19-09-2012, 04:02 PM
Wireless MAC protocols
Wireless MAC.ppt (Size: 184 KB / Downloads: 104)
FDMA (Frequency Division Multiple Access)
Similar to broadcast radio and TV, assign a different carrier frequency per call
Modulation technique determines the required carrier spacing
Each communicating wireless user gets his/her own carrier frequency on which to send data
Need to set aside some frequencies that are operated in random-access mode to enable a wireless user to request and receive a carrier for data transmission
TDMA(Time Division Multiple Access)
Each user transmits data on a time slot on multiple frequencies
A time slot is a channel
A user sends data at an accelerated rate (by using many frequencies) when its time slot begins
Data is stored at receiver and played back at original slow rate
Cellular Systems Terms
Base Station (BS) – includes an antenna, a controller, and a number of receivers
Mobile telecommunications switching office (MTSO) – connects calls between mobile units
Two types of channels available between mobile unit and BS
Control channels – used to exchange information having to do with setting up and maintaining calls
Traffic channels – carry voice or data connection between users
Duplex techniques
Separates signals transmitted by base stations from signals transmitted by terminals
Frequency Division Duplex (FDD): use separate sets of frequencies for forward and reverse channels (upstream and downstream)
Time Division Duplex (TDD): same frequencies used in the two directions, but different time slots
RECC Access Protocol
A RECC is used by multiple endpoints to request a traffic channel
But before a mobile issues call origination request on an RECC, it needs to make sure that the RECC is idle
First, endpoints tune into FOCC; find the strongest one and then determine
whether corresponding RECC is busy or idle
Broadcasts on FOCC
When a mobile handset is switched on, it locks into the strongest FOCC
If in the band of the “other” service provider, then “roaming” signal is displayed
If there are no strong signals, display “no service”
Basestation broadcasts busy/idle status of each RECC on its corresponding FOCC
Basestation sends global action message specifiying
maximum number of tries if busy (MAXBUSY)
maximum number of attempts to seize the RECC (MAXSZTR)
Messages on control channels
Sent on FOCC
System parameter
identifies provider, indicates number of forward and reverse control channels
Global action
indicates MAXBUSY and MAXSZTR
Registration ident
specifies frequency to be used by terminal for sending registration messages
Other messages: Page, Initial voice channel, Release, Confirm registration, etc.