Hidden and Exposed Terminal Scenarios

Hidden Terminal Scenario

• In a hidden terminal scenario, A cannot detect C and vice versa, leading to collisions at B.
• A sends to B, C cannot hear A, and C senses a "free" medium, starting transmission, causing a collision at B.
• A is "hidden" from C, and C is "hidden" from A, resulting in collisions.

Exposed Terminal Scenario

• In an exposed terminal scenario, B sends to A, and C wants to send to another terminal outside the range.
• C senses the carrier and detects the medium as busy, postponing transmission until it detects the medium as idle again.
• However, A is outside the radio range of C, making waiting unnecessary.
• C is "exposed" to B, resulting in unnecessary delay.

Near and Far Terminals

• A and B are both sending with the same transmission power, but signal strength decreases proportionally to the square of the distance.
• B's signal drowns out A's signal, making C unable to receive A's transmission.
• If C is an arbiter for sending rights, B's signal overpowers A's signal on the physical layer, making C unable to hear A.

Space Division Multiple Access (SDMA)

• SDMA is used to allocate a separate space to users in wireless networks.
• A typical application involves assigning an optimal base station to a mobile phone user.
• A MAC algorithm decides which base station is best, taking into account frequencies, time slots, or code availability.

P-persistent CSMA

• P-persistent CSMA is a system where nodes sense the medium and transmit with a probability of p.
• The station defers to the next slot with a probability of 1-p, i.e., access is slotted in addition.

CSMA with Collision Avoidance (CSMA/CA)

• CSMA/CA is an access scheme used in wireless LANs, following the IEEE 802.11 standard.
• It combines carrier sensing with a back-off scheme to achieve fairness among competing stations.

Demand Assigned Multiple Access (DAMA)

• DAMA is a scheme that increases channel efficiency by implementing reservation mechanisms and combining them with fixed TDM patterns.
• It has a reservation period followed by a transmission period, and stations can reserve future slots during the reservation period.
• Collisions may occur during the reservation period, but the transmission period can be accessed without collision.
• DAMA is suitable for voice traffic and data traffic in batch mode.

Packet Reservation Multiple Access (PRMA)

• PRMA is an implicit reservation scheme where slots can be reserved implicitly.
• A base station broadcasts the status of each slot to all mobile stations, indicating which slots are occupied and which are free.
• Stations can compete for free slots in Aloha fashion, and the base station returns the reservation status, indicating whether the reservation was successful or not.

Reservation TDMA

• Reservation TDMA combines fixed TDM with reservation, allowing each station to reserve up to k data-slots.
• This guarantees each station a certain bandwidth and a fixed delay.
• Other stations can use unused data-slots, either using a simple round-robin scheme or uncoordinated using an Aloha scheme.

Multiple Access with Collision Avoidance (MACA)

• MACA is a random access Aloha scheme with dynamic reservation, solving the hidden terminal problem.
• It does not require a base station and can be used in a decentralized manner.

Randomly Addressed Polling

• A base station signals readiness to all mobile terminals, and terminals ready to send transmit random numbers without collision using CDMA or FDMA.
• The base station chooses one address for polling from the list of all random numbers, and the cycle starts again after polling all terminals on the list.

Inhibit Sense Multiple Access (DSMA)

• DSMA is used in the Cellular Digital Packet Data (CDPD) system in the AMPS mobile phone system.
• The base station signals a busy medium via a busy tone (BUSY/IDLE indicator) on the downlink.
• After the busy tone stops, accessing the uplink is not coordinated any further, and the base station acknowledges successful transmissions.

Code Division Multiple Access (CDMA)

• CDMA systems apply codes with certain characteristics to separate different users in code space and enable access to a shared medium without interference.
• CDMA systems enable multiple users to share the same bandwidth and timeframe.