Chapter 6: Multiple Radio Access

1. Chapter 6:Multiple Radio Access Associate Prof. Yuh-Shyan Chen Dept. of Computer Science and Information Engineering National Chung-Cheng University

2. Introduction • A channel • can be exclusively assigned • shared among numerous subscribers • As users need to access the channel at random times and for random periods • It is not desirable to allocate a channel permanently • even if there is a controller like a BS, users need to use control channel to inform the BS before using the traffic channel or information channel

3. Introduction • Such exchange of facts necessitates shared access of a control channel, for which each MS has to compete • Such contention is exclusively present in ad hoc networks wherein the same frequency is used to enable all wireless devices • to tune into the same band ad listen to each other

4. Multiple access of shared medium in a wireless network

5. Typical scenario • MSs (node) and have to compete for a shared multiple access medium • Each node has a transmitter/receiver that communicate with other nodes or stations • In a general, • Transmission from any node can be received by all other nodes in the neighborhood • If more than one node attempts to transmit at one time on the channel, collision occurs, where in signals in the medium are garbled

6. Typical scenario (con.) • Nodes receiving information cannot interpret or differentiate what is being transmitted • These situation are called collision in the medium or multiple access issues • Collision must be avoided • To determine which node has exclusive access to the shared medium at a given time and for a given duration so that the node can transmit and other nodes can receive

7. Two types of protocols • Contention-based protocol • resolve a collision after it occurs • Conflict-free (or collision free) protocol • Ensure that a collision never occurs

8. Mudium sharing techniques

9. Multiple Radio Access Protocols • Existing LANs (local area networks), MANs (metropolitan area network), PRNs (packet radio networks), PANs (personal area networks), and satellite networks do utilize broadcast channels rather than point-to-point channels for information transmission • A simple modification of OSI model is done by adding the so-called MAC (medium access control) sublayer in data link layer • Are multiple access protocols

10. Classification of multiple access protocols Busy tone multiple access Idle signal multiple access

11. Contention-Based Protocols • In a contention-based protocol • A terminal in the system may transmit its message at any time it wished, hoping that no other terminals will transmit at the same time • Since collisions may exist in a contention-based protocol, the protocol has to have a provision to make collided messages retransmitted efficiently • A terminal is allowed to transmit the collided message only after a random delay

12. (Pure) ALOHA • Developed in 1970s for a packet radio network at the university of Hawaii • Whenever a sender has data to send, it transmit the data right away • The sender side also waits to see whether transmission is acknowledge by the receiver • No response within a specified period of time indicates a collision with other transmission • If the presence of a collision is determined by the sender, it retransmits after some random wait time

13. Collision mechanism in pure ALOHA

14. (Pure) ALOHA • The random time is different for different transmitters, thereby avoiding collision during retransmission cycle • The worst case collision period if two times the length of each packet, assuming the packets to be of equal length

15. Slotted ALOHA • Slotted ALOHA improves on pure ALOHA by cutting the vulnerable period for packet collision in half • Time is slotted and packet must be transmitted within a slot • If a terminal has a packet to transmit, it waits until the beginning of the next slot before sending

16. Slotted ALOHA • The terminal listens to the broadcast and checks if the packets was transmitted successfully • If there was a collision, indicated by the absence of acknowledge message from the receiver within a prespecified period of time, the terminal waits for a random number of slots and attempts to send it again

17. Collision mechanism in slotted ALOHA

18. Throughputs of pure ALOHA and slotted ALOHA

19. Performance of ALHOA and slotted ALOHA • From the performance curves, the maximum throughput of ALOHA and slotted ALOHA are 0.184 and 0.368, respectively. • It needs to find another way of improving throughputs and supporting high speed potential collision by simply listening to the channel before transmitting a packet • Collision could be avoided • Carrier sense multiple access protocol

20. CSMA (Carrier sense multiple access protocol) • Each terminal can sense the transmission of all other terminals, and the propagation delay is small as compared with the transmission time

21. Collision mechanism in CSMA

22. Types of CSMA protocols

23. Nonpersistent CSMA protocol • The terminal sense the medium first whenever the terminal has a packet to send • If the medium is busy, the terminal waits for a random amount of time and senses the medium again • If the medium is idle, the terminal waits for a random amount of time and starts all over again • The packets can be sent during slotted period (slotted nonpersistent CSMA) and can be transmitted at any arbitrary time (unslotted nonpersistent CSMA)

