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Open Systems Interconnections Model

Computer systems interact via networks using 7 layers, as described by the OSI model.

In the 1980s, all significant computer and telecommunications companies accepted it. It's the first industry standard for network communications.

In the year 1984, the International Organization for Standardization (ISO) created OSI. It has a 7-layer architecture, with each layer carrying out a particular duty. To transport data from one person to another across the world, all 7- layers must work together.

Open Systems Interconnections Model Layers

Physical Layer

It's in charge of the wired / wireless connections that physically connect network nodes.

It specifies the connection / electrical cable / wireless technology that connects the devices.

It regulates the bit rate and the transmission of the raw data, which is just a stream of 0s and 1s.

It creates and breaks connections between two network nodes.

Packets get disintegrated into Frames, which are then sent from the source to the destination.

This layer comprises of 2 components: Media Access Control (MAC), and Logic Link Control (LLC).

MAC utilises MAC addresses to link devices. It establishes permissions to send and receive data.

LLC identifies network protocols, does error checking, and synchronises frames.

Network Layer

The network layer serves two primary purposes. One is dividing segments into network packets, which are then put back together at the other end. The other is packet routing with the best route. To route packets to a target node, the network layer needs network addresses (IP addresses).

Transport Layer

Data sent at the session layer gets divided into "segments" by the transport layer. It happens at the receiving end.

On the receiving end, it's in charge of putting the segments back together. It provides data that the session layer may use.

The transport layer performs error control. It determines if data gets transmitted at an incorrect rate.

It also performs flow control. It checks if data gets transmitted at the same rate as the receiving device.

Session Layer

The session layer creates communication channels, called sessions, between devices. It's responsible for opening sessions. It ensures the sessions remain open and functional while data is getting transferred. It also closes the sessions when communication ends.

This layer can set checkpoints during a data transfer for any session interruption.

Devices can resume data transfer from the last checkpoint.

Presentation Layer

The presentation layer prepares data for the application layer. It defines how two devices should encode, encrypt, and compress data. It's done so that the received data correctly on the other end.

The presentation layer takes the data transmitted by the application layer and prepares it for transmission over the session layer.

Application Layer

End-user applications like web browsers and email clients operate at the application layer.

It offers protocols that let computer programs send and receive data.

Network Latency, Application Latency and SIP Latency

1.Network Latency refers to the delay in data transmission over a network. It's the time taken for data to travel from the source to the destination through the network infrastructure.

Components Description
Propagation Delay Time taken for a signal to travel through the physical medium, for example fiber optic cables, wireless links etc
Transmission Delay Time required to transmit a packet of data over the network
Processing Delay Time spent by routers and switches processing packets
Queueing Delay The time packets spend waiting in queues at network devices
Causes Explanation
Distance Signals travel faster over shorter distances
Network Congestion High traffic can cause delays as packets wait in queues
Network Quality Poorly maintained or low-quality networks can introduce additional latency
Protocol Overhead Additional overhead from network protocols like TCP/IP can add to latency

Examples

  1. A user accessing a website hosted on a server located on the other side of the world may experience higher network latency due to the longer distance the data needs to travel.
  2. Video streaming services may suffer from network latency, causing buffering or lag.

2.Application Latency refers to the delay between when an application receives a request and when it responds with the requested data or action. It includes all processing steps within the application itself, excluding network transmission times.

Components Description
Server Response Time Time taken by the server to process the request and generate a response
Database Query Time If the application relies on databases, this includes the time taken to execute queries and retrieve data
Computation Time Time spent on computational tasks such as data processing, algorithm execution, etc
Resource Allocation Time spent allocating resources like memory, CPU cycles, etc
Causes Explanation
Server Load High server load can slow down response times
Database Performance Slow database queries can significantly impact application latency
Code Efficiency Inefficiently written code can lead to longer processing times
Resource Constraints Limited resources such as RAM or CPU power can cause delays

Example

  1. A web application that takes several seconds to load a page after clicking on a link is experiencing high application latency.
  2. A mobile app that takes a long time to process user input (e.g., logging in) is suffering from application latency.

3.SIP Ping Latency refers specifically to the delay in sending and receiving SIP messages, which are used in VoIP and other real-time communication services. This type of latency is crucial for ensuring timely and reliable communication.

Components Description
SIP Message Transmission Time taken for SIP messages (e.g., INVITE, ACK, BYE) to be transmitted between endpoints
Server Response Time Time taken by SIP servers to process and respond to SIP requests
Causes Explanation
Network Latency Underlying network conditions can affect SIP message transmission times
Server Load High load on SIP servers can slow down response times
Configuration Issues Mis-configured SIP settings or firewalls can introduce delays

Examples

  1. In VoIP calls, high SIP ping latency can cause delays in call setup and tear-down, leading to poor user experience.
  2. In video conferencing, delayed SIP messages can result in synchronization issues between audio and video streams.

Relationships Between Latency Types

  1. Network Latency affects both SIP ping latency and Application latency because it influences how quickly data (including SIP messages and application requests) can be transmitted over the network.

  2. Application Latency can be influenced by both network latency (for data transmission) and server-side processing times. High network latency can exacerbate this by delaying the transmission of requests and responses.

  3. SIP Ping Latency is particularly sensitive to underlying network conditions. Therefore, high network latency will directly impact SIP ping latency.

Important Note

SIP Ping Latency= Network Latency + Application Latency

Summary

  1. Network Latency measures data travel delay.

  2. Application Latency is primarily server-side processing delay, independent of network transmission times.

  3. SIP Ping Latency combines network latency and SIP server processing delays.

VoIP Latency

The time delay between the time a voice packet gets delivered and when it arrives at its destination is latency in VoIP.

For VoIP conversations, a delay of 20 ms is common. A latency of up to 150 ms is perceptible and hence acceptable. But once you get over that, the quality starts to suffer. When it reaches 300 ms or more, it's unacceptable.

VoIP call quality gets affected by high latency, leading to:

  1. Slow and interrupted phone conversations
  2. Conflicting and overlapping noises
  3. Echo
  4. Disruption of speech and other data type synchronization, notably during video conferencing