on Ethernet
1. Ethernet is simple and flexible
- reliable, cheap, easy to maintain, easy
to install
2. Ethernet interworks easily with TCP/IP
3. Ethernet has been able to evolve in certain crucial ways
- speeds gone up
- hub and switches introduced

81. Wireless LANs The 802.11 Protocol Stack The 802.11 Physical Layer
The MAC Sublayer Protocol
The Frame Structure
Services

82. Part of the 802.11 protocol stack.
FHSS: Frequency Hopping Spread Spectrum (dwell time < 400ms)
DSSS: Direct Sequence Spread Spectrum (up to 2 Mb/s)
OFDM: Orthogonal Frequency Division Multiplexing ( up to 54 Mb/s)
HR-DSSS: High Rate Direct Sequence Spread Spectrum (11Mb/s)
Part of the protocol stack.

83. 802.11 MAC Sublayer Protocol (a) The hidden station problem.
(b) The exposed station problem.

84. The 802.11 MAC Sublayer Protocol (2)
NAV: Network Allocation Vector
The use of virtual channel sensing using CSMA/CA.

85. MAC Sublayer Protocol
supports two modes of operation: DCF and PCF
Distributed Coordination Function (DCF)
uses CSMA/CA (CSMA / with Collision Avoidance)
(a) The first mode supported by CSMA/CA
(1) When a station wants to transmit, it senses the channel
(2) If it idle, it just starts transmitting (The sender does not sense the channel while transmitting)
(3) If the channel is busy, the sender defers until it goes idle and then starts transmitting
(4) If a collision occurs, it wait a random amount of time (exponential back off) and then try again later

86. (b) The second mode of CSMA/CA is based on MACAW
(Multiple Access with Collision Avoidance for Wireless)
(1) When a station wants to transmit, it senses the channel
(2) If the channel is idle longer than SIFS interval it transmits an
RTS (Request to Send, 30 bytes) which contains the length
of the data frame
(3) After received the RTS frame, the receiving station replies
with a CTS (Clear to Send) frame which contains the data
length (copied from the RTS frame)
(4) Upon receipt of the CTS, the sender transmits the frame
(5) All stations heard the RTS frame should remain silent for a
period of time (an estimation based on the information of RTS)
(6) All stations heard the CTS frame should remain silent for a
period of time (an estimation based on the information of CTS)
(7) If the channel is busy, the sender goes to step1

88. Point Coordination Function (PCF) (contention free)
(1) When a station wants to gain control of the medium, it sends out a beacon at the end of PIFS.
The beacon frame contains system parameters such as
hopping sequences, and dwell time (for FHSS), clock
synchronization, length of the contention free period, etc.
(2) All other stations heard the beacon will keep silent and wait for polling sign up frame
(3) After gained control, it invites new stations to sign up for polling service
(4) At the end of the contention free period, all station return to DCF mode

89. 802.11 MAC Sublayer Protocol (3)
• To deal with the problem of noise channels, allows frames to be
fragmental into smaller pieces, each with its own checksum and ack.
(using stop-and-wait protocol)
A fragment burst.

90. 802.11 MAC Sublayer Protocol (4)
PCF and DCF can coexist within one cell
Interframe spacing in

91. Frame Structure
The data frame.

92. Version: Protocol version
Type: data, control, or management
Subtype: RTS, CTS, ack, …
To DS and from DS: to or from inter cell distribution
system (e.g. Ethernet)
MF: more fragments
Retry: retransmission
Power management: put the receiver into sleep state or
take it out
More: additional frames coming
W: wired equivalent privacy
O: processed strictly in order
Duration: time length of the frame and ack
Addresses1.2.3 and 4: Source, destination, the source and
destination base stations for intercell traffic
Sequence: Sequence No.

93. 802.11 Services Association: To connect to a base station
Distribution Services
(managing cell membership, and interacting
with station outside the cell)
Association: To connect to a base station
Disassociation: To disconnect from a base station
Reassociation: To change its preferred base
station
Distribution: How to route frames (local or intercell)
Integration: Translation from to other
protocol frame format

94. 802.11 Services Intracell Services Authentication
Deauthentication (leave the network)
Privacy: managing the encryption
and decryption
Data Delivery

95. Broadband Wireless Comparison of 802.11 and 802.16
The Protocol Stack
The Physical Layer
The MAC Sublayer Protocol
The Frame Structure

96. Protocol Stack
The Protocol Stack.

97. The 802.16 transmission environment.
Physical Layer
The transmission environment.

98. Frames and time slots for time division duplexing.
Physical Layer (2)
Frames and time slots for time division duplexing.

99. 802.16 MAC Sublayer Protocol Service Classes Constant bit rate service
Real-time variable bit rate service
Non-real-time variable bit rate service
Best efforts service

100. (a) A generic frame. (b) A bandwidth request frame.
Frame Structure
(a) A generic frame. (b) A bandwidth request frame.

