Mumbai University > Computer Engineering > Sem6 > Mobile Communication and Computing

Marks: 10M

Year: May 2015

• The major functions related to system integration are handled by the MAC management layer in a 802.11 station.

• Out of the many important functions handled by it, let us discuss the synchronization function in depth. Its main task is to keep all the nodes in a network synchronized (i.e make their internal clocks have the same time)

• The various jobs done by this functional group includes finding a wireless LAN, synchronization of internal clocks and generation of beacon signals.

• Every node of an 802.11 network has an internal clock. To synchronize the clocks of all nodes, there is a timing synchronization function (TSF)

• Inside a BSS (Base Station System) the timing is conveyed by the (quasi)periodic transmission of a beacon frame. A beacon has a timestamp along with other information meant to be used for power management and roaming.

• A node is not required to pay heed to all the beacons it receives but has the duty to keep its internal clock synchronized.

• Also the beacon is not transmitted in a periodic fashion (i.e. exactly at equal intervals) as the medium may also be busy.

For infrastructure based network:

• Here the access point performs the synchronization by transmitting the (quasi)periodic beacon signal.

• The other wireless nodes in the network adjust their local time to this timestamp signal.

• If the channel (i.e. medium) is busy, then the access point holds or delays the beacon signal.

• However, the access point always tries to schedule transmissions according to the expected beacon interval(i.e. there are fixed intervals at which beacons have to be transmitted shown by the vertical lines marked A,B,C,D,C)

• Note that only at point D in the figure, the beacon is transmitted at correct instant. Elsewhere the beacon is delayed.

• Also, the beacon interval is not altered if the previous beacon was delayed. (Compare C and D; beacon at C was delayed but beacon at D was at time…).

• The beacon always transmits the real-transmit time, not the scheduled time.

For ad-hoc based network:

• Here in ad-hoc mode there is no access node for beacon transmission as in case of infrastructure mode.

• Here each node maintains its own synchronization timer.

• After each beacon interval, each node starts transmission of beacon signal.

• But this will result in confusion (as all nodes a sending beacon signals together). Therefore the standard random back off algorithm is applied.

• Because of this algorithm, only one beacon will win and its real time time-stamp will be transmitted. The rest all nodes suppress their beacon for the current cycle (i.e. the beacon interval) and adjust their clocks to the transmitted signal.