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May 2019

Media Access Control Method

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A way data moves from one terminal to another and how the computer terminal on a network gains and controls the transfer of data packets over the network through the cables forming the communication link called the Media Access Control Method. A collision can occur, and data may be corrupted if two or more computers send data simultaneously, except a method resolves the collision gracefully. Media access control methods ensure the smooth traffic flow on a network and prevent or deal with collisions. Media access control methods are implemented at the data-link layer of the Open Systems Interconnection (OSI) reference model. There are four main media access control methods in Networking:

 

Layer 2 protocols identify packet encapsulation into a frame and the method for getting the encapsulated packet on and off each medium. The method also used to get the frame on and off the media is called the media access control method.

As packets travel from the source to the destination, they traverse different physical networks. These physical networks can consist of different types of physical media, such as copper wires, optical fibers, and wireless networks consisting of electromagnetic signals, microwave and radio frequencies, and satellite links.

Without the data link layer, network layer protocols such as IP would have to make provisions for connecting to every media type that could exist along a delivery path. Moreover, IP must adapt whenever a new network technology or medium is developed. This process would slow down protocol and network media innovation and development. This is also a key reason for using a layered approach to networking.

Providing Access to Media

During a single communication, different media access control methods may be required. Every network environment has different characteristics. For example, Ethernet LAN, WLAN, and serial links have different characteristics.

Router interfaces encapsulate the packet into a suitable frame, and a proper media access control method is used to access each link. In any given exchange of network layer packets, there may be several data link layers and media transitions. At each hop along the path, a router does the following:

  • Accepts a frame from a medium
  • De-encapsulates the frame
  • Re-encapsulates the packet into a new frame
  • Forwards the new frame to the medium of that segment of the physical network

Controlling Access to The Media

The media access control layer (data link sub-layer) standardizes the placement of data frames onto the media. Media access control is the same as traffic rules that control the entry of vehicles onto a roadway. The lack of media access control would be the equivalent of vehicles ignoring all other traffic and entering the road without regard to the other vehicles. On the other hand, not all roads and entrances are the same. Traffic can enter the road by merging, waiting for its turn at a stop sign, or obeying signal lights. A driver follows a different set of rules for each type of entrance.

In the same way, there are different methods to control placing frames onto the media. The protocols at the data link layer define the rules for access to different media. These media access control techniques describe if and how the nodes share the media. The real media access control method used depends on the topology and media sharing.

What is a Datalink Layer?

What is a Datalink Layer?

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The OSI model’s data link layer (Layer 2) handles the moving of data in and out across a physical link in a network. It performs the node-to-node delivery of data. It takes a frame from the network layer, forms frames, and gives it to the physical layer. The data link layer also synchronizes the information transmitted over the data. The main responsibilities of this layer are the following:-

  • The data link layer encodes bits into packets before transmission and then decodes the packets back into bits at the destination.
  • Allowing the upper layers to access the media
  • Responsible for logical link control, media access control and also responsible for hardware addressing,
  • Handling and defining physical layer standards.
  • Preparing network data for the physical network and also controlling data placement and receiving on the media
  • Exchanging frames between nodes over a physical network media, such as UTP or fibre-optic
  • Receiving and directing packets to an upper-layer protocol
  • Performing error detection

The Layer 2 notation for network devices connected to a common media known as a node. Nodes build and send frames. The OSI data link layer exchanges Ethernet frames between source and destination nodes over a physical network media.

The data link layer effectively separates the media transitions that occur as the packet is forwarded from the communication processes of the higher layers. The data link layer receives packets from and directs them to an upper-layer protocol, in this case, IPv4 or IPv6. This upper-layer protocol does not need to know which media the communication will use.

Datalink Sub-Layers

As we know, the OSI model’s data link layer(Layer 2) is the protocol layer and handles moving data in and out across a physical link in a network. The data link layer is theoretically divided into two sublayers. Logical link control (LLC) and media access control (MAC) layer. This division is based on the architecture used in the IEEE 802 Project, the IEEE working group responsible for creating the values describing many networking technologies.

  • Logical Link Control (LLC)
  • Media Access Control (MAC)

Logical Link Control (LLC)

This upper sublayer is Logical Link Control(LLC), which communicates with the network layer. It places information in the frame that identifies which network layer protocol is being used for the frame. This information allows multiple Layer 3 protocols, such as IPv4 and IPv6, to use the same network interface and media. It provides services to the network layer above it and hides the rest of the details of the data link layer to allow different technologies to work seamlessly with the higher layers. Most local area networking technologies use the IEEE 802.2 LLC protocol.

