Computer Network Tutorial

Introduction of Computer Network Types of Computer Network Network Topology Computer Networking Architecture Transmission Modes (Data Flow) Basic Networking Devices Integrate Services Digital Network (ISDN)

Model

OSI Model TCP/IP Model

Physical Layer

Digital Transmission Analog Transmission Transmission Media Switching

Data Link Layer

Error detection and Error correction Data Link Control Multiple Access Aloha

Network Layer

Network Layer - Logical Address Address Mapping Unicast Routing Protocol

Transport Layer

Process to Process Delivery User Datagram Protocol Transmission Control Protocol Stream Control Transmission Protocol Session Layer and Presentation Layer

Application Layer

Domain Name System Application Protocol E-mail Cryptography

Misc

Classes of Routing Protocols Classification of Routing Algorithms Controlled Access Protocols in Computer Networks Differences between IPv4 and IPv6 Fixed and Flooding Routing Algorithms Advantages and Disadvantages of Fibre Optics Cable APIPA Difference between Active and Passive FTP Fiber Optics and its Types Method of Joining and Fusion of Fiber Optic Cable Define Framing in Computer Network Disadvantages of Computer Network Mesh Topology Diagram in Computer Network Ring Topology in Computer Network Star Topology in Computer Networks 4G Mobile Communication Technology Advantages and Disadvantages of LAN Advantages and Disadvantages of MAN Advantages and Disadvantages of WAN Application Layer in OSI Model Cyclic Redundancy Check Example Data link layer in OSI model Difference between Transport and Network Layer Hamming Code Example Network Layer in OSI Model Session Layer in OSI Model Transport Layer in OSI Model Two Port Network in Computer Networks Uses of Computer Networks What is Computer Network What is Framing in a Computer Network Advantages and Disadvantages of Bus Topology Difference between Star Topology and Bus Topology Subnetting in Computer Network Subnetting Questions and Answers What is Bus Topology What is Network Topology and Types in Computer Networks Access Control in Networking Basic Characteristics of Computer Network Benefits of SOCKS5 Proxy in Computer Networks Computer Network viva Questions Difference between BOOTP and RARP Difference Between Network Topologies and Network Protocols Difference between NFC and RFID Difference Between Point-to-Point Link and star Topology Network Differences Between MSS and MTU Differences Between Trunk Port and Access Port Different Modes of Communication in Computer Networks MIME Protocol in Computer Networks Modes of Communication in Computer Networks Network Attack in Computer Network Port Address in Networking Simplest Protocol in Computer Network Sliding Window Protocol in Computer Network Stop And Wait Protocol in Computer Networks TCP 3-Way Handshake Process in Computer Networks What is a Proxy Server What is APPN What is ICMP Protocol What is Point-to-Point Protocol What is Port Address in Networking What is the HDLC Protocol What is VRRP Protocol Difference Between Analog and Digital Signals Difference Between Hub and Repeater Difference between Repeater and Switch Difference Between Transparent Bridge and Source Routing Bridge Source Routing Bridge in Computer Networks Transparent Bridge in Computer Networks Transport Protocol in Computer Networks Types of CSMA in Computer Networks What is Wired and Wireless Networking Network Security in Computer Network Disadvantages of Extranet Difference Between TELNET and FTP Define Protocol in Computer Networks Guided Transmission Media in Computer Network What is a Gateway in a Computer Network IGMP in Computer Networks LAN Protocols in Computer Networks MAN Meaning in Computer Modulation Techniques in Computer Networks Switching in DCN TCP/IP Applications What is IGMP? What is Modem in Networking What is Non-Persistent CSMA Difference between Cell Splitting and Cell Sectoring Forouzen Computer Network

Multiple Access Control

Multiple Access Control: The data link layer is separated into two sub-layers. The upper sub-layer is responsible for flow control and error control that is called the logical link control layer. The lower sub-layer is responsible for multiple access resolution that is called media access control (MAC). The Sub-layer of the data link layer is shown in the figure below.

Multiple Access Control

The media access control protocol is divided into three protocols: The Random access, the Controlled-access, and the Channelization protocol.

Multiple Access Control

Random Access Protocol

In the Random access protocol, all systems are equal. No anyone system can depend and control another system. However, if more than one station attempts to transmit the data, there is an access conflict—collision, due to which the frames are either lost or changed.

Random access protocol is divided into two categories; firstly, aloha, and second CSMA (carrier sense multiple access). CSMA is later further divided into two parallel methods; CSMA/CD and CSMA/CA. When a collision is detected, CSMA/CD tells the station what to do. CSMA/CA attempts to stop a collision.

