| Applicable Versions | Academic | Standard | Pro |
| Applicable Releases | v13.0 | v13.1 |
The DSRC IEEE1609 protocol involves switching between the Control Channel(CCH) and the Service Channel(SCH) periodically as per the SCH and CCH time set in the device properties. Further, there is a Guard Interval that can be defined for the start of each channel.
BSM traffic is sent only during CCH and not during SCH. This adds to the delay for packets reaching the MAC layer during the SCH.
By default, the CCH and SCH times are set to 100000 Microseconds. If the BSM application Inter Packet Arrival time(IAT) is set to the same value, the delay experienced by the packets will be as explained below:
BSM traffic arrives at intervals of 100000 Microseconds either during the SCH interval or the CCH interval. Packets arriving during the CCH interval go through after the regular DIFS, carrier sense and backoff mechanism, etc. However, the packets arriving during SCH will have to wait till the next CCH interval which adds to the delay.
For instance, if the SCH and CCH time is set to 50000 Microseconds instead, you will find a linear increase in the delay experienced by the BSM messages.
This behavior is expected as per the explanation; the maximum delay that a packet can experience can be the SCH time plus the backoff duration.
For instance, if a packet reaches the MAC layer right at the end of a CCH interval then it has to wait for the guard interval followed by the entire SCH duration to start carrier sense and then complete backoff to get transmitted. The gradual increase in the delay is based on packet arrival times at the MAC layer as explained below:
- At the simulation start, there is a CCH interval for a duration of 50000 Microseconds. (Simulation time 0 to 50000 Microseconds)
- This is followed by a Guard Interval of 100 Microseconds for Channel Switch. (Simulation time 50000 to 50100 Microseconds)
- Next is an SCH interval for a duration of 50000 Microseconds. (Simulation time 50100 to 100100 Microseconds)
- Just before the end of the SCH interval above, Packet ID 1 arrives at 100000 Microseconds in the MAC layer.
- The end of SCH interval is followed by a Guard Interval of 100 Microseconds for Channel Switch. (Simulation time 100100 to 100200 Microseconds)
- Next is a CCH interval for a duration of 50000 Microseconds. (Simulation time 100200 to 150200 Microseconds)
- Carrier Sense starts right at the beginning of the CCH interval at 100200 Microseconds followed by a backoff for 43 Microseconds after which the packet is sent to the Physical layer. (Packet waits from 100000 to 100243 Microseconds).
- This is followed by a Guard Interval of 100 Microseconds for Channel Switch. (Simulation time 150200 to 150300 Microseconds)
- Next is an SCH interval for a duration of 50000 Microseconds. (Simulation time 150300 to 200300 Microseconds)
- Before the end of the SCH interval, Packet ID 2 arrives at 200000 Microseconds in the MAC layer.
- The end of SCH interval is followed by a Guard Interval of 100 Microseconds for Channel Switch. (Simulation time 200300 to 200400 Microseconds)
- Next is a CCH interval for a duration of 50000 Microseconds. (Simulation time 200400 to 250400 Microseconds)
- The CCH interval carrier sense for Packet ID 2 starts at 200400 Microseconds followed by a backoff for 124 Microseconds after which the packet is sent to the Physical layer. (Packet waits from 200000 to 200524 Microseconds).