Media Access Control (MAC)

This lower sublayer defines the media access processes performed by the hardware. It also provides data link layer addressing and access to various network technologies.

The figure above illustrates how the data link layer is divided into the LLC and MAC sublayers. The LLC communicates with the network layer, while the MAC sublayer allows various network access technologies. For instance, the MAC sublayer communicates with Ethernet LAN technology to send and receive frames over copper or fibre-optic cable. It also communicates wirelessly with wireless technologies such as WiFi and Bluetooth to send and receive frames.

Also, Read Physical Layer (Layer-2) of OSI Model

Wireless Media – Exclusive Introduction

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Wireless media carry data through electromagnetic signals using radio or microwave frequencies. It provides the best mobility options, and the number of wireless-enabled devices continues to increase. Wireless is quickly gaining popularity in enterprise networks as network bandwidth options increase. The first wireless transmitter went on air in the 20th century using Morse Code.

Nowadays, Cellular phones, Global Positioning Systems (GPS), Cordless Mouse, Cordless Keyboard, Cordless telephone sets, remote controls, Satellite televisions, Wireless LANs and some monitoring devices, such as intrusion alarms, employ acoustic waves at frequencies above the range of human hearing also classified as wireless. Wireless does have some important points to consider before planning:-

  • Coverage area: Wireless data communication technologies work well in open environments. However, certain construction materials in buildings, structures, and the local terrain will limit effective coverage.
  • Interference: Wireless Media is at risk of intrusion and can be disrupted by common devices such as household cordless phones, fluorescent lights, microwave ovens, and other wireless communications.
  • Security: Wireless Media communication coverage requires no access to a physical media strand. Thus, devices and users not authorized to access the network can gain access to the transmission. Network security is the main component of wireless network administration.
  • Shared medium: WLAN works in half-duplex, which means just one device can be sent or received at a time. The wireless medium is shared among all wireless users. The more users need to access the WLAN simultaneously, the less bandwidth each user will need.

Types of Wireless Media

The IEEE and telecommunications industry standards for wireless data communications cover the data link and physical layers. Cellular and satellite communications can also provide data network connectivity. But, we are not discussing these wireless technologies in this chapter. In each of these standards, physical layer specifications are applied to areas that include:

  • Transmission Frequency
  • The transmission power of the transmission
  • Data to radio signal encoding
  • Signal reception and decoding requirements
  • Antenna design and construction

Wi-Fi is a trademark of the Wi-Fi Alliance. The certified product uses that belong to WLAN devices that are based on the IEEE 802.11 standards. Different standards are the following:-

WI-FI standard IEEE 802.11

WLAN technology is commonly referred to as Wi-Fi. WLAN uses a protocol called Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). The wireless NIC must first listen before transmitting to decide if the radio channel is clear. If another wireless device is transmitting, the NIC must wait until the channel is clear. We will briefly discuss CSMA/CA.

Bluetooth standard IEEE 802.15

Wireless Personal Area Network (WPAN) standard, commonly known as “Bluetooth”, uses a device pairing process to communicate over distances from 1 to 100 meters.

WI-MAX Standard IEEE 802.16

Usually known as Worldwide Interoperability for Microwave Access (WiMAX), it uses a point-to-multipoint topology to provide wireless broadband access.

Wireless LAN (WLAN)

General wireless data implementation wireless LAN requires the following network devices:

  • Wireless Access Point (AP): In a wireless local area network (WLAN), an access point (AP) is a station that transmits and receives data. An access point also connects users to other users within the network and can serve as the interconnection point between the WLAN and a fixed wire network. Each access point can serve multiple users within a defined network area; as people move beyond the range of one access point, they are automatically handed over to the next one. A small WLAN may only need a single access point; the number required increases the function of the number of network users and the physical size of the network.
  • Wireless NIC adapters: Provide wireless communication ability to each network host.

As technology has developed, several WLAN Ethernet-based standards have emerged. So, care needs to be taken when purchasing wireless devices to ensure compatibility and interoperability.

The benefits of wireless data communications technologies are clear, particularly the savings on costly premises wiring and the convenience of host mobility.

