CSMA with Reservation by Interruptions (CSMA/RI) Reducing Collisions in CSMA/CD

Overview of CSMA/RI T: the duration of a slot  : maximum propagation delay between any two stations in the network. The minimum T = 2 

CSMA/CD Protocol (R1) If the channel is sensed idle, a ready station transmits its packet immediately. (R2) If the channel is sensed busy, a ready station keeps monitoring the channel status. (R3) Upon detection of a successful transmission, each station reads the data from the ongoing packet transmission into its local buffer. (R4) If a collision is detected, each ready station reschedules the retransmission individually to some later time.

CSMA/RI Protocol (R1*) is the same as (R1) (R2*) If the channel is sensed busy If busy due to successful transmission apply (R2a*) Else busy due to collisions apply (R2b*)

CSMA/RI Protocol (R2a*) A ready station may interrupt the packet transmission to make a reservation (becoming an RI station) if no other station made reservation before, otherwise it becomes a backlogged station. Upon the completion of the transmission: 1. RI station transmits into the next slot. 2. Backlogged station remains silent.

CSMA/RI Protocol (R2b*) If a ready station becomes ready during a collision, it transmits into the next slot as soon as the channel becomes idle as in CSMA/CD (R2).

CSMA/RI Protocol (R3*) Upon detection of a successful transmission, in addition to CSMA/CD rule (R3): 1. Each station (failed RI and backlogged) will attempt to make a reservation after some waiting time. 2. The waiting time is randomly chosen according to packet transmission time. 3. During the waiting time, the station is required to monitor the channel to detect if other stations make reservations.

CSMA/RI Protocol (R4*) is the same as (R4). (R5*) For a backlogged station, following the completion of a successful transmission, if the channel remains idle for at least a slot, it becomes a ready station and transmits its packet into the idle channel immediately as in (R1*).

Implementation Issues A. Pasting Fragmented Packets. 1) The first part—this is the part of the packet that was transmitted before the interruption. 2) The pasted part—the remainder of the packet.

Implementation Issues B. The Packet Size CSMA/RI allows variable packet size; however, in this case, the following must be implemented: 1) The packet size should be specified in the header of the packet. 2) There must be a minimum packet size.

Implementation Issues C. Power Up A station which is powered up in the middle of an ongoing packet transmission is not allowed to interrupt that packet transmission. Reasons: 1) May not aware of any existing reservations 2) May not know the remainder length of the packet

Implementation Issues D. Power Down It is possible that a station makes a reservation to become an RI station and shuts down due to an internal failure. In this case, (R5*) will apply to all backlogged stations after the channel is sensed idle for a minimum of one slot.

Performance CSMA/RI vs CSMA/CD b: packet size

Performance Packet Size M: Burst size

Performance Disaster Scenario

Performance CSMA/RI vs Token Ring

Conclusions CSMA/RI performs better than CSMA/CD by reducing collisions The performance of CSMA/RI is close to G/D/1 and Token Ring