1. Wireless & Mobile Networking: Multiple Radio Access Azizol Bin Abdullah azizol@fsktm.upm.edu.my (A2.04) Rujukan: Text Book Chapter 6

2. Chapter 6: Multiple Radio Access 1)Introduction 2)Multiple Radio Access Protocols 3)Contention-Based Protocols

3. Multiple Radio Access: Intro - In wireless networks, one needs to access a channel to communicate - The channel can either: 1) exclusively assigned(allocated permanently) 2) shared among other subscribers Medium Sharing Techniques Dynamic medium access control Static Channelization Scheduled Random - The 1 st solution is not feasible; a channel is accessed at random times for random periods. Thus, the best is to share the resources.

4. Multiple Radio Access: Intro (cont.) - Resource sharing process based on predefined rules or algorithms – if no central authority - In sharing environment, user needs to compete for the resources. - Collision is bound to happen when > one node attempts to transmit at one time on the same channel. Collision messes up the signals. - Thus, collision MUST be avoided and we need to follow some protocols to determine which nodes has the exclusive right at that particular moment with certain duration.

5. Multiple Radio Access: Protocols - In this chapter, we will focus only on “contention-based” Multiple Access Protocols Contention-based Conflict-free Random accessCollision resolution ALOHA, CSMA, BTMA, ISMA, etc TREE, WINDOW, etc FDMA, TDMA, CDMA, Token Bus, etc

6. Multiple Radio Access: Contention-based Protocols -A terminal may transmit it message at any time it wishes, hoping that no other terminal will transmit at the same time. -If more than one terminal transmit messages in one time – collision. -To resolve: 1) Random-access: - def: a terminal is allowed to transmit collided message only after a random delay - ALOHA-type, CSMA(Carrier Sense Multiple Access), BTMA(Busy Tone Multiple Access), ISMA(Idle signal multiple access) etc.. 2)Collision Resolution - def: more sophisticated way(than random delay) to control the retransmission process - WINDOW, TREE etc

8. Multiple Radio Access: Contention-based Protocols (cont.) Aloha - developed in 1970 for a packet radio network at University of Hawaii - whenever a sender has data to send, it transmits the data right away and wait for response. If no ack within a specified period of time, indicate collision. - Retransmit after some random wait time. Slotted Aloha - improve pure Aloha by using slotted time, packets must be transmitted within a slot. - if a terminal has a packet to transmit, it waits until the beginning of next slot before sending. - Collision occurs, the terminal waits a random number of slots and attempts to send it again.

9. Multiple Radio Access: Contention-based Protocols (cont.) CSMA - Listening to the channel before transmitting – avoid potential collision - Types of CSMA : Nonpersistent CSMA – sense, if busy, wait for a random amount of time and sense again before transmitting 1- Persistent CSMA – sense, if busy, keep sensing and transmit imediately after the medium become idle. p- Persistent CSMA – time is slotted, sense, if busy wait until next slot and sense again. If idle, terminal transmits with probability p. CSMA/CD - Transmission is terminated immediately detection of a collision.

10. Multiple Radio Access: Contention-based Protocols (cont.) CSMA - Listening to the channel before transmitting – avoid potential collision - Types of CSMA : Nonpersistent CSMA – sense, if busy, wait for a random amount of time and sense again before transmitting 1- Persistent CSMA – sense, if busy, keep sensing and transmit imediately after the medium become idle. p- Persistent CSMA – time is slotted, sense, if busy wait until next slot and sense again. If idle, terminal transmits with probability p. CSMA/CD - Transmission is terminated immediately detection of a collision.

11. Multiple Radio Access: Contention-based Protocols (cont.) CSMA/CA - Listening to the channel before transmitting – transmit it data if the channel is idle for time interval that exceeds the distributed interframe space (DIFS) - Types of CSMA/CA : CSMA/CA with ACK – employ an immediate positive ack to indicate a successful reception of each data frame. CSMA/CA with RTS and CTS – send the request to send (RTS) and clear to send (CTS)