Fiber vs Copper

Fiber vs Copper

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Choosing the perfect solution for your cabling infrastructure is not easy. However, once you know the different properties of copper and fiber, your solution may seem clearer. There are many advantages to fiber vs. copper cable, as explained below.

Fiber vs Copper Data Bandwidth

Bandwidth is the main character of communication, so here, first, we are going to compare fibre vs copper for bandwidth. Fiber-optic can support data bandwidth from 10 Mbps to 10 Gbps. However, copper wire infrastructure and TDM technology are limited. Because it was originally designed to transmit voice calls only, the demand for bandwidth wasn’t high. So, it supports 10 Mbps to 100 Gbps data only.

Distance Covered

The distance is another important consideration for comparing fiber vs copper. The signal travel on copper wire networks degrades with the length of the cable. So the distance covered by copper wire is very Short, which is from 1 meter to 100 meters. However, the signal travel on fiber optic cable does not degrade as the signal degrades in copper wire. The distance covered by fiber optic is very long, from 1-100,000 meters.

Security

The security difference between fiber and copper is important to consider. We can easily intercept copper cable only by connecting taps to a line to pick up the electronic signals. So it is very easy to compromise your security and difficult to trace compromise cable. But Putting a tap on a fibre-optic cable to intercept data transmissions is very difficult. It’s also easy to quickly find compromised cables, which visibly emit light from transmissions.

Immunity to EMI and RFI

Copper wire is a conductor, so it is very low in immunity to EMI and RFI. However, fiber optic cable is a non-conducting material, and electrical signals do not interfere with it. So, the immunity to EMI and RFI is very High (Completely immune)

Immunity to electrical hazards

The immunity to electrical hazards in copper wire is very low. However, fibre optics have high immunity to electrical hazards because of their non-conducting materials.

Size of Cable

The speed via copper cable is directly associated with the weight of the cable used. More cable is required to achieve a higher speed, which requires more space in a system room. However, fiber cable’s speed is not associated with its size, and it is far lighter than copper. This renders it easier to use and less demanding of limited space in small rooms.

Media and connector costs—The cost of the copper cable is low for the connector and media, but the cost of fiber optic-related media equipment and connectors is very high.

Installation skills required—The installation skills are most important for fiber vs. copper cables. Copper cable requires only some installation skills, but fiber optic cable requires highly skilled technicians and engineers for installation.

Safety precautions- The copper wire safety precautions are low. But fiber optic required high safety precautions.

Power over Ethernet (PoE)—Copper cable provides the facility of power over Ethernet(POE), but fiber optic does not provide such facility.

Flexibility – Copper wire is more flexible than fiber optic cable.

Reliability- Copper wire is less reliable than fiber optic cable.

Fiber Optic Connectors

Fiber Optic Connectors

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Fibre optic connectors terminate both ends of an optical fibre. A variety of fibre optic connectors are available. The differences between the types of connectors are the size and the coupling methods. There are four types of fibre optic connectors in use. The list of all types of fibre optic connectors is as follows: We should discuss the first four fibre optic connectors in detail.

  1. ST Connector (ST)
  2. Standard Connector (SC)
  3. Lucent Connector (LC)
  4. Ferrule Core Connector (FC)
  5. Bionic Connector
  6. SMA Connector
  7. E2000 Connector
  8. Enterprise Systems Connection Connector (ESCON)
  9. Plastic Fiber Optic Cable Connectors
  10. MT Connector
  11. Opti-Jack Connector
  12. Fiber Distributed Data Interface Connector (FDDI)
  13. LX-5 Connector
  14. Volition Connector
  15. MU Connector
  16. MT-RJ Connector

ST Connectors

ST Connectors_Fiber optic Connectors

The ST connector was one of the first connector types broadly implemented in fiber optic networking applications. It was the most popular connector for the multimode network. It was originally developed by AT&T. ST stands for Straight Tip connector. ST connectors use a 2.5mm ferrule with a round plastic or metal body. The connector stays in place with a “twist-on/twist-off” bayonet-style mechanism.

Although extremely popular for many years, the ST connector has slowly been replaced with smaller, denser connectors in many installations. The SC/FC/FDDI/ ESCON connectors have the same ferrule size—2.5 mm or about 0.1 inch—so they can be mixed and matched to each other using hybrid mating adapters. ST connectors are spring-loaded, so it is necessary to make sure they are seated properly. If you have a high loss, open the connector and reconnect it to see if it makes a difference.

