Medium access control (MAC) issues in mobile computing

 Medium access control (MAC, also called media access control)

In IEEE 802 LAN/MAN standards, the medium access control (MAC, also called media access control) sublayer is the layer that controls the hardware responsible for interaction with the wired, optical or wireless transmission medium. The MAC sublayer and the logical link control (LLC) sublayer together make up the data link layer. Within the data link layer, the LLC provides flow control and multiplexing for the logical link (i.e. EtherType, 802.1Q VLAN tag etc), while the MAC provides flow control and multiplexing for the transmission medium.

These two sublayers together correspond to layer 2 of the OSI model. For compatibility reasons, LLC is optional for implementations of IEEE 802.3 (the frames are then "raw"), but compulsory for implementations of other IEEE 802 physical layer standards. Within the hierarchy of the OSI model and IEEE 802 standards, the MAC sublayer provides a control abstraction of the physical layer such that the complexities of physical link control are invisible to the LLC and upper layers of the network stack. Thus any LLC sublayer (and higher layers) may be used with any MAC. In turn, the medium access control block is formally connected to the PHY via a media-independent interface. Although the MAC block is today typically integrated with the PHY within the same device package, historically any MAC could be used with any PHY, independent of the transmission medium.

wireless MAC issues

There are three important MAC issues are

1. Half Duplex operation
2. Time varying Channel
3. Burst channel errors

1. Half Duplex operation:-

         In wireless , it s difficult to receive data when the transmitter is sending the data , because (either send or receive but not both at a given time ) when node is transmitting a large fraction of signal energy leaks into the receiver path.

the transmitted and receiver power levels can differ by order of magnitude.

      2.Time varying Channel

           Three mechanisms  for radio signal propagation

Reflection:-

Occurs when a propagating wave impinges upon an object that has very large dimensions than the wavelength of the radio wave.

e.g. refletion occurs from the surface of the earth and from buildings and walls.

Diffraction:- 

Occurs when the radio path between the transmitter and the receiver is obstructed by surface with sharp edges.

Scattering:-

Occurs when the medium through which the wave travels consists of object.

       3. Burst Channel errors:-

           As a consequence of time varying channel and varying signals strength errors are introduced in the transmission for wire line network the bit error rate (BER) is the probability of packet error is small.

Functions performed in the MAC sublayer

According to IEEE Std 802-2001 section 6.2.3 "MAC sublayer", the primary functions performed by the MAC layer are:

• Frame delimiting and recognition
• Addressing of destination stations (both as individual stations and as groups of stations)
• Conveyance of source-station addressing information
• Transparent data transfer of LLC PDUs, or of equivalent information in the Ethernet sublayer
• Protection against errors, generally by means of generating and checking frame check sequences
• Control of access to the physical transmission medium

In the case of Ethernet, according to 802.3-2002 section 4.1.4, the functions required of a MAC are:

• receive/transmit normal frames
• half-duplex retransmission and backoff functions
• append/check FCS (frame check sequence)
• interframe gap enforcement
• discard malformed frames
• prepend(tx)/remove(rx) preamble, SFD (start frame delimiter), and padding
• half-duplex compatibility: append(tx)/remove(rx) MAC address

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