Interference among wireless links

In this section, we define a metric to measure interference between a pair of wireless links. We assume that nodes communicate using the IEEE 802.11 protocol; parameters such as transmit power, data rate etc. are all set to fixed values; and the background noise level is constant. We also assume that RTS/CTS handshake is disabled for all nodes, which is the default behavior for most wireless cards.

We start by defining what constitutes a wireless link. Unlike in a wired network, the links in a wireless network are not well-defined. For the purposes of this paper, we define wireless links using packet loss rate. We say that a link from node $A$ to node $B$, denoted by $L_{AB}$, exists if the packet loss rate in either direction does not exceed some threshold. We defer a detailed discussion of the definition until Section 4.2.

We now define a metric to measure interference between a pair of links. Consider links $L_{AB}$ and $L_{CD}$. For some fixed packet size, let $U_{AB}$ denote the unicast throughput of the link $L_{AB}$, when no other links are active in the network. Similarly define $U_{CD}$ for link $L_{CD}$. Now assume that both $L_{AB}$ and $L_{CD}$ are active simultaneously. Let their respective unicast throughput be denoted by $U_{AB}^{AB, CD}$ and $U_{CD}^{AB, CD}$. Define the link interference ratio as:

$$\small LIR_{AB, CD} = \frac{U_{AB}^{AB, CD}+U_{CD}^{AB, CD}}{U_{AB}+U_{CD}}$$ (1)

Thus, $LIR$ is the ratio of aggregate throughput of the links when they are active simultaneously, to their aggregate throughput when they active individually.

$LIR$ takes values between 1 and 0. The maximum value of $LIR$ is 1, which means that the aggregate throughput does not decrease when the links are active simultaneously. Thus, $LIR = 1$ implies that the links do not interfere. A value of $LIR$ less than 1 means that the aggregate throughput of the links decreases when they operate simultaneously. Thus, $LIR < 1$ implies that the links interfere with each other. The links can interfere with each other due to several reasons, listed below. Consider two links, $L_{AB}$ and $L_{CD}$:

Carrier Sense: The 802.11 protocol requires the sender to monitor the radio channel for signs of activity, prior to transmitting a packet. If any activity is detected, transmission is deferred until a later time ¹. This is known as carrier sensing. If the two senders, $A$ and $C$ are within the carrier sense range of each other, then only one of them will transmit at a time. Otherwise, they may both transmit, and one of the following may occur.

Data-Data Collision: The transmission by $C$ may generate sufficient noise at $B$ to interfere with reception of the packet being sent by $A$. A similar ``collision'' may occur at $D$. This is known as the hidden terminal problem.

Data-ACK Collision: For unicast communication, the 802.11 protocol requires the receiver of a packet to transmit an acknowledgment to the sender. If node $D$ successfully receives the data packet sent by $C$, it will transmit an ACK. This transmission may interfere with ongoing reception of data packet at $B$. A similar collision may occur at $D$.

ACK-Data Collision: The data packet sent by $C$ may interfere with ongoing reception of ACK sent by $B$ at $A$. A similar collision may occur at $C$.

ACK-ACK Collision: The ACK sent by $D$ may interfere with the reception of ACK sent by $B$ at $A$. A similar collision may occur at $C$.

A typical value of $LIR$ is 0.5, which means that the aggregate throughput of the links is halved when they are active together. This usually (but not always) happens when the senders are within carrier sense range of each other. The minimum value of LIR is 0. This means that the links get zero throughput when they operate together. This can happen if the senders are not within the carrier sense range of each other, and collisions at the receiver are frequent.

In practice, we see a range of $LIR$ values instead of just the three described above. They can result from packet losses, variable nature of background noise etc. Many of the simple heuristics used to estimate link interference only predict whether a pair of links interfere with each other or not. In other words, they only predict $LIR$ is less than 1. We term this as the ``binary'' notion of interference.