Hidden and Exposed Terminal Scenarios

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Questions and Answers

What is the primary difference between P-persistent CSMA and CSMA/CA?

The use of back-off scheme

The access to the medium

The transmission probability (correct)

The type of wireless LAN

What is the standard that governs CSMA/CA in wireless LANs?

IEEE 802.10

IEEE 802.11 (correct)

IEEE 802.15

IEEE 803.12

What is the channel efficiency of Aloha?

18% (correct)

10%

50%

36%

What is the purpose of the reservation period in DAMA?

To reserve future slots in the transmission period Signup and view all the answers

What is the advantage of DAMA in satellite systems?

It increases the number of users in a pool of satellite channels Signup and view all the answers

How does DAMA assign channels to users?

Based on requests issued from a user Signup and view all the answers

What is the benefit of DAMA in terms of resource utilization?

It utilizes the resources only in proportion to the occupied channels Signup and view all the answers

What type of traffic is DAMA suitable for?

Both voice and data traffic in batch mode Signup and view all the answers

What is the scenario where A cannot detect C and vice versa?

Hidden terminal scenario Signup and view all the answers

What happens when C senses the carrier in the exposed terminal scenario?

C postpones its transmission until it detects the medium as being idle again Signup and view all the answers

What is the effect of a hidden terminal on the network?

It causes collisions Signup and view all the answers

What is the reason for C being unable to receive A's transmission in the near and far terminals scenario?

B's signal drowns out A's signal Signup and view all the answers

What is the purpose of Space Division Multiple Access (SDMA)?

To allocate a separated space to users in wireless networks Signup and view all the answers

What is the typical application of SDMA?

Assigning an optimal base station to a mobile phone user Signup and view all the answers

What is the effect of an exposed terminal on the network?

It causes unnecessary delay Signup and view all the answers

What is the relationship between signal strength and distance in the near and far terminals scenario?

Signal strength decreases proportional to the square of the distance Signup and view all the answers

What happens to future slots when a station succeeds in reserving a slot?

They are implicitly reserved for the same station Signup and view all the answers

What is the purpose of mini-slots in a fixed TDM scheme?

To reserve data-slots Signup and view all the answers

What is the advantage of the Reservation TDMA scheme?

It guarantees each station a certain bandwidth and a fixed delay Signup and view all the answers

What is the problem that MACA solves?

The hidden terminal problem Signup and view all the answers

What happens when a station senses the medium and finds it free?

It starts sending immediately Signup and view all the answers

What is the characteristic of traffic in a Reservation TDMA scheme?

It is a combination of isochronous and best-effort traffic Signup and view all the answers

What is the consequence of the hidden terminal problem?

The sender is unable to detect the collision Signup and view all the answers

What is the function of the base station in the slotted aloha scheme?

It indicates idle slots Signup and view all the answers

What is the type of reservation scheme used in DAMA?

Explicit reservation Signup and view all the answers

What happens to the data transmission during collisions in the reservation phase of DAMA?

Data transmission is unaffected Signup and view all the answers

What is the purpose of the satellite in DAMA?

To collect and send back reservation lists Signup and view all the answers

How do stations know which slot is occupied and which slot is free in PRMA?

Through a satellite broadcast Signup and view all the answers

What happens to the occupied slots in PRMA when a station wants to transmit?

They are not touched Signup and view all the answers

What is the pattern of reservation and transmission in DAMA?

Fixed TDM pattern Signup and view all the answers

What is the purpose of synchronization in DAMA?

To maintain a fixed TDM pattern Signup and view all the answers

What happens when more than one station wants to access a free slot in PRMA?

A collision occurs Signup and view all the answers

What is the purpose of the contention phase in the master-slave polling scheme?

To create a list of stations wishing to transmit Signup and view all the answers

In the Randomly Addressed Polling scheme, how does the base station choose a terminal to poll?

By randomly selecting a terminal from the list Signup and view all the answers

What is the purpose of the BUSY/IDLE indicator in Inhibit Sense Multiple Access?

To indicate when the medium is busy Signup and view all the answers

In CDMA systems, how are different users separated in the code space?

Applying codes with specific characteristics Signup and view all the answers

What happens when two mobile terminals choose the same address in the Randomly Addressed Polling scheme?

A collision occurs Signup and view all the answers

What is the purpose of the acknowledgement in Inhibit Sense Multiple Access?

To confirm successful transmissions Signup and view all the answers

What is the name of the scheme used in the AMPS mobile phone system for packet data transmission?

Digital Sense Multiple Access Signup and view all the answers

What is the purpose of the back-off and retransmission mechanisms in Inhibit Sense Multiple Access?

To handle cases of collisions Signup and view all the answers

Study Notes

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.

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Description

Understand the concepts of hidden and exposed terminals in computer networks, including their effects on transmission and collision detection. Learn about the limitations of carrier sense and collision detection mechanisms.