Data Comm: MAC Protocol, Round Robin, Reservation, Contention, & Channel Allocation, Slides of Data Communication Systems and Computer Networks

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Data Communication

C O N N E C T I V I T Y 1 0 10 1 0 1 0 0

0 0 1 0 1 0 1 0 1 0 0 1 0 1

0 10 1 0 0 1@ 0 1 0 11 0

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Medium Access Control

• Usually for shared network like LAN
• Devices need to know  Can multiple devices send at the same time?  When to transmit the data?  For how long it can transmit the data?
• Devices require access control mechanism [MAC-protocol]
• How to control the access?  Synchronous Access Control - Specific capacity is dedicated to each device - Same as circuit switching, packet switching, TDM and FDM - Not Optimal for LANs as requirements are unpredictable  Asynchronous Access Control - Can handle immediate demands for medium access - Round Robin - Reservation - Contention

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medium access control..cont..

• Where to implement Access Control Mechanism

 Centralized Control

• A central controller is used like hub or switch
• Stations ask permission from the controller  Distributed Control
• Each station contributes to implement the control mechanism
• Mutual co-ordination  Which one is better Centralized or Distributed ..?????
• Greater control, priorities are possible [Centralized ]
• Simple logic for stations [Centralized ]
• No co-ordination is required [Centralized ]
• Single point of failure [Centralized ]
• Bottlenecks [Centralized ]

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MAC-Frame Format

• MAC layer performs medium access control for higher layers
• The access control is implemented through MAC-PDU called MAC- Frame
• A general MAC-Frame

 The Control Field contains protocol control information like priority  Destination MAC address  Source Destination MAC address  LLC: Data from LLC  CRC – Cyclic Redundancy Check for error control

MAC-Control Destination MAC-Address Source MAC-Address LLC-PDU CRC

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The Channel Allocation Problem

• Broad cast network

• Medium is shared, also termed as broadcast channel or multi access channel
• MAC (Medium Access Control) is needed for proper access

• Channel Allocation Schemes

• Static Channel Allocation in LANs and MANs
  • The channel is not utilized properly
  • Example-
    • A Telephone exchange with N channel
    • In case number of users < N , the channels are under utilized
    • In case number of users > N , some users may not get the channel
  • Example-
    • FDM
    • Same reasonng

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Channel Allocation -

• Example-
  • FMD
    • In case all capacity of a channel is given to a user the through put will be maximum but other users will not be serviced
    • In case the channel is divided into more channels the capacity of each channel will be much smaller - In such case if there are N sub channels - If number of users < N medium is under utilized - If number of users > N some of the users will not be entertained
• Conclusion

 Static channel is not suitable for all shared medium in all cases

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Multiple Access Protocol

• ALOHA

• Carrier Sense Multiple Access Protocols

• Collision-Free Protocols

• Limited-Contention Protocols

• Wavelength Division Multiple Access Protocols

• Wireless LAN Protocols

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Aloha

• Developed in 1970’s
• Initially used for ground based radio broadcasting
• Is applicable to any system in which uncoordinated users are competing for the use of single shared channel  Pure Aloha  Slotted Aloha

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Pure ALOHA

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

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Pure ALOHA -

Vulnerable period for the shaded frame.

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Slotted ALOHA

• Time in uniform slots equal to frame transmission time
• Need central clock (or other sync mechanism)
• Transmission begins at slot boundary
• Frames either miss or overlap totally
• Max utilization 37%

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CSMA

• If medium is idle, transmit
• If busy, listen for idle then transmit immediately
• If two stations are waiting, collision

 1-Persistent CSMA  Non-Persistent CSMA  P-Persistent CSMA

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Non-Persistent CSMA

• Works same as 1-persistent CSMA expect the following

 If medium is busy wait random amount of time

• Utilization is better but delays are longer

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P-Persistent CSMA

• Used for slotted channels
• Sense the medium if idle

 Transmit with probability p  Defer for q=1-p for next slot  If next slot is also empty transmit

• If medium is idle

 Wait for the slot