Multiple Access Protocols Channel Partitioning Access Protocols Random

Multiple Access Protocols Channel Partitioning Access Protocols Random Access Protocol Taking Turns Controlled LANs: Ethernet, Token Ring, FDDI LAN addresses and ARP

• All nodes are capable of transmitting frame. • More than two nodes can transmit frames at the same time. • If so, the transmitted frames collide at all of the receiver. Multiple Access Protocols • When there is a collision, none of the receiving nodes can make any sense of any of the frames that were transmitted. • In a sense, the signals of the colliding frames become inextricably tangled together. • Thus, all frames involved in the collision are lost, and the broadcast channel as wasted during the collision interval. • To rectify this problem, Multiple Access Protocol was implemented. Er. Sulav Paudel | MSc

Multiple Access Protocols (cont. ) • Multiple access protocol is used to coordinate access to the link. • Classification of Multiple Access Protocols ü Channel-Partitioning Protocols ü Random Access Protocols ü Taking-Turns Protocols Er. Sulav Paudel | MSc

Channel Partitioning Protocols • Available bandwidth of a link is shared in time, frequency, or through code, between different stations. • The three channelization protocols are ü Frequency Division Multiple Access (FDMA) ü Time Division Multiple Access (TDMA) ü Code Division Multiple Access (CDMA) Er. Sulav Paudel | MSc

Frequency Division Multiple Access (FDMA) Er. Sulav Paudel | MSc

• Derived from a compound of Hawaiian words Random Access Protocol ALOHA ü ALO means “share” ü HA means “essence of life” • Contention is a media access method that is used to share a broadcast medium • In contention, any computer in the network can transmit data at any time (first come first served) • The ALOHAnet solution was to allow each client to send its data without controlling when it was sent, with an acknowledge/retransmission scheme used to deal with collisions Er. Sulav Paudel | MSc

• Use of two distinct frequencies in a hub/star configuration. ALOHA Basic Idea Behind ALOHA • The hub machine broadcasting packets to everyone on the “outbound” channel. • Various client machines sending data packets to the hub on the “inbound” channel. • If data was received correctly at the hub, a short acknowledge packet is sent to the client. • If an acknowledge was not received by a client machine after a short wait time, it would automatically retransmit the data packet after waiting a randomly selected time interval. Er. Sulav Paudel | MSc

ALOHA Basic Idea Behind ALOHA Use of two distinct frequencies in a hub/star configuration Er. Sulav Paudel | MSc

ALOHA Basic Idea Behind ALOHA Er. Sulav Paudel | MSc

ALOHA Basic Idea Behind ALOHA Er. Sulav Paudel | MSc

Version of ALOHA Protocol • Pure ALOHA (Original Version of ALOHA) • Slotted ALOHA (Modified version, Division in time slots) Er. Sulav Paudel | MSc

• Whenever a station has a data, it transmits i. e. frames are transmitted at completely arbitrary times. Pure ALOHA • 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. Er. Sulav Paudel | MSc

Frame 1. 1 Frame 1. 2 Frame 2. 1 Frames in a pure ALOHA network Frame 3. 1 Frame 1. 3 Frame 2. 2 Frame 3. 2 Frame 4. 1 Collision Duration Er. Sulav Paudel | MSc Collision Duration

Start Set back off to Zero Protocol Flow Chart for ALOHA Send the Frame Wait back off time Wait No Reached Limit? Yes Abort Er. Sulav Paudel | MSc Increase back off No Received ACK? Yes Success

Thank You Er. Sulav Paudel | MSc