Asad Ijaz

About Asad Ijaz

https://networkustad.com/

Posts by Asad Ijaz:

unicast

Unicast, Multicast, and Broadcast Addresses

, , , , , , , , , , , , , , , , , , , , , 1 Comment

In Ethernet, broadcast and multicast addresses always describe a group of recipients. But the unicast is used to explain communication where data is sent from one point to another point.

Unicast Address

In case of a unicast address, a packet has sent from a single source to a specific destination. All Ethernet and IP networks support unicast transmission. A unicast MAC address is the unique address uses for sending a  packet from a single transmitting device to a single destination device.

For sending a unicast packet from source to destination, a destination IP address must be in the IP packet header. A destination MAC address must also be present in the Ethernet frame header. The IP address and MAC address combine to deliver data to a specific destination

Multicast Address

A multicast address is a logical identifier for a group of hosts in a computer network that is available to process datagram or frames intended to be multicast for a designated network service. The Data link layer of the OSI model also uses a multicast address.

The multicast MAC address is a special value that begins with 01-00-5E in hexadecimal. The range of multicast MAC addresses is from 01-00-5E-00-00-00 to 01-00-5E-7F-FF-FF for Ethernet and Fiber Distributed Data Interface (FDDI) media access control (MAC) addresses. The remaining portion of the multicast MAC address is created; by converting the lower 23 bits of the IP multicast group address into 6 hexadecimal characters.

The range of IPv4 multicast addresses is 224.0.0.0 to 239.255.255.255. By the help of multicast addresses; a source device can send a packet to a group of devices that belong to a multicast group. These devices assign a multicast group of IP address. The source will also be a unicast address.

The online games also use multicast addresses, where many players connected remotely but playing the same game. Another use of the multicast address is in distance learning through video conferencing, where many students connected to the same class.

Broadcast Address

When a single computer sends messages to many at once, is called a broadcast. The television broadcasting is the most common example of a broadcast. The radio transmission is another example of broadcasting.

In computer networking, a broadcast means that the network sends one copy of a packet to each destination. The broadcast is used very frequently in the networking world. The terms broadcast IP address and broadcast MAC address is common in networking.

A broadcast packet also contains a destination IPv4 address that has all ones (1s) in the host portion (see 172.17.255.255) in IP Packet. This numbering in the address means that all hosts on that local network (broadcast domain) will receive the packet.

Several network protocols, such as ARP and DHCP, use broadcasts. When the IPv4 broadcast packet is encapsulated in the Ethernet frame, the destination MAC address is the broadcast MAC address of FF-FF-FF-FF-FF-FF in hexadecimal which is 48 ones in binary.

MAC Address

MAC Address

, , , , , , , , , , , , , , , 5 Comments

A media access control address or a MAC address of a device is a worldwide unique identifier assigned to a network interface controller. It is also known as “hardware address” or “physical address” and very important for communication within network segments. Most IEEE 802 network technologies, including Ethernet, Wi-Fi, and also a Bluetooth uses Media Access Control address for within network segment communication.

It is a 48-bit binary value expressed as 12 hexadecimal digits. As we know that decimal is a base ten “(xxx)10” number system and Hexadecimal is a base sixteen “(xxx)16” number system. The base sixteen numbers system uses the number from 0 to 9 as well as letters from A to F. Where A= 10, B=11, C=12, D=13, E=14, and F=15.

Ethernet Identity

In Ethernet LAN, each network device connected to share media. Therefore all nodes would receive every frame transmitted over the shared media. So, to stop too much overhead involved in the processing of every frame, MAC addresses identify the actual source and destination. Media Access Control addressing also provides a method for device identification.

MAC Address Structure

Vendor Develop Ethernet Card and also assign it a unique address followed by IEEE standards. IEEE required registration of any organization with them that manufactures Ethernet Devices and NIC cards.

After registration, IEEE assigns a 3-byte code term Organizational Unique Identifier(OUI). So, each MAC Addresses assigned to an Ethernet device must have a vendor OUI and also a unique serial number assigned to them.