SC Connector

SC Connectors_Fiber optic Connectors

Simple, rugged and low-cost, SC connectors use a ceramic 2.5 mm ferrule to deliver accurate alignment of the SMF. They use a push-on/pull-off mating mechanism, which is usually easier to use than the twist-style ST connector when in tight spaces. The connector body of an SC connector is square-shaped.

Two SC connectors are usually held together with a plastic clip (a duplex connection). Japanese telecommunications company NTT originally developed SC, which is believed to be an abbreviation for Subscriber Connector or possibly Standard Connector.

LC connector

LC Connectors_Fiber optic Connectors

The Lucent Connector, sometimes called the Little Connector, is the popular Small Form Factor (SFF) connector. Lucent Technologies developed this interface. It uses a retaining tab mechanism, similar to a phone or RJ-45 connector, and the connector body resembles the square-type shape of an SC connector.

LC connectors are normally held together in a duplex configuration with a plastic clip. The ferrule of an LC connector is 1.25mm. There are 3 different types of LC connectors:

  • Single Mode LC APC
  • Single Mode LC UPC
  • Multi-Mode LC UPC

 Ferrule Core Connector (FC)

FC Connectors_Fiber optic Connectors

Ferrule Core Connector (FC) was the most popular single-mode connector for several years. It uses a 2.5 mm ferrule core, but some early connectors used ceramic inside stainless steel ferrules.

It screws on firmly, but you must make sure you have the key aligned; in the slot properly before tightening the FC connector is a pretty common choice, for example, in Video over Fiber Transmission Equipment.

Unshielded Twisted Pair(UTP) Cabling

Unshielded Twisted Pair(UTP) Cabling

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Unshielded twisted pair (UTP) cables are widely used in the computer network and in the telecommunications industry as Ethernet cables and telephone wires.  Unshielded twisted-pair (UTP) cabling consists of four pairs of colour-coded copper wires twisted together and then enclosed in a flexible plastic sheath. The twist of the copper cables around each other cancels electromagnetic interference (EMI) from external sources. Unshielded means no additional shielding like meshes or aluminium foil, which add bulk, are used. It is small, which is helpful during installation. Unshielded twisted-pair (UTP) cable does not use shielding to counter EMI and RFI effects. Cable designers discovered that they can limit the negative effect of crosstalk in the following ways:-

Cancellation

When two wires in an electrical circuit are placed close jointly, their magnetic fields are the exact opposite of each other. So, the two magnetic fields cancel each other and cancel out any outside EMI and RFI signals.

Varying the several twists per wire pair

To improve the cancellation effect of paired circuit wires, designers vary the several twists of each wire pair in a cable. UTP cable must follow precise specifications governing how many twists or braids are permitted per cable meter. Notice in the figure below that the orange/orange-white pair is twisted more than the green/green-white pair. Each coloured pair is twisted a different number of times.

Standards of Unshielded Twisted Pair (UTP) Cabling

Unshielded Twisted Pair (UTP) cabling conforms to the standards recognized by the TIA/EIA. Specifically, TIA/EIA-568 stipulates the commercial cabling standards for LAN installations and is the standard most commonly used in LAN cabling. Some of the elements defined are below:-
1. Cable types
2. Cable lengths
3. Connectors
4. Cable termination
5. Methods of testing cable

Electrical and Electronics Engineers (IEEE) define the electrical characteristics of copper cabling. IEEE categories cables based on their ability to carry higher bandwidth rates. For example, Category 5 (CAT5) cable is commonly used in 100BASE-TX Fast Ethernet installations. Other Categories include Enhanced Category 5 (CAT5e) cable, Category 6 (CAT6), and Category 6a.
Cables in higher Categories support higher data rates. As new gigabit-speed Ethernet technologies are developed and adopted, CAT5e is now the minimally acceptable cable type.

CAT1 Cable

CAT1 cable is usually used for telephone wire. It does not support computer networking. The cable is not twisted and is also a single-pair cable, so it has zero capability to cancel the EMI and RFI. It supports a maximum of up to 1 Mbps data.

CAT2 Cable

It is a two-pair twisted-pair network cable supported by up to 4 Mbps data rate. The maximum working length of CAT2 cable is 100 meters. It is also used in telephone networks.

CAT3 Cable

CAT3 is a twisted four-pair network-supported cable with up to 10 Mbps data rate. The maximum data rate of CAT3 cable is 10 Mbps. Public telephone networks also use CAT3 cable. The maximum working length of CAT3 cable is 100 meters.

