Latency is common in both wired and wireless systems. It is a quality of service (QoS) parameter – it acts as one of the performances determining factors of the wired or wireless communication link of interest. Different applications require different levels of latency. In simpler terms, latency refers to how quick the data is transferred from the source (or the sender) to the destination (or the receiver). Latency takes the unit of time. Link speeds are often specified in milliseconds (ms), microseconds (µs), nanoseconds (ns) and so on.
Latency Is a Quality-of-Service Parameter
Let us look into an example of latency with a numerical.
In a communication taking place between a transmitter and receiver 10 km apart, files of 20 megabits of data are transmitted. What will be the propagation delay considering free space medium.
Soln. The propagation delay, considering free space medium, is expressed as follows −
The link bandwidth that is available for data transmission plays a major part in determining the system latency. This applies for both wired and wireless systems. In wired systems, the physical transmission media can include optical fiber cable, coaxial cable and LAN cables. In wireless systems, the physical transmission media Latency Is a Quality-of-Service Parameter can include air, glass, and concrete and so on. Congestion in links increases the latency.
In a link shared by N users, if the existing bandwidth is increased, the bandwidth available to each user will be more and per user latency will be less
We have multimedia content in the form of text, image, video, audio, and animation that consumes more bandwidth and reduces transmission time. However, the realtime bandwidth is limited and cannot be allocated to one single applications. A shared-bandwidth system is followed in real-time data transmission. Often, the data is compressed prior to transmission and the same is decompressed at the receiver.
The transmission time of data depends on the size of the data and the bandwidth allocated for it. A text file may not require the entire allocated bandwidth, while some transmission systems follow the concept of ‘fair sharing of bandwidth’.
This method allows all the applications to utilize same bandwidths during each transmission period incorporating the concept of ‘priority’ and allocating bandwidths based on the data type and the demand from the receiver.
More Bandwidth Allocation ⇒ Less Congestion ⇒ Less Transmission Time
Data rate is defined as the number of bits transmitted bit (symbol) duration. Having a high data rate implies more numbers of bits are transmitted per symbol. The information carried per symbol duration (capacity) is high. Hence, the transmission time is reduced. However, it is important to note that the wireless channel delay (propagation delay) still has its part to play in determining the final transmission time.
Channel propagation delay is one of the major delays in the context of wireless communication links. The propagation delay is denoted by τp. The wireless propagation delay is a function of the physical distance between the transmitter and the receiver and, the velocity of the electromagnetic (EM) wave. EM waves encounter several media such as air, glass, concrete and so on.
The velocity of the EM wave depends on the refractive index of the medium and this relation is an inverse one. A medium having lower refractive index will pass an EM wave relatively quick through it when compared to a medium having relatively higher refractive index. In order to combat the reflection, scattering and other radio propagation mechanisms, the transmitting structures (antennas) are placed at greater altitudes from the ground level on top of elevated structures.
The channel propagation time is expressed as follows −
Here, L denotes the distance between the transmitter and the receiver while vm denotes the velocity of the electromagnetic wave in the medium of propagation.
Like how each medium is associated with a refractive index, each medium also associates itself with a relative permittivity which is a real number like refractive index.
The following relation follows with respect to the velocity of EM waves in a medium.
For air medium, εr = 1. Thus, for air medium, the velocity of propagation of the EM wave is equal to the speed of light, which is, 3 x 108 m/s.
Question − A 4 MHz bandwidth signal is transmitted from A and received at B. The medium has a refractive index of 1.1 and the distance between A and B is 10 km. Find the propagation delay involved in the transmission.
Solution − We find the velocity of the signal using the relation −
We determine the propagation delay using the relation −
The channel propagation delay is approximately 37μs
Jitter is defined as the variations in the latencies. It takes the units of time. Prior to transmission, the packets are arranged logically at the transmitter. If there are ten packets, the order of transmission will be from 1 all the way up to 10. It is expected that the packets will arrive in sequential order form 1-10.
Jitter is variation in latency
The sequence will be broken if this order is not followed. If packet 4 arrives earlier than packet 2, the logical flow of the data is affected. This effect is particularly undesirable for video data transmission such as video calls and video programs being broadcasted or telecasted. This causes Jitter. Jitter is an important design parameter to be taken care of especially in wireless communications.