Media Access Control Addresses assigned to an Ethernet device must have a vendor OUI and a unique serial number assigned to them. Hardware and software manufacturers represent the Physical address or Media Access Control address in different hexadecimal formats that are 12 digits (6 byte or 48 bits), as shown in the following:

  • MM:MM:MM:SS:SS:SS
  • MM-MM-MM-SS-SS-SS
  • MMM.MMM.SSS.SSS

The leftmost 6 digits (24 bits) called an “OUI” which associated with the adapter manufacturer. Every manufacturer registers and obtains MAC OUI as assigned by the IEEE. the manufacturer often possesses many OUI numbers associated with their different products.

The rightmost digits of a Media Access Control address represent an identification number for the particular device. Devices manufactured by the same vendor OUI, each is given their own unique 24-bit number. The figure below illustrates the Media Access Control Address. The figure below illustrates the Media Access Control address OUI as well as Interface controller-specific number.

MAC Address

IEEE requires a vendor to follow two simple rules:-

  • MAC addresses assigned to a NIC and also to another Ethernet device must use that vendor’s assigned OUI as the first 3 bytes.
  • All MAC addresses with the same OUI must be assigned a unique value in the last 3 bytes.

Note: Duplicate Media Access Control addresses are possible to exist due to mistakes during manufacturing or in some virtual machine implementation methods. In either case, it will be necessary to modify the MAC address with a new NIC or in software.

How to find MAC address for Personal Computer

On a Windows host, we can find the MAC address using the command “ipconfig /all” as shown in the figure below. This command identifies the MAC address of an Ethernet adapter. The steps for finding MAC address are the following:-

  1. Click Windows Start or press the Windows key.
  2. In the search box, type cmd, and press Enter
  3. The command prompt you should see now.
  4. Type ipconfig /all. and press Enter
  5. A Physical Address displays for each adapter. The Physical Address is your device’s MAC address.

ipconfig

Note:- We often see various representations of MAC addresses which depend on devices and operating system. Cisco routers and switches also use the address in the form of XXXX.XXXX.XXXX where X is a hexadecimal character.

Ethernet frame structure

Ethernet Frame Structure (2020)- Easy Explanation

, , , , , , , , , , , , , , , , , , , , , , , , , , , , 26 Comments

The basic Ethernet frame structure is defined in the IEEE 802.3 standard. However several optional Ethernet frame formats are being used to extend the protocol’s basic capability. The Early versions of frames structure were comparatively slow.

The latest versions of Ethernet operate at 10 Gigabits per second. This is the fastest version of Ethernet. At the data link layer, the frame structure is almost the same for all speeds of Ethernet.

What is in an ethernet frame’s trailer and header

The frame structure adds headers and trailers around the Layer 3 Protocol Data Unit (PDU) to encapsulate the message.  Ethernet-II is the frame format uses in TCP/IP networks.  It starts with the Preamble which works at the physical layer.

Ethernet header contains both Source and Destination Media Access Control address, after which the payload of the frame is present. The last field is CRC (Cyclical Redundancy Checking) which is used to detect the error. The figure below illustrates the frame structure and fields.

Ethernet frame structure

The minimum Ethernet frame size is 64 bytes and the maximum size of the frame is 1518 bytes. This includes all bytes from the Destination MAC Address field through the  Frame Check Sequence (FCS) field excluding the Preamble field. The Preamble field is not included when describing the size of a frame.

Every less than 64 bytes in length has been considered a “runt frame/ runt packet” or “collision fragment” and the receiving stations automatically discard these frames and frames with more than 1500 bytes of data considered “jumbo” or “baby giant frames”. If the frame is less than or greater than above mention size, the receiving device drops and discard the frames.

Preamble and SFD Fields

This preamble field has 7 bytes and Start Frame Delimiter (SFD) field has 1 byte. The Start Frame Delimiter (SFD), also called the start of frame( 1 Byte) and the Preamble field has (7 bytes).

Sending and receiving nodes and devices use both fields for synchronization. The first eight bytes of the frame acquire the attention of the receiving nodes. These first few bytes tell the receivers to get ready to receive a new frame.