101. Bluetooth Bluetooth Architecture Bluetooth Applications
The Bluetooth Protocol Stack
The Bluetooth Radio Layer
The Bluetooth Baseband Layer
The Bluetooth L2CAP Layer
The Bluetooth Frame Structure

102. Bluetooth Architecture
Two piconets can be connected to form a scatternet.

103. Bluetooth Applications
The Bluetooth profiles.

104. Bluetooth Protocol Stack
The version of the Bluetooth protocol architecture.

105. Bluetooth Frame Structure
A typical Bluetooth data frame.

106. Data Link Layer Switching
Bridges from 802.x to 802.y
Local Internetworking
Spanning Tree Bridges
Remote Bridges
Repeaters, Hubs, Bridges, Switches, Routers, Gateways
Virtual LANs

107. Data Link Layer Switching
Multiple LANs connected by a backbone to handle a total load higher than the capacity of a single LAN.

108. Bridges To connect multiple LANs Operate in data link layer
bridges do not examine network layer header: copy IP, IPX, OSI packets equally well
Six reasons why single organization with multiple LANs
autonomy: university and corporate departments
the organizations may be spread over several buildings
load: each LAN with its own file server to restrict traffic locally
physical distance is too great
reliability: discretion about what is forwarded
security
How it works: Fig two-port bridge

109. Operation of a LAN bridge from 802.11 to 802.3.
Bridges from 802.x to 802.y
Operation of a LAN bridge from to

110. The IEEE 802 frame formats. The drawing is not to scale.
Bridges from 802.x to 802.y (2)
The IEEE 802 frame formats. The drawing is not to scale.

111. Local Internetworking
When the bridges are first plugged in, they use the flooding algorithm and the backward learning algorithm to establish the routing table
A configuration with four LANs and two bridges.

112. Routing Procedure of Bridges
If destination and source LANs are the same, discard the frame
If the destination and source LANs are different, forward the frame
If the destination LAN is unknown, use flooding

113. Two parallel transparent bridges.
Spanning Tree Bridges
To prevent looping
Looping
Two parallel transparent bridges.

114. Spanning Tree Bridges Spanning Tree Abstraction (See Fig. 5-44)
exactly one path from every LAN to every other LAN: no loops
root: the bridge with lowest serial number
a tree of shortest paths from root to every other bridge and LAN
can adapt to dynamic topology

115. Procedure to Establish a Spanning Tree
1. Take the bridge with lowest serial number as the root
Compute the shortest path from the root to ever bridge and LAN
Connect these shortest paths to from a tree (no looping)

116. Spanning Tree Bridges (2)
(a) Interconnected LANs. (b) A spanning tree covering the LANs. The dotted lines are not part of the spanning tree.

117. Remote bridges can be used to interconnect distant LANs.
To connect two (or more) distance LANs
Remote bridges can be used to interconnect distant LANs.

118. Repeaters, Hubs, Bridges, Switches, Routers and Gateways
(a) Which device is in which layer.
(b) Frames, packets, and headers.

119. Repeaters, Hubs, Bridges, Switches, Routers and Gateways (2)
When two frames arrive simultaneously at a hub, they will collide
Bridges and switched will route the frames based on their destination addresses
Bridges connect LANs
(a) A hub. (b) A bridge. (c) a switch.

120. A building with centralized wiring using hubs and a switch.
Virtual LANs
A building with centralized wiring using hubs and a switch.

121. Reasons for Virtual LANs
Fitting into the organization structure
Loading partition
Relieving broadcast storm

122. Virtual LANs (2)
VLANs are based on specially-designed VLAN-aware switches (bridges)
(a) Four physical LANs organized into two VLANs, gray and white, by two bridges. (b) The same 15 machines organized into two VLANs by switches.

123. Each VLAN is assigned a distinct color
Each VLAN is assigned a distinct color. Three methods are used to distinguish the color of an incoming frame
Every port is assigned a VLAN color (when a host moved, the port must be reassigned)
Every MAC address is assigned a VLAN color
Every layer 3 protocol or IP address is assigned a VLAN color
(The payload must be examined by the data link layer, which violates the rule: independence of the layers. When the layer 3 protocol changed, the switch fails.)

124. There are some issues for VLAN
What is the VLAN field format?
How to identify VLAN field?
Who generates the VLAN field?
What happens to frames that are already the maximum size?
The 802.1Q will solve these problems

125. IEEE 802.1Q Standard To support VLAN, switches must be VLAN-aware.
Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded symbols are VLAN aware. The empty ones are not.

126. The 802.3 (legacy) and 802.1Q Ethernet frame formats.
IEEE 802.1Q Standard (2)
Priority: This field makes it possible to distinguish hard real-time traffic from soft
real-time traffic from time-insensitive traffic.
CFI (Canonical Format Indicator): To indicate that the payload contains
VLAN Identifier: To indicate which VLAN the frame belong to.
The (legacy) and 802.1Q Ethernet frame formats.

127. Channel allocation methods and systems for a common channel.
Summary
Channel allocation methods and systems for a common channel.