Hidden Terminal and Exposed Terminal problem and its solution

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Hidden Terminal Problem:

A wireless network with lack of centralized control entity, sharing of wireless bandwidth among network access nodes i.e. medium access control (MAC) nodes must be organized in decentralized manner. The hidden terminal problem occurs when a terminal is visible from a wireless access point (APs), but not from other nodes communicating with that AP. This situation leads the difficulties in medium access control sublayer over wireless networking.

In a formal way hidden terminals are nodes in a wireless network that are out of range of other node or a collection of nodes. Consider a wireless networking, each node at the far edge of the access point’s range, which is known as A, can see the access point, but it is unlikely that the same node can see a node on the opposite end of the access point’s range, C. These nodes are known as hidden. The problem is when nodes A and C start to send packets simultaneously to the access point B. Because the nodes A and C are out of range of each other and so cannot detect a collision while transmitting, Carrier sense multiple access with collision detection (CSMA/CD) does not work, and collisions occur, which then corrupt the data received by the access point. To overcome the hidden node problem, RTS/CTS handshaking (IEEE 802.11 RTS/CTS) is implemented in conjunction with the Carrier sense multiple accesses with collision avoidance (CSMA/CA) scheme. The same problem exists in a MANET.

Consider the scenario of wireless networking with three wireless devices (e.g. mobile phones) as shown below.

Hidden Terminal Problem
Hidden Terminal Problem

The transmission range of access point A reaches at B, but not at access point C, similarly transmission range of access point C reaches B, but not at A. These nodes are known as hidden terminals. The problem occurs when nodes A and C start to send data packets simultaneously to the access point B. Because the access points A and C are out of range of each other and resultant they cannot detect a collision while transmitting, Carrier sense multiple access with collision detection (CSMA/CD) does not work, and collisions occur, which then corrupt the data received by the access point B due to the hidden terminal problem.

The hidden terminal analogy is described as follows:

  • Terminal A sends data to B, terminal C cannot hear A
  • Terminal C wants to send data to B, terminal C senses a “free” medium (CS fails) and starts transmitting
  • Collision at B occurs, A cannot detect this collision (CD fails) and continues with its transmission to B
  • Terminal A is “hidden” from C and vice versa.

IEEE 802.11 uses 802.11 RTS/CTS acknowledgment and handshake techniques over wireless networks to transferring packets that partly overcome the hidden node problem. RTS/CTS is not a proper and permanent solution and may decrease throughput even further, but adaptive acknowledgments from the base station can help too.

Some other technology that can be employed to solve hidden node problem are :

Increase Transmitting Power from the Nodes: With the enhancement of the transmission power of access point can solve the hidden terminal problem by allowing the cell around each node to increase in size, encompassing all of the other nodes.

Use Omni directional antennas: Since nodes using directional antennas are nearly invisible to nodes that are not positioned in the direction the antenna is aimed at, directional antennas should be used only for very small networks.

Remove obstacles: Keep away the obstacles that affect the performance of access point accessibility.

Move the node: Provide the mobility features to the nodes.

Use protocol enhancement software: Pooling and token passing strategy should be used before start data transformation.

Exposed Terminal Problem:

In wireless networks, when a node is prevented from sending packets to other nodes because of a neighboring transmitter is known as the exposed node problem.

Exposed Terminal Problem
Exposed Terminal Problem

Consider the below wireless network having four nodes labeled A, B, C, and D, where the two receivers are out of range of each other, yet the two transmitters (B, C) in the middle are in range of each other. Here, if a transmission between A and B is taking place, node C is prevented from transmitting to D as it concludes after carrier sense that it will interfere with the transmission by its neighbor node B. However note that node D could still receive the transmission of C without interference because it is out of range from B. Therefore, implementing directional antenna at a physical layer in each node could reduce the probability of signal interference, because the signal is propagated in a narrow band.

The exposed terminal analogy is described as follows:

  • B sends to A, C wants to send to another terminal D not A or B
  • C senses the carrier and detects that the carrier is busy.
  • C postpones its transmission until it detects the medium as being idle again
  • But A is outside radio range of C, waiting is not necessary
  • C is “exposed” to B

Note: Hidden terminals cause collisions, where as Exposed terminals causes’ unnecessary delay.

Hidden vs. Exposed Terminal Problem:

Let us consider the following arguments:

  • In the case of hidden terminal problem, unsuccessful transmissions result from collisions between transmissions originated by a node such as node A which cannot hear the ongoing transmissions to its corresponding node B. The probability of such a collision is proportional to the total number of terminals hidden from node A.
  • In the case of exposed terminal, unsuccessful transmissions result from nodes such as node A being prevented from transmitting, because their corresponding node is unable to send a CTS. Again such unsuccessful transmissions are proportional to the number of exposed terminals. Both these events lead to degradation of a node’s throughput.

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