Destination MAC Address Field

The Destination MAC Address Field has 6-byte, the identifier for the recipient. The address in the frame and the MAC address of the device is compared. If there is a match, the device accepts the frame. The MAC address can be a unicast, multicast or broadcast.

Source MAC Address Field

Source MAC Address, the Media Access Control address of the outgoing network interface card. This 6-byte field identifies the originating device. It must be a unicast address.

Ether Type Field

This field size is 2-byte long and it identifies the upper-layer protocol encapsulated in the frame. Common values are, in hexadecimal, 0x800 for IPv4, 0x86DD for IPv6 and 0x806 for ARP.

Data Field

This field is containing the original encapsulated data from a higher layer. This field size is 46 – 1500 bytes. All frames must be at least 64 bytes long. If an encapsulated packet is small in size, additional bits called a pad increase the size of the frame to this minimum size.

Frame Check Sequence (FCS)

The Frame Check Sequence (4 bytes)  detect errors in the frame. It uses a cyclic redundancy check (CRC). The sending device includes the results of a cyclic redundancy check (CRC) in the frame check sequence (FCS) field of the frame.

The receiving device receives the frame and generates a cyclic redundancy check (CRC) to look for errors. If the calculations match, no error occurred.

Calculations that do not match are an indication that the data has changed, therefore, the frame is dropped. A change in the data could be the result of a disruption of the electrical signals that represent the bits.

[qsm quiz=5]

MAC and LLC

2 Sublayers – LLC and MAC Important Functions

, , , , , , , , , , , , , , , , , , , , , 0 Comments

The data link layer is split into logical sublayers, the upper sublayer, known as LLC sublayer, that communicates with the network layer and the lower sublayer, known as MAC sublayer, that communicates with the physical layer below. This article presents a brief function both sublayers of the data link layer. The Figure below illustrates the structure of LLC and MAC Sublayers۔

Sublayers

LLC Sublayer Overview and Function

The upper sublayer of the data link layer is known as LLC sublayer. It communicates with upper layers of the OSI model. It gets the network protocol data, which is usually an IPv4 packet. LLC sublayer also adds control information to help deliver the packet to the destination. The LLC sublayer communicates with the upper layers of the application and transition the packet to the lower layers for delivery.

LLC is implemented in software, and its implementation is independent of the hardware. The driver software of computer NIC and be considered as LLC. The driver is a software program and instruction that interacts directly with the computer NIC and other hardware. Driver software also enables the communication between the network device, computer, and operating system as well as with other network computers and network devices.

Multiplexing/De-Multiplexing

As I mentioned earlier that LLC communicates with the uppers i.e network layer. When the data link layer received a frame from the physical layer, the LLC is looking at the Layer 3 Protocol type and handing over the datagram to the correct Layer 3 protocol. It is called the de-multiplexing process.

When Layer 3 sends a packet, LLC takes it from different Layer 3 protocols like IP, IPX, ARP etc., and forward it over to the MAC layer after adding the Layer 3 protocol type in the LLC header portion of the frame multiplexing. This process is called the multiplexing process.

LLC optionally present reliable frame forwarding.  By the help of LLC the sending node numbering each frame during transmission called sequence number, at the other hand, the receiving node acknowledged each received frame using the acknowledgement number.

If the frame lost during transmission, the sending node retransmits lost frames.  The LLC also provides flow control optionally by allowing the receivers to control the sender’s speed through control frames known as Receive Delay and Receive Not Delay. LLC provides the following types of services.

Unacknowledged Connectionless Service

Connectionless service is also known as the best effort service e.g. IP datagram service, without prior connection establishment between source and destination and without acknowledgement. In the connectionless service, the data is sent to the peer directly without any connection establishment. The connectionless service is also unreliable because reliability, flow control and error control are provided at the transport layer. It is provided flow control optionally.

Connectionless Acknowledged Service

In this type of service, the data is directly sent between Layer2 peers without any prior logical link establishment. But the difference with previous is that this service mark each frame with its sequence numbers and the peer acknowledges each frame received using an Acknowledgment number field. This service mode is used where data reliability is needed.