CAT4 Cable

CAT4 cable has four twisted pair cables supported up to 16 Mbps data rate and is mostly used in token ring networks. Telephone networks also use CAT4 cable for their services. The maximum working length of the cable is 100 meters.

CAT5 Cable

CAT5 cables are 4 pairs of twisted copper wire and have more twists per inch than CAT4, and CAT3, therefore, can run at higher speeds. The “twist” effect of each pair in the cables ensures any interference in one cable is cancelled with the twist of the partner cable. This type of wire can support computer networks and also telephone traffic for higher speeds of up to 100 Mbps. It was a popular cable in networking.

CAT5e Cable

The more popular CAT5 wire was later on replaced by the CAT5e, which provides improved crosstalk specification and allows it to support speeds of up to 1Gbps.

CAT6 Cable

CAT6 wire was designed to support Gigabit Ethernet, although there are standards that allow gigabit transmission over CAT5e wire. It is similar to CAT5e wire but contains a physical separator between the four pairs to reduce electromagnetic interference further. The maximum working length to support speeds of 1Gbps is 100 meters. The cable also supports 10Gbps speed for lengths of up to 55 meters.

CAT6A Cable

CAT6A supported 10G speed and its higher specification of presenting better immunization to crosstalk and electromagnetic interference.

CAT7 Cable

CAT7 cable is a newer copper cable designed to support speeds of 10Gbps at lengths of up to 100 meters. To achieve this, the cable features four individually shielded pairs plus an additional cable shield to protect the signals from crosstalk and electromagnetic interference (EMI). Due to the extremely high data rates, all components like patch panels, cords, jacks, and RJ-45 connectors must be CAT7 cable certified. CAT7 cable is usually used in Data Centers for backbone connections between servers, network switches, and storage devices.

Types of UTP cable by use

Different situations required UTP cable wiring with different conventions. This means that the individual wires in the cable have to connect in different orders to different sets of pins in the RJ-45 connectors. Following are the main cable types using specific wiring conventions:

Ethernet Straight-through is the most common type of networking cable. A straight-through cable generally connects the host to a switch and a switch to a router.

Ethernet Crossover: A cable used to connect similar devices. For example, to connect a switch to a switch, a host to a host, or a router to a router.

Rollover: A Cisco proprietary cable connects a workstation to a router or switch console port.
Figures 3-7 show the UTP cable type, related standards, and typical applications of these cables. They also identify the individual wire pairs for the TIA-568A and TIA-568B standards.

Using a crossover or straight-through cable incorrectly between devices may not damage them, but connectivity and communication between them will not take place. This is a common error in the lab and when checking device connections.

Cable TypeStandard
Ethernet Straight-throughBoth ends T568A, or Both ends T568B
CrossoverOne end T568A, another end T568B
RolloverRollover Cisco proprietary
Datalink and Network Layer Addresses

Datalink and Network Layer Addresses

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Data Delivery from the source device to a destination device is the primary responsibility of the network layer and data link layer. Protocols at both layers contain a source and destination address, but their addresses have different purposes.

Network Layer Addresses

Network Layer addresses deliver the IP packet from the source device to the destination device.  The destination may be on the same network or may be on the remote network. An IP address is also known as the network layer address. The IP address is also known as a logical address.  Any IP packet contains two network layer addresses:-

  • Source IP address– The IP address of the sending device and the packet’s source.
  • Destination IP address– The IP address of the receiving device and the packet’s final destination.

The figure below illustrates the source and destination IP address in the packet sent over the network.

network layer addresses

Data Link Layer Addresses

The data link layer delivers the data link frame from one network interface card (NIC) to another network interface (NIC) on the same network.  The address working on the data link layer is called the physical address or MAC address.

It has a different role than the IP address. It delivers the data link frame from one network interface card to another on the same network. Before an IP packet is sent over a wired or wireless network, it is encapsulated in a data link frame and transmitted over the physical medium.

The figure below illustrates the data link layer address or L2 address. As the IP packet travels from host to router, router to router, and finally router to host at each point, the IP packet is encapsulated in a new data link frame. Each data link frame has the source data link address of the NIC card sending the frame and the destination data link address of the NIC card receiving the frame.

