Computer Networks - Hybrid Channel Access Link

2. Hybrid Multiple Access Techniques
GSM (Global System for Mobile Communication)
• Combines FDD (Frequency Division Duplex) with
TDMA & FDMA.
 FDD – Transmitter & receiver operate at different carrier
frequencies.
• FDD is used to prevent interference between outward
and return channel.
• FDMA & TDMA is used for multiple handsets to
work in a single cell.
• GSM with GRRS (General Packet Radio Service)
combines the FDD & FDMA with slotted ALOHA
for reservation inquiries and Dynamic TDMA for
transferring the actual data.

3. Bluetooth
• Combines frequency hopping with CSMA/CA
 FHSS (Frequency Hopping Spread Spectrum)
o method of transmitting radio signals by rapidly switching a carrier
among many frequency channels using pseudo random sequence.
o This pseudo random sequence is known to both sender and receiver.
• CSMA/CA for multiple access.
Wireless LAN (IEEE 802.11b)
• Based on FDMA & DS-CDMA (Direct Sequence
CDMA) combined with CSMA/CA
Hybrid Multiple Access Techniques

4.  DS-CDMA:
o Multiple access scheme based on DSSS (Direct Sequence Spread
Spectrum) by spreading the signals from / to different users with
different codes.
• FDMA & DS-CDMA is used to avoid the interference
among adjacent Wireless LAN cells (or) access point
• CSMA/CA for multiple access with in a cell.
HIPERLAN / 2
• Combines the FDMA with dynamic TDMA, meaning
that resource reservation is achieved by packet
scheduling.
Hybrid Multiple Access Techniques