The data sender can track lost or damaged frames and retransmit these frames to accomplish reliability. The wireless links use connectionless acknowledged service, where the quality of the link is not good.

Connection-Oriented Service

Before sending data, a logical connection is established between peers in this type of service. Before start data transfer, the service exchanges the control frames known as Supervisory Frames to establish a logical connection. Actual data transfer starts after the connection establishment phase is over. There are two types are used in connection-oriented service, one is without acknowledgement and another is with acknowledgement.

Connection-oriented service Without Acknowledgment

In this type of service a logical link is established before actual data transfer start but without any concept of frames being numbered and acknowledged through Sequence number and acknowledgement number fields. The example of this service is HDLC, PPP and LAPB.

Connection-oriented service with Acknowledgment

In this type of service a logical link is established before actual data transfer start, but with the concept of frames being numbered and acknowledged through Sequence number and acknowledgement number fields. Reliability and flow control services are also provided by the LLC sublayer through the use of sequence number, acknowledgement number and retransmission of lost frames. Sliding window mechanism provides Flow control.  Most of the internet use this type of services because the Internet uses TCP,

The function of MAC sublayer

MAC is the lower sublayer of the data link layer. It is listed in the IEEE 802.3 standards. The figure above lists common IEEE Ethernet standards. It also shows how the data link layer divided into the LLC and MAC sublayer. The LLC sublayer 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. The MAC sublayer also communicates with wireless technologies such as Wi-Fi and Bluetooth to send and receive frames wirelessly.

As we know that the Media Access Control (MAC) layer is between the LLC sublayer and physical layers, The MAC sublayer has the following key tasks:

  • Data encapsulation
  • Media access control

Data encapsulation

The data encapsulation process includes frame assembly from the transmission node and disassembly on the receiving node. To form the frame, the MAC layer adds a header and trailer to the network layer PDU. Data encapsulation provides three key functions:

  • Data Framing-The framing process provides a sequence of one or more characters to recognize a group of bits that make up a frame. These sequences bits provide synchronization between the transmitting and receiving nodes.
  • Addressing -The data link layer is the lowest in the OSI model concerned with addressing. Data Link Layer receive Layer 3 PDU. The encapsulation process in the data link layer provides data link layer address as well as labelling information with a particular destination location. Every device on a network has a unique number, generally called a physical address or MAC address, that is used by the data link layer protocol to ensure that data intended for a specific machine get to it properly.
  • Error detection –The data link each frame contains a trailer that used to detect any errors in transmissions.

Media Access Control

The Media Access Control sub-layer second responsibility is to control access to the media. This sub-layer is also responsible for the placement of frames on the media and the removal of frames from the media. The MAC sub-layer communicates directly with the physical layer.

[qsm quiz=4]

Ethernet Standards

Different Ethernet Standards

, , , , , , , , , , , , , , , , , , , , , , , , , , 7 Comments

Most widely used technology in networking is Ethernet. As we say in the early lesson that Ethernet work both in the data link layer and the physical layer. Ethernet standards define both the Layer 2 protocols and the Layer 1 technologies. For the Layer 2 protocols, as with all 802 IEEE standards, Ethernet relies on the two separate sublayers of the data link layer to operate, the Logical Link Control (LLC) and the MAC sublayers.

Other LAN types include Token Ring, Fast Ethernet, Gigabit Ethernet, 10 Gigabit Ethernet, Fiber Distributed Data Interface (FDDI), Asynchronous Transfer Mode (ATM) and Local Talk. There are different Ethernet standards that support different band of data:-

 

Ethernet standards

Ethernet is the most popular physical layer LAN technology in use now a day. A standard Ethernet network can broadcast data at a rate up to 10 Mbps. Ethernet is popular as it strikes a good balance between speed, cost and ease of installation.

