Multiple Access Protocols Chapter 6 of Hiroshi Harada Book Khurram Masood 200806100

Introduction Multiple access control channels Each node is attached to a transmitter/receiver which communicates via a channel shared by other nodes Transmission from any node is received by other nodes Shared Multiple Access Control Channel to BS Node 4 Node 3 Node 2 Node 1 … Node N

Introduction (Cont’d) Multiple access issues If more than one node transmit at a time on the control channel to BS, a collision occurs How to determine which node can transmit to BS? Multiple access protocols Solving multiple access issues Different types: Contention protocols resolve a collision after it occurs. These protocols execute a collision resolution protocol after each collision Collision-free protocols (e.g., a bit-map protocol and binary countdown) ensure that a collision can never occur.

Packet Communication System Configuration 4/21/2017

Classification of Multiple Access Protocols Contention-based Conflict-free Random access Collision resolution ALOHA, CSMA, BTMA, ISMA, etc TREE, WINDOW, etc FDMA, TDMA, CDMA, Token Bus, DQDB, etc BTMA: Busy Tone Multiple Access ISMA: Internet Streaming Media Alliance DQDB: Distributed Queue Dual Bus

Contention Protocols ALOHA Slotted ALOHA Developed in the 1970s for a packet radio network by Hawaii University. Whenever a station has a data, it transmits. Sender finds out whether transmission was successful or experienced a collision by listening to the broadcast from the destination station. Sender retransmits after some random time if there is a collision. Slotted ALOHA Improvement: Time is slotted and a packet can only be transmitted at the beginning of one slot. Thus, it can reduce the collision duration.

Contention Protocols (Cont’d) CSMA (Carrier Sense Multiple Access) Improvement: Start transmission only if no transmission is ongoing CSMA/CD (CSMA with Collision Detection) Improvement: Stop ongoing transmission if a collision is detected CSMA/CA (CSMA with Collision Avoidance) Improvement: Wait a random time and try again when carrier is quiet. If still quiet, then transmit CSMA/CA with ACK CSMA/CA with RTS/CTS

ALOHA 4/21/2017

ALOHA 4/21/2017

Collision mechanism in ALOHA Waiting a random time Node 1 Packet Node 2 Packet Retransmission Retransmission 1 2 3 3 2 Time Collision Node 3 Packet Collision mechanism in ALOHA

Collisions in (Pure) ALOHA

Throughput of ALOHA The probability that n packets arrive in two packets time is given by n ( ) ! n (2G) P e 2 G - = where G is traffic load. The probability P(0) that a packet is successfully received without collision is calculated by letting n=0 in the above equation. We get We can calculate throughput S with a traffic load G as follows: The Maximum throughput of ALOHA is

Unslotted ALOHA Unslotted ALOHA (a.k.a. Pure ALOHA) was the precursor to slotted ALOHA. In Pure ALOHA, each node transmits a new packet immediately upon receiving, rather than waiting for a slot boundary. If a packet is involved in a collision, it is retransmitted after a random delay.

Unslotted ALOHA (cont.) A frame (red frame) will be in a collision if and only if another transmission begins in the vulnerable period of the frame Vulnerable period has the length of 2 frame times

Unslotted ALOHA (cont.) Since arrivals are independent, Psucc=e-2G Since attempted transmissions occur at rate G(n), the throughput = Ge-2G The MAX throughput of a Pure ALOHA system = 1/(2e), achieved when G=0.5. If λ is very small and the mean retx time is very large, the system can be expected to run for long periods w/o major backlog buildup. The main adv. of pure ALOHA is that it can be used with variable-length packets.

Collision mechanism in slotted ALOHA Node 1 Packet Nodes 2 & 3 Packets Retransmission Retransmission 1 2&3 2 3 Time Collision Slot Collision mechanism in slotted ALOHA

Slotted ALOHA The basic idea: Each unbacklogged node simply transmit a newly arriving packet in the first slot after packet arrival. Slotted ALOHA risks occasional collisions but achieves very small delay if collisions are rare. Contrast to TDM systems, which avoids collisions at the expense of large delays.

Collisions in S-ALOHA

Slotted ALOHA (cont.) When a collision occurs, each node sending one of the colliding packets discovers the collision at the end of the slot and becomes backlogged. Such nodes wait for some random number of slots before retransmitting.

Slotted ALOHA (cont.) The MAX departure rate occurs at G=1 and is 1/e ≈ 0.368. If G<1, too many idle slots are generated. If G>1, too many collisions are generated.

Throughput of Slotted ALOHA The probability of no collision is given by The throughput S is The Maximum throughput of slotted ALOHA is

Throughput 0.368 S Slotted Aloha 0.184 Aloha G

Comparison of ALOHA and S-ALOHA

CSMA 4/21/2017

CSMA 4/21/2017

CSMA 4/21/2017

CSMA (Carrier Sense Multiple Access) Max throughput achievable by slotted ALOHA is 0.368. CSMA gives improved throughput compared to Aloha protocols. Listens to the channel before transmitting a packet (avoid avoidable collisions).

Collision Mechanism in CSMA Node 5 sense Node 1 Packet Node 2 Packet Delay Node 3 Packet 1 2 3 4 5 Time Delay Collision Node 4 sense

Kinds of CSMA Unslotted Nonpersistent CSMA Slotted Nonpersistent CSMA Unslotted persistent CSMA Persistent CSMA Slotted persistent CSMA 1-persistent CSMA p-persistent CSMA

Non persistent CSMA 4/21/2017

Slotted non persistent ISMA 4/21/2017

Computer Simulation Configuration 4/21/2017

Layout of access point and user terminals 4/21/2017

Simulation and results Pure ALOHA Slotted ALOHA Non persistent CSMA Non persistent ISMA Results Throughput Average delay time 4/21/2017

Pure ALOHA 4/21/2017

Pure ALOHA 4/21/2017

Slotted ALOHA 4/21/2017

Slotted ALOHA 4/21/2017

Non persistent CSMA 4/21/2017

Non persistent CSMA 4/21/2017

Non persistent ISMA 4/21/2017

Non persistent ISMA 4/21/2017