The Medium Access Control (MAC) Sublayer

1
The Medium Access Control (MAC)Sublayer

• Chapter 4

2
The Channel Allocation Problem

• Static Channel Allocation in LANs and MANs
• Example FDM, where each link may be assigned a
  different frequency.
• Works ok for only a small number of links
• Performance is poor
• Dynamic Channel Allocation in LANs and MANs
• A common physical channel is to be shared
• How to support effective sharing, e.g., which
  link should use the channel when and for how
  long, etc.

3
Performance
4
Dynamic Channel Allocation in LANs and MANs

1. Station Model.
2. Single Channel Assumption.
3. Collision Assumption.
4. (a) Continuous Time.(b) Slotted Time.
5. (a) Carrier Sense.(b) No Carrier Sense.

5
Multiple Access Protocols

• ALOHA
• Carrier Sense Multiple Access Protocols
• Collision-Free Protocols
• Limited-Contention Protocols
• Wavelength Division Multiple Access Protocols
• Wireless LAN Protocols

6
Pure ALOHA

• In pure ALOHA, frames are transmitted at
  completely arbitrary times.

7
Pure ALOHA (2)

• Analysis background
• Poisson arrivals
• Frames arrive at random times
• Random ? probabilistic behavior
• N frames arriving in time t?
• N is random (Poisson distribution)
• Problem probability of no arrival in a time
  interval 2 frames ?
• No collisions ? no arrivals in this interval
• Time interval between two successive arrivals is
  Exponentially distributed

• Vulnerable period for the shaded frame.

8
Pure ALOHA (3)

• Slotted Aloha Channel is divided into time
  slots.
• Any sender can access the channel at the
  beginning of a time slot
• Collision window t G

• Throughput versus offered traffic for ALOHA
  systems.

t
t
2t
3t
4t
9
Carrier Sense Multiple Access (CSMA)

• Slotted Aloha can achieve a maximum of 37
  channel utilization
• Why poor utilization? Senders do not bother to
  monitor the channel
• This leads to higher collision probability
• All senders monitor the channel to find out any
  one is already using the channel ? Carrier sense
  
• If someone is using the channel what to do
• Sender keeps itself busy in listening to the
  channel all the time (Persistent CSMA)
• Sender backs off (sleeps) for random time before
  trying again (non-persistent CSMA)
• Try again with probability p (lt 1) (p-persistent
  CSMA)

10
Persistent and Nonpersistent CSMA

• Comparison of the channel utilization versus load
  for various random access protocols.

11
CSMA with Collision Detection

• CSMA/CD can be in one of three states
  contention, transmission, or idle.

12
Collision-Free Protocols
t0
t1
t2
t6
t7

• The basic bit-map protocol.

• Low numbered stations are at a disadvantage (they
  have less time to react)
• Overhead d/(Nd) (d frame size, N contention
  bits)
• Low load overhead is higher
• High load overhead Nd/(NNd) d/(d1)
• Does not scale well i.e., as N becomes large.

13
Collision-Free Protocols (2)

• N stations can be coded by log2N bits (instead of
  N)
• Channel efficiency d/(d log2N)

• The binary countdown protocol. A dash indicates
  silence.

14
Limited-Contention Protocols

• Broadly, two methods for shared access
• CSMA
• Collision Free (CF)
• Performance small delay at low load, high
  channel utilization at high loads
• Low load ALOHA or CSMA have low delay CF high
  delay
• High load ALOHA, CSMA too many collisions
• High load collision free, but at the expense of
  increased overhead
• Mix the two Limited contention protocols

• Acquisition probability for a symmetric
  contention channel.

15
Adaptive Tree Walk Protocol

• The tree for eight stations.

• Assume previous frame was sent successfully. In
  the following contention slot 0
• All stations are permitted to contend (I.e., we
  are node 1, level 0)
• If only one (of 8) is ready, it acquires the
  channel.
• Else, there was a contention and slot 1 is
  offered to stations under node 2
• If stations under node 2 incur a collision,
  offer slot 3 to stations under 4 etc.
• If none of the stations under 2 want the
  channel, next slot (3) is offered to node 3
• Essentially, depth first traversal of the binary
  tree.