The Institute for Electrical and Electronic Engineers developed an Ethernet standard known as IEEE Standard 802.3. This standard also defines rules for configuring an Ethernet network as well as how the elements in an Ethernet network interact with each other. By adhering to the IEEE standard, network equipment and network protocols can communicate efficiently.

Fast Ethernet

The Fast Ethernet standard has been established for Ethernet networks that need higher transmission speeds. The standard defines for fast Ethernet is IEEE 802.3u. This standard extends the Ethernet speed limit from 10 Mbps to 100 Mbps with little changes to the existing structure. It also provides a better result for video, multimedia, graphics, Internet surfing, and stronger error detection and correction. Fast Ethernet has three types:-

  • 100BASE-TX using Cat-5 and Cat-6 UTP Cable.
  • 100BASE-FX using Fiber Optic Cable.
  • 100BASE-T4 using Cat-3 UTP Cable.

Gigabit Ethernet

Gigabit Ethernet is more for faster than Fast Ethernet networks with applications such as multimedia and VoIP. The Gigabit Ethernet speed is 10 times faster than Fast Ethernet. The Gigabit Ethernet is defined in the IEEE 802.3 standard as a backbone. Ethernet and Fast Ethernet can feed into a Gigabit Ethernet to interconnect high-performance switches, routers, and servers.

10 Gigabit Ethernet

This is the fastest as well as recent of the Ethernet standards. IEEE 802.3ae defines a version of Ethernet with a nominal rate of 10 Gbits/s. The  10 Gigabit Ethernet is 10 times faster than Gigabit Ethernet. The difference with other Ethernet is that it is based entirely on the use of optical fibre connections.

Asynchronous Transfer Mode (ATM)

ATM is the abbreviation of Asynchronous Transfer Mode. It is a cell-based fast-packet communication technique that supports data-transfer rates from sub-T1 speeds to 10 Gbps. It can be integrated into an existing network as needed without any further update the existing network. The technology was basically for the high-speed transmission of all forms of media from basic graphics to full-motion video.

Due to the small size of cells, ATM equipment can transmit large amounts of data over a single connection. It also allows ISPs to assign limited bandwidth to each customer. While this may seem like a downside for the customer, it actually improves the efficiency of the ISP’s Internet connection, causing the overall speed of the connection to be faster for everybody.

Power over Ethernet (PoE)

Power over Ethernet (PoE) is a solution in which an electrical current is run to networking hardware over the Ethernet Category 5 or Category 6 cable. This minimizes the amount of cable needed as well as eliminates the difficulties and cost of installing extra outlets for input AC or DC power to the equipment.

Ethernet Standards

NIC Card and Ethernet Protocol

, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , 16 Comments

Network Interface Card (NIC) is also referred to as an Ethernet card. It also called a network adapter. It is a circuit board that is installed in a computer so that it can be connected to a network.  A network interface card provides the computer with a dedicated, full-time connection to a network. Personal computers and workstations on a local area network typically contain a network interface card specifically designed for the LAN transmission technology. Most new computers have a network interface built-in directly into the motherboard. Network Interface Card enables a computer to connect to a network; such as a home network, or the Internet using an Ethernet cable with an RJ-45 connector and wires.

Types of Network Interface

There is a different form of the network card. But the two main ones are wired and wireless. Wireless NICs Uses wireless technologies to connect the network, so they have one or more antennas. Wired NICs just use an RJ45 port since they have an Ethernet cable attached to the end. This makes them much flatter than wireless network cards.

Speed of Network Interface Card (NIC)

All network interface cards have a different speed rating, such as 10 Mbps, 11 Mbps, 54 Mbps or 100 Mbps, and 1000 Mbps. The speed describes the general performance of the interface card.  It’s important that the speed of the NIC does not necessarily determine the speed of the internet connection. This is due to reasons like available bandwidth and the speed you’re paying for. For example, if your DSL speed is 16 Mbps and your NIC Speed is 100 Mbps, So network interface card will not increase the speed of your internet.  but, if your DSL speed is 16 Mbps and your NIC is 10 Mbps, So your internet speed will also slow down to 10 Mbps.