In layer 2, the data link protocol is only used to deliver the packet from NIC-to-NIC on the same network. The router removes the Layer 2 information it received on one NIC and adds new data link information before forwarding out the exit NIC on its way toward the final destination. The IP packet is encapsulated in a data link frame that has data link information, including:

  • Source data link layer addresses – The physical address of the device’s NIC sending the data link frame.
  • Destination data link layer addresses—The physical address of the NIC receiving the data link frame. This address is either the next-hop router or the final destination device.
Data Link Layer Addresses
TCP/IP Protocol Suite

TCP/IP Protocol Suite

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The TCP/IP protocol Suite includes many protocols, as shown in the Figure below. Each layer has its own working protocol. Application layer protocols include the Domain Name System(DNS), host configuration protocols such as Dynamic Host Configuration Protocol (DHCP) and Bootstrap Protocol (BOOTP), Email protocols (POP, SMTP, and IMAP), file transfer protocols, and HyperText Transfer Protocol. The Transport Layer includes the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

The Internet layer working protocols are Internet Protocol (IP), Network Address Translation (NAT), Internet Control Messaging Protocol (ICMP), and routing protocol. The Network Access Layer includes Address Resolution Protocol (ARP), Point to Point Protocol (PPP), Ethernet, and Interface Drives.

TCP/IP Protocol Suite

The lower layer protocols in the network access layer are responsible for delivering the IP packet over the physical medium. Standards organizations, such as IEEE have developed these lower-layer protocols.

The Protocol suite is implemented; as a TCP/IP stack on both the sending and receiving hosts to provide end-to-end delivery of applications over a network. The Ethernet protocols are used to transmit the packet over the physical medium used by the LAN.

TCP/IP Communication Process

1. Host-1 sends data using the Hypertext Markup Language (HTML) page to Host-2.

2. The application protocol HTTP header was added to the front of the HTML data, as shown in the figure below. The header has various information, such as the IP header, Ethernet header, TCP header, and the HTTP version the server is using, as well as a status code representing it that has information for the web client.

 

The application layer protocol HTTP forwards the HTML-formatted web page data to the transport layer (Layer 4 of the OSI Model). The Transport Layer then adds a TCP header to the HTTP data and converts the data into a segment. In this example, the transport layer also handles each conversation between Host-1 and Host-2.

4. Next, the IP information is added to the front of the TCP information and converted to the packet segment. IP assigns the source and destination IP addresses.

5. The Ethernet protocol, known as a data link frame, adds information to both ends of the IP packet. This frame is delivered to the nearest router along the path towards the web client. This router removes the Ethernet information, analyzes the IP packet, determines the best path, inserts the packet into a new frame, and sends it to the next neighbouring router towards the destination. Each router removes and adds new data link information before forwarding the packet.

6. This data is transported through the internet through media and intermediary devices.

At the receiving end, the client receives the data link frames that contain the data. Each protocol header is processed and then removed in the opposite order it was added. After removing all headers, the user receives and sees the original data, as shown in the figure below.

TCP/IP Protocol Suite

[qsm quiz=9]

Transmission Control Protocol

Transmission Control Protocol (TCP)

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The transmission control protocol (TCP) is a network communication protocol designed to send data packets over the Internet. It is a transport layer protocol in the OSI model. It creates a connection between remote computers by transporting and ensuring the delivery of messages over supporting networks and the Internet.

When the application layer requires sending a large amount of data, it sends the data to the transport layer for transmission control protocol (TCP) or User Datagram Protocol (UDP) to transport it across the network.

The transmission control protocol (TCP) first establishes a connection between the source and destination in a three-way handshake process. After connection establishment, it breaks the data into segments, adds a header to each segment, and sends them to the Internet layer. The transmission control protocol (TCP) header is 20 to 24 bytes in size, and the format is shown in the Figure below.

Transmission Control Protocol

When the Application layer sends data to the transport layer, TCP sends the data across using the following sequence:

Transmission Control Protocol (TCP) three-way handshake

TCP uses a three-way handshake to establish a connection between client and server. The three-way handshake has three steps. It uses the SYN and ACK flags in the Code Bits section of the header. This process is necessary to start the sequence and acknowledgement number fields, which are important for TCP. A three-way handshake is also known as a TCP handshake. The following figure illustrates the TCP three-way handshake.