16
Wavelength Division Multiple Access Protocols

• Wavelength division multiple access.

17
Wireless LAN Protocols

• A wireless LAN. (a) A transmitting. (b) B
  transmitting.
• Essentially, CSMA approach
• What really matters is the interference at the
  receiver.
• (a) node C may not hear node A, even though
  B can (hidden node problem)
• (b) node B talking to A which can be heard
  by C, thus preventing C from talking to D,
  even though, C can not be heard by A (Exposed
  node problem)

18
Wireless LAN Protocols (2)

• The MACA protocol. (a) A sending an RTS to B.
• (b) B responding with a CTS to A.

19
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 Ethernet

20
Ethernet Cabling

• The most common kinds of Ethernet cabling.

21
Ethernet Cabling (2)

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

22
Ethernet Cabling (3)

• Cable topologies. (a) Linear, (b) Spine, (c)
  Tree, (d) Segmented.

23
Manchester Coding (Clock Embedding)

• (a) Binary encoding, (b) Manchester encoding,
  (c) Differential Manchester encoding.

• Each bit needs to be sampled in the middle
• Because and noise and clock drift, middle point
  is difficult do establish
• Manchester coding embeds clock within the data
  itself
• Clock can be recovered from the data (strong
  signal at 2f)
• Effective bandwidth is halved.

24
IEEE 802.2 Logical Link Control

• (a) Position of LLC. (b) Protocol formats.

25
Ethernet MAC Sublayer Protocol
(a)

• Frame formats
• (a) DIX Ethernet, (b) IEEE 802.3.

(b)

• Ethernet Address 6 bytes or 48 bits long
  (b0..b47)
• Point to point address
• Broadcast address FFFFFFFFFFFF
• Multicast address range starting from 01005e
  b470

26
Ethernet MAC Sublayer Protocol (2)

• Collision detection can Take as long as 2t

• Minimum packet size considerations
• Collision (if occurring) must be detected while
  sender is in the transmission state
• That is, frame duration should be gt 2t
• Minimum frame t/bit rate
• Bit rate 10Mbps, Max. cable length 2500m 4
  repeaters, 2t ? 50 µsec
• Min. frame size 500 bits or 512 bits 64 bytes
  (hence the need for padding)
• Collision ? binary exponential back off algorith
• After collision, time is discretized into slots
  of duration 2t 51.2 µsec
• 1st collision wait 0 or 1 slot time 2nd
  collision wait 0, 1,2,3 3rd 0..7 x 51.2 µsec
• Max 1023 slots

27
Ethernet Performance

• p prob. that a station contends
• A prob. that some station is successful
• Assume that ith station is successful
• only the ith station wants to transmit (p)
• none of the other (k-1) stations xmit

• prob. that contention interval j slots?
• First (j-1) slots had collisions, jth slot
  success qj A(1-A)j-1
• Mean of collisions before success

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

• For optimal p, mean contention interval

28
Switched Ethernet

• A simple example of switched Ethernet.

29
Fast Ethernet

• The original fast Ethernet cabling.

30
Gigabit Ethernet

• (a) A two-station Ethernet. (b) A multistation
  Ethernet.

31
Gigabit Ethernet (2)

• Gigabit Ethernet cabling.

32
Wireless LANs

• The 802.11 Protocol Stack
• The 802.11 Physical Layer
• The 802.11 MAC Sublayer Protocol
• The 802.11 Frame Structure
• Services

33
The 802.11 Protocol Stack

• Part of the 802.11 protocol stack.

34
The 802.11 MAC Sublayer Protocol

• (a) The hidden node problem.
• (b) The exposed node problem.

• Wireless technology inherently can not support
  CSMA/CD
• The CD part is not possible (receiver will get
  saturated with its stations transmitter)
• 802.11 instead uses CSMA/CA (collision avoidance)
• Collision avoidance use of DCF(mandatory) and
  PCF (optional uses WAP)

35
802.11 Operation Modes
Ad-hoc mode
Infrastructure mode
36
Coordination Modes

• DCF Distributed coordination function stations
  communicate among themselves
• PCF Point coordination function controlled by
  AP used as a base station
• AP polls all stations that want to communicate
• AP stores and forwards frames
• Reservation is implied via polling, hence
  collision free
• PCF is optional

37
The 802.11 MAC DCF

• The use of virtual channel sensing using CSMA/CA.