Network Interface Card Driver

All hardware devices installed in computers need device drivers in order to work with the software on the computer. If installed network card isn’t working, it’s mean that the driver is missing, corrupted or outdated. The updating of driver software required in internet connection in order to download the driver. But the problem is that your network card is not working and you cannot access the internet. So you can download a driver on a computer that is connected to the internet from the card manufacturers site and then transfer using a USB drive or CD.

 

Ethernet

The Ethernet is both a data link and the physical layer main component for local area networks (LANs). Robert Metcalfe and D.R. Boggs invented this in 1972. Ethernet is a network protocol that controls how data transmitted over a LAN. Technically it is referred to as the IEEE 802.3 protocol. It is also used in contrast with WAN which spans larger geographical areas.

The Ethernet protocol standards describe many aspects of network communication as well as frame format, frame size, timing, and encoding. It’s telling, how networked devices can format data for transmission to other network devices on the same network segment, and describe how to put that data out on the network connection.

It is a contention-based method of networking. A contention-based method means that any device can try to send data across the shared medium when it has data to send. The Carrier Sense Multiple Access/Collision Detection (CSMA/CD) process uses in half-duplex Ethernet LANs to detect and resolve collisions. Today’s Ethernet LANs use full-duplex switches, which allow multiple devices to send and receive simultaneously with no collisions.

The OSI model describes separately both the data link layer function and the physical standards of the media. The Ethernet standard defines both the Layer 2 protocols and the Layer 1 technologies. According to specification, It supports different media, bandwidths, and other Layer 1 and 2 variations. The basic frame format and address scheme are the same for all types of Ethernet.

Framing

Framing in Data Link Layer

, , , , , , , , , , , , , , , , , , , , , , , , 1 Comment

Data-link layer framing is an important subject in the network. Data-link layer takes the packets from the Network Layer and encapsulates them into frames. If the frame size becomes too large, then the packet may be divided into small-sized frames to make efficient flow control and error control.

Frames are data-link layer data unit that is transmitted between network points. It contains complete addressing, necessary protocol, and control information. Before transmission of the frame, the data link layer protocol encapsulates the frame with trailer and header.

There are a lot of different data link layer protocols that describe data link layer frames. At the receiving end again data link layer protocols explain and de-encapsulate this frame. The figure below Illustrate frame which has three basic parts:-

  • Header
  • Data
  • Trailer

Framing

The frame fields

The general frame as shown in the above mention image but, the structure of the frame contains fields in the header and trailer varies according to the protocol. For example, if examine the fields of PPP which is different from the field of HDLC. There is no framing structure that meets the requirements of all data transportation across all type of media.

Depending on the background, the size of control information in the data link layer framing varies to match the access control requirements of the media and logical topology. So frame has no standard size its varies according to the media and environment.

 Typical framing fields are following also illustrated in the image below:-

Framing in Data Link Layer 2

  • Start and stop indicator fields – These fields  explain the start and end restrictions of the datalink framing
  • Addressing fields– indicate the source nodes and destination nodes.
  • Type  – this field explains layer3 protocols in the data field.
  • Control – identify special flow control services like QoS (Quality of Service).
  • Data -The frame payload (Network layer packet)
  • Error Detection– This frame detects error and has included after the data to form the trailer.

[qsm quiz=3]

Carrier Sense Multiple Access

Carrier Sense Multiple Access (CSMA)

, , , , , , , , , , , , , , , , , 1 Comment

CSMA is the abbreviation of Carrier Sense Multiple Access. it is a networking protocol that listens to network signals on the carrier/medium before transmitting data on the media. CSMA implemented in Ethernet networks with more than one computer or network device attached to it. CSMA is part of the Media Access Control (MAC) protocol. The protocol decreases the chances of collisions when two or more stations start sending their signals over the data-link layer. It requires that each station first check the state of the medium before sending.

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

Carrier Sense Multiple Access/Collision Detection (CSMA/CD); is the most widely used transmission method in half-duplex Ethernet networks.  On Ethernet, any device can attempt to send a frame at any time. Each device senses whether the line is idle and available for use. If it is, the device begins to send its first frame. If another device has tried to send at the same time, a collision occurs and the frames then discarded. Then each device waits a random amount of time and retires until successful in getting its transmission again.