24. 1-Persistent CSMA protocol • The terminal senses the medium when the terminal has a packet ready to send • If the terminal is busy, the terminal keeps listening to the medium and transmits the packet immediately after the medium becomes idle • 1-persistent • Terminal transmits with a probability of 1 whenever if finds the medium to be idle • There will always be a collision if two nodes want to retransmit at the same time

25. p-Persistent CSMA Protocol • The protocol is considered as an optimal access strategy • The time is slotted • The terminal senses the medium when the terminal has a packet ready to send • If the terminal is busy, the terminal keeps listening to the medium and transmits the packet immediately after the medium becomes idle • If the medium is idle, the terminal transmit with probability p or defers with probability (1-p) until the next slot • If a collision occurs, the terminal waits for a random amount of time and starts all over again

26. The throughput of different ALHOA and CSMA protocols is compared

27. CSMA/CD (Collision Detection) • In typical CSMA protocol • If two terminals begin transmitting at the same time, each will transmit its complete packet even though they collide • This wastage of the medium for an entire packet time can be addressed by CSMA/CD • CSMA/CD • The terminal senses the medium when terminal has a packet to send

28. CSMA/CD • If the medium is idle, the terminal transmit its packet immediately • If the medium is busy, the terminal waits until the medium becomes idle • If a collision is detected during the transmitting, the terminal aborts its transmitting immediately and the terminal attempts later after waiting for a random amount of time

29. Collision mechanism in CSMA/CD

30. CSMA/CD • CSMA protocol minimizes the number of collisions while CSMA/CD can further reduce the effect of collisions as it renders the medium ready to be used as soon as possible • The collision detection is two times end-to-end propagation delay

31. CSMA/CA • The IEEE 802.11 MAC is called the distributed foundation wireless MAC (DFW-MAC) • The access mechanism is based on a modified version of the CSMA/CD access protocol • CSMA with collision avoidance (CSMA/CA) • The IEEE 802.11 wireless LAN standard supports operation in two separate modes • A distributed coordination mode • A centralized point-coordination mode

32. A basic collision avoidance scheme

33. Basic CSMA/CA • All terminals listen to the medium same as CSMA/CD • A terminal that is ready to transmit data senses the medium and will transmit its data if the medium is idle for time interval that exceeds the distributed interframe space (DIFS) • Otherwise; if the medium is idle, it waits for an additional predetermined time period, denoted as DIFS, and then picks a random backoff period within its contention window to wait before transmitting its data

34. Basic CSMA/CA • The backoff counter can count down only when the medium is idle • Otherwise; it is frozen when the medium is busy • After a busy period, the counting down of the backoff counter resumes only after the medium has been free longer than DIFS. • The terminal can start transmitting its data when backoff counter becomes zero • Collisions can occur only when two or more terminals select time slot in which to transmit their frames

35. Basic CSMA/CA

36. CSMA/CA with ACK • In this scheme • An immediate positive acknowledge is employed to indicate a successful reception of each data frame • This is accomplished by making the receiver send an acknowledgment frame immediately after a time interval short interframe (SIFS) • SIFS is smaller than DIFS, and following the reception of the data frame, the receiver transmit acknowledgment without sensing the state of the medium as no other terminal or device is expected to use the shared medium the time • In case an ACK. Is not received, the data frame is presumed to be lost and a retransmission is automatically scheduled by the transmitter

37. CSMA/CA with ACK.

38. CSMA/CA with RTS and CTS • The DCF (Distributed Coordination Function) provides an alternative way of transmitting data frames by using special hand-shaking mechanism • It send RTS (Request To Send) and CTS (Clear To Send) frames prior to the transmission of the actual data frame • A successful exchange of RTS and CTS frames attempts to reserve the medium for the entire time duration required to transfer the data frame under consideration

39. CSMA/CA with RTS and CTS • The rule for the transmitting of a RTS frame are the same as those for a data frame under basic CSMA/CA • The transmitter sends a RTS frame after the medium has been idle for a time interval exceeding DIFS • On receiving a RTS frame, the receiver responds with a CTS frame • The CTS frame ACK. The successful reception of an RTS frame • Which can be transmitted after the medium has been idle for a time interval exceeding SIFS

40. CSMA/CA with RTS and CTS • After the successful exchange of RTS and CTS frames, the data frame can be sent by the transmitter after waiting for a time interval DIFS • RTS is retransmitted following the backoff rules as specified in the CSMA/CA with ACK procedures

41. CSMA/CA with RTS and CTS