38
802.11 MAC Frame Fragmentation

• Need to fragment a frame wireless channel
  exhibits high BER, Perr p 10-4
• Ethernet frame 1518 bytes ? n12,144 bits
• Prob. Of entire frame incurring no error (1-p)n
  lt 0.30
• Why not make n smaller ? entire is broken down
  into smaller fragments
• Each fragment is individually ACKed
• Higher probability of fragment being received
  correctly
• Less time overhead to re-transmit fragments.

• A fragment burst.

39
802.11 MAC Inter-frame Timings

• Interframe spacing in 802.11.

40
The 802.11 Frame Structure
12-bit frame, 4-bit fragement
Re-xmitted fragment (lost ACK?)
WEP in use
Data Control Mgmt
POLL RTS CTS ACK etc.
A1 A2 A3 A4
DA, SA, BSSID, N/A DA,
BSSID SA N/A BSSID SA
DA N/A RAAP TAAP DA
SA

• 0 0
• 0 1
• 0
• 1 1

More fragments of this frame are remaining
Used only by AP to inform that it has more
frames (buffered) for the dest. station

• The 802.11 data frame.

41
802.11 Services
Services 5 distribution service 4 station
services

• Distribution service

• Association
• Disassociation
• Re-association
• Distribution (wireless, routing..)
• Integration (Bridging)

42
802.11 Services

• Intracell (station) Services

• Authentication
• Deauthentication (on leaving)
• Privacy (must be encrypted)
• Data Delivery

43
Broadband Wireless

• 802.16 is being discussed by the IEEE
• Connecting building using broadband wireless
• 802.11 concerned with connecting mobile stations
• Higher bandwidth (16-55 Ghz)
• Security
• Various traffic classes

44
The 802.16 Protocol Stack

• The 802.16 Protocol Stack.

45
The 802.16 Physical Layer

• The 802.16 transmission environment.

46
The 802.16 Physical Layer (2)

• Frames and time slots for time division duplexing.

47
The 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

48
The 802.16 Frame Structure

• (a) A generic frame. (b) A bandwidth request
  frame.

49
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

50
Bluetooth Architecture

• Two piconets can be connected to form a
  scatternet.

51
Bluetooth Applications

• The Bluetooth profiles.

52
The Bluetooth Protocol Stack

• The 802.15 version of the Bluetooth protocol
  architecture.

53
The Bluetooth Frame Structure

• A typical Bluetooth data frame.

54
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

55
Data Link Layer Bridging

• Multiple LANs connected by a backbone to handle a
  total load higher than the capacity of a single
  LAN.

56
Bridges from 802.x to 802.y

• Transparent bridging stations should be
  oblivious of segmentation
• Different segments may using different MAC/LLC

• Operation of a LAN bridge from 802.11 to 802.3.

57
Bridges from 802.x to 802.y (2)

• The IEEE 802 frame formats. The drawing is not
  to scale.

58
Local Internetworking

• A configuration with four LANs and two bridges.

59
Spanning Tree Bridges

• Two parallel transparent bridges.

60
Spanning Tree Bridges (2)

• (a) Interconnected LANs. (b) A spanning tree
  covering the LANs. The dotted lines are not part
  of the spanning tree.

61
Remote Bridges

• Remote bridges can be used to interconnect
  distant LANs.

62
Repeaters, Hubs, Bridges, Switches, Routers and
Gateways

• (a) Which device is in which layer.
• (b) Frames, packets, and headers.

63
Repeaters, Hubs, Bridges, Switches, Routers and
Gateways (2)

• (a) A hub. (b) A bridge. (c) a switch.

64
Virtual LANs

• A building with centralized wiring using hubs and
  a switch.

65
Virtual LANs (2)

• (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.

66
The IEEE 802.1Q Standard

• Transition from legacy Ethernet to VLAN-aware
  Ethernet. The shaded symbols are VLAN aware.
  The empty ones are not.

67
The IEEE 802.1Q Standard (2)

• The 802.3 (legacy) and 802.1Q Ethernet frame
  formats.

68
Summary

• Channel allocation methods and systems for a
  common channel.