TCP

As shown in the above figure, the source starts the three-way handshake by sending a TCP header to the destination with the SYN flag set. The destination responds with the SYN and ACK flag sent. Examine that the destination uses the received sequence number plus 1 as the Acknowledgement number. This is because it is assumed that 1 byte of data was contained in the exchange. The source responds with only the ACK bit set in the final step. After this, the data flow can commence.

Data Segmentation

The data size transmitted across a single Internet layer PDU is limited by the protocol used in that layer. This limit is called the maximum transmission unit (MTU). The application layer may send data much larger than this limit; hence, TCP has to break down the data into smaller segments. Each segment is limited in size to the MTU. Sequence numbers are used to identify each byte of data. The sequence number in each header signifies the byte number of the first byte in that segment.

Flow Control

Flow control ensures the rate at which a sender is transmitting is proportional to the receiver’s receiving capabilities.  It manages the flow of data/packets among two different nodes, especially in cases where the sending device can send data much faster than the receiver can take in.

The TCP process initializes when the source sends data in groups of segments. The Window bit in the TCP header (Check-in TCP header Image) determines the number of segments that can be sent simultaneously to avoid an irresistible destination. At the start of the session, the window is small, but it increases over time.

The destination host can also decrease the window to slow down the flow. Hence, the window is called the sliding window. When the source has finished the number of segments allowed by the window, it cannot send any further segments until an acknowledgement is received from the destination.

The figure below illustrates how the window increases during the session. Notice the Destination host increasing the Window from 800 to 1000 simultaneously when it sends an ACK back to the source. This process is called windowing.

 Reliable Delivery with Error recovery

When the destination receives the last segment in the agreed window, it must send an acknowledgement to the source. It sets the ACK flag in the header, and the acknowledgement number is set to the sequence number of the next byte expected. If the destination does not receive a segment, it does not send an acknowledgement back. This tells the source that some segments have been lost, and it will re-transmit the segments.

The above figure illustrates how windowing and acknowledgement are used in the TCP process. Notice that when the source does not receive acknowledgement for the segment with sequence number 2000, it re-transmits the data. Once it receives the acknowledgement, it sends the next sequence according to the window size.

 Ordered Delivery

TCP transmits data in the order received from the application layer and uses the sequence number to mark the order. The data may be received at the destination in the wrong order due to network conditions. Thus, TCP at the destination orders the data according to the sequence number before sending it to the application layer at its end. This order delivery is part of TCP’s benefit and one of the purposes of the Sequence Number.

Connection Termination

When all data has been successfully transferred, the source initiates a four-way handshake to close the session. To close the session, the FIN and ACK flags are used.  FIN and ACK will be discussed in the coming articles.

6 Important Network Devices – Exclusive Details

6 Important Network Devices – Exclusive Details

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Network devices are physical devices required for communication and interaction between hardware on a computer network. They work as a group and have a single purpose: to securely transfer data as fast as possible. Several network devices, such as repeaters, bridges, hubs, switches, and routers, are used to meet this goal.

Hub

Hub is one of the basic network devices that works at the physical layer and hence connects networking devices physically together. It is not used in modern networks. In networking, it is just studied because it is helpful to understand switches. If somebody understands it, then he can easily understand a switch.

A hub is a device that simply copies data received on any port to all its ports. So, if a packet of data arrives on interface 1 of a 5-port hub, it will blindly copy that data out to interfaces 2 through 5. The hub is a common connection point for network devices in a network. Different segments of LAN are commonly connected to it. It was a cheap and quick way to link up multiple computers in the early days.

The hub utilizes Carrier Sense Multiple Access with Collision Detect (CSMA/CD) to control Media access. The Ethernet hub communicates in a half-duplex mode, where data collisions are inevitable. The main issue with hubs is that only one computer can talk at a time.

So, if 2nd computers are going to talk at the same time, their traffic will join as it echoes out the other interfaces. This is called a collision, and it would corrupt the data being transmitted by both computers. So, each computer would have to try again after a random period. This becomes a real problem when the network gets busy or when more than a handful of computers are on a network. A switch solves the collision issue. Hub is a single broadcast and single collision domain. It has two types:-

Passive Hub

They Just point contact for the wires to be built into the physical network. They have nothing to do with modifying the signals.

Active Hub

Active hubs are smarter than passive hubs. They regenerate, concentrate, and strengthen the original signals before sending them to their destinations. Active hubs are also termed ‘repeaters’.