1.     Aloha

ALOHA is a Multiple Access Protocol which is used for random access in the network. Aloha was designed for the wireless local area network (WLAN), which is also known as the radio broadcast. In this method, any user can transmit the data at any time.

Type of Aloha

1. Pure Aloha

2. Slotted Aloha

Pure Aloha: Pure aloha is also called the original aloha protocol. It's a simple but elegant protocol, i.e., whenever the system has a data frame to send, it transmits the data frame continuously. Due to which the risk of collision is very high in this aloha method. Shown in below pure aloha.

Multiple Access Control
  • The maximum throughput occurs at G = 1/2, which is 18.4%.
  • In Pure Aloha, the probability of successful transmission of the data frame is S= G* e^-2G
  • In Pure Aloha, vulnerable time is: 2 * Tfr

Slotted Aloha: Slotted aloha was developed to improve the efficiency of the Pure Aloha. In this Aloha, the time of the systems is divided into slots so that the system can send only one frame to a slot, and this frame can only be sent at the beginning of the slot. If a system cannot send a frame at the beginning of the slot, then it has to wait for the next slot to start. If two systems try to transmit the frame at the beginning of a time slot. But it is better than pure Aloha because it has less chance of collision. Shown in below slotted aloha.

Multiple Access Control
  • In Slotted Aloha, maximum throughput occurs at G = 1, which is 37%.
  • In Slotted Aloha, the probability of successful transmission of the data frame: S=G* e^-G
  • In Slotted Aloha, vulnerable time is: Tfr

2.     CSMA (Carrier Sense Multiple Access)

CSMA was developed to improve performance and minimize the chance of collision. Each station is required to test the state of the medium before transmission. In other words, CSMA is based on the principle “sense before transmit.” CSMA can reduce the risk of collision, but it cannot remove it.

CSMA primary access mode:

1-Persistent: The 1-persistent method is very easy and quick. In this method, after the station finds the line empty, it immediately transmits its frame. The chances of the collision are very high in this method because two or more stations immediately transmit their frames as soon as the line is found empty.

Non-Persistent: In the non-persistent method, if found the line is empty, it transmits the frames immediately. If the line isn't clear, it waits for a random period and detects the line again. This approach decreases the risk of a collision.

P-Persistent: This method is a combination of 1-Persistent and Non-Persistent advantages. The p-persistent approach decreases the risk of collision and increases performance. 

In the P-Persistent approach, the following steps follow after the station finds the line-empty:

  1. With probability (p), the station transmits its frame.
  2. With probability (q = 1 ? p), the station waits for the starting of the next time slot and re-test the line.
  3. If the line is empty, it goes to step 1.
  4. If the line is not empty, it behaves as though a collision has happened, and it uses the back-off process.

In later, CSMA is divided into two parallel methods: CSMA/CD and CSMA/CA. When a collision is detected, CSMA/CD tells the station what to do, and CSMA/CA attempts to stop a collision.

Multiple Access Control

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

CSMA/CD is a carrier communication network protocol that operates in the Multiple Access Control layer.

In this approach, the station tracks the medium after transmitting a frame to see whether the transmission was successful or not. If so, the station work is completed. But, if there is a collision, the frame is re-sent.

CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)

CSMA/CA is used in wireless networks because CSMA cannot detect the collision, so CSMA/CA is the only solution to collision avoidance.

In CSMA/CA, Collisions are avoided by the use of three techniques:

  1. Interframe space
  2. Contention window
  3. Acknowledgment
  • Interframe space: If a station detects the idle channel, the station does not send the frame immediately. It waits for a time period. That time period is called IFS time. IFS is often used to describe the station's priority.
  • Contention window: The contention window is the sum of time that is split into slots. When a station is ready to send a frame, it can choose a random number of slots as the Wait time. According to the binary exponential back-off strategy, the number of slots in the window changes. There are shown in the figure below.
  • Acknowledgment: Positive acknowledgment or time-out timer helps to guarantee a successful transmission.

Controlled Access

In this method, stations contact each other to see which channel is appropriate for transmission. A station may not send a frame until other stations approve it. There are three types of controlled access: Reservation, Polling, and Token Passing.

Channelization

In this process, the usable bandwidth of the network is shared between several stations in time, distance, or through-code. There are three types of Channelization: FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), and CDMA (Code Division Multiple Access).

FDMA figure is shown below.

Multiple Access Control

TDMA figure is shown below.

Multiple Access Control