Repeater

A repeater is an electronic device that operates at the physical layer. It has two Ethernet ports. The repeater amplifies the received signal and retransmits it in the same network before it becomes too weak or corrupted, extending the length to which the signal can be transmitted over the same network. When the signal weakens, the repeater copies it bit by bit and regenerates it at its original strength.

Bridge

If a router connects two different types of networks, then a bridge connects two sub-networks as part of the same network. Bridges’ essential role in network architecture is storing and forwarding frames between the different segments that they connect.

Bridgeworks are at the Physical and Data Link Layer of the OSI Model. They connect different networks and develop communication between them. They connect two local-area networks, two physical LANs, into larger logical LANs or two segments of the same LAN that use the same protocol.

We can also use the bridge to divide larger networks into smaller sections by sitting between two physical network segments, managing the data flow, and reducing the broadcast between them.

Bridge uses MAC addresses for transferring frames. Bridges can forward the data or block it from crossing by looking at the MAC address of the devices connected to each segment. Bridges can also connect two physical LANs into a larger logical LAN. There are three main types of bridges:-

Transparent Bridge

A transparent bridge maintains a list of MAC addresses and appears transparently to other network devices. The other devices are ignorant of its existence. It only blocks or forwards data according to the MAC address. Transparent bridges also save and maintain the source-route addresses of incoming frames by listening to all the connected bridges and hosts. They use a transparent bridging algorithm to accomplish this.

Source Route Bridge

A form of routing is used to establish connections between pairs of nodes on different token rings. The source route bridge uses the path the packet takes through the network and is implanted within the packet.

Translational Bridge

Translational bridges reorder source and destination address bits when translating between Ethernet and Token Ring frame formats. They convert the data format of one networking system to another.

network devices

Switch

An Ethernet Switch is a device used to connect multiple computers and network devices within a LAN. It works at the OSI model’s Layer Two (Data Link Layer). Some switches also work at Layer 3( Network Layer). These switches are referred to as Layer 3 switches or multilayered switches.

The Basic Function of a Network Ethernet Switch and a Network Ethernet Hub is the same: forwarding Layer 2 packets (Ethernet frames) from the source device to the destination device. But a Network switch is more intelligent than a hub. An Ethernet switch uses MAC addresses to make forwarding decisions. It does not know about the protocol in the data portion, such as an IPv4 packet. The switch makes forwarding decisions based only on the Layer 2 Ethernet MAC addresses.

Unlike an Ethernet hub that repeats bits from all ports except the incoming port, an Ethernet switch consults a MAC address table to make a forwarding decision for each frame. The MAC address table is sometimes called a content addressable memory (CAM) table. Network switches for different input and output bandwidths are available. Today’s Ethernet Network Switches can have bandwidths of 10, 100, 1000, or 10,000 Megabits per second.

Switch Features and Advantages

  • Connect network devices in a Local Area Network (LAN).
  • It learns Layer 2 (MAC) addresses and forwards Layer 2 packets (Ethernet frames); to the exact destination with the help of the device’s MAC address.
  • It’s control of who has access to various parts of the network.
  • Provision to monitor network usage.
  • High-end switches have pluggable modules.
  • Allows to connect multiple devices and port can be managed VLAN can create security also can apply
  • First broadcast; then unicast & multicast as needed.
  • Switches use content-accessible memory CAM table, typically accessed by ASIC (Application Specific integrated chips).
  • Half/Full duplex
  • Connecting two or more nodes in the same network or a different network
  • The switch has one broadcast domain [unless VLAN is implemented]

Router

The router is a network device that selects the best path for a data packet. It is located at any gateway (where one network meets another). It forwards data packets from one network to another based on the address of the destination network in the incoming packet and an internal routing table. It also determines which port (line) to send the packet (ports typically connect to Ethernet cables).

Routers also require packets formatted in a routable protocol, the global standard being TCP/IP, or simply “IP.” Routers operate at Layer 3 (network layer) of the OSI model, and they use the destination IP address in a data packet to determine where to forward the packet. The router stores the IP address in a Routing table and maintains an address on its own.

Gateway

Gateways usually work at the Transport layer and Session layer of the OSI model. It connects two networks that may work on different networking models. Gateway takes data from one system, interprets it, and transfers it to another. It is also a protocol converter and can operate at any network layer. Gateways are generally more complex than switches or routers. Gateway deals with numerous protocols and standards from different vendors. It performs all of the functions of routers. A router with added translation functionality is a gateway.

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