Carrier Sense Multiple Access with Collision Avoidance

CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) is a method for carrier transmission in 802.11 networks. If a collision has occurred in the wired network then the energy of received signal almost doubles and the station can sense the possibility of collision. In the case of wireless networks, most of the energy; is used for transmission and the energy of received signal increases by only 5-10% if a collision occurs. It can’t be used by the station to sense collision.

Therefore CSMA/CA has specially designed for wireless networks. It uses a method similar to CSMA/CD to detect if the media is clear. CMSA/CA also uses additional techniques. This method does not detect collisions but attempts to prevent collisions before happen. Each device that transmits includes the time duration that it needs for the transmission. All other wireless devices receive this information and know how long the medium will be unavailable.

In CSMA/CA, when a node receives traffic for sending, it checks to sure that the channel is clear (no other node is transmitting at the time). The packet is sent when the channel is clear. If the channel is not clear, the node waits for a randomly chosen period of time and then checks again to see if the channel is clear. This period of time called the back off factor and counted down by a backoff counter.

If the channel is clear when the backoff counter reaches zero, the node transmits the packet. If the channel not clear and the backoff counter reaches zero, the backoff factor reset again, and the process repeated. Carrier sense multiple access with collision avoidance (CSMA/CA) is,  the least popular of the access methods. The WLANs now uses this access method.

half-duplex

Transmission Modes -Simplex, Half-Duplex and Full-Duplex

, , , , , , , , , , , , , , , , , , , , , 4 Comments

Transmission Modes refers to the direction of data transmission between two devices. For example, your cell phone is a full-duplex because both parties can talk at once but, a walkie-talkie is a half-duplex device because only one party can transmit at a time. Radio is the example of simplex communication, where radio only can receive the signal and never transmit. Half-duplex communications limit the exchange of data to one direction at a time while full-duplex allows the sending and receiving of data at the same time.

Simplex Communication

In simplex transmission, the communication between sender and receiver only occurs in one direction.  Simplex in one way communication in which the signals can travel only in one direction. The sender only can send and receiver only can receive nor send. Radio Broadcasting, television broadcasting computer to printer communication, and keyboard to computer connections are the examples of simplex communication.

Half-duplex communication

In half-duplex communication, one side can talk at a time another side should be on listening. When one side has complete transmitting its data, the other side can reply. Only one node can talk at a time. If both try to talk at the same time, a collision will take place on the network. So both devices can transmit and receive on the media but cannot do so at the same time.

The half-duplex mode is used in legacy bus topologies and with Ethernet hubs. WLANs also operate in half-duplex. Half-Duplex is used with contention-based access methods. This method of communication is not very efficient and requires more time to send/receive larger amounts of data. Older networks mostly based in half-duplex mode, due to the constraints of the network medium (coax cable) and hardware equipment (hubs) as shown in the figure below.

half-duplex

Full-duplex communication

Full-duplex is the type of communication where Both devices can transmit and receive on the media at the same time. The data link layer assumes that the media is available for transmission for both nodes at any time without any danger of a collision. So, the transfer of data occurs much faster.

Ethernet switches operate in full-duplex mode by default but can operate in half-duplex if connecting to a device such as an Ethernet hub. Below figure illustrates the full-duplex communication.

Full-Duplex

Today, all networks make use of switches instead of hubs and UTP Ethernet cabling instead of co-axial cabling, which allow full-duplex communication between all connected hosts. It is important that both interconnected interfaces operate using the

Host forwarding decision

5 Types of Network Topology

, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , 9 Comments

The network topology is the schematic arrangement or relationship of the network devices, including its nodes and connecting lines. The network really has two shapes or two types of network topology; one is physical network topology and the other is logical network topology.

The physical network topology

The physical topology of a network is the actual physical and geometric layout of the network that we can see, such as, devices like routers, switches, and wireless access points, nodes and cables. So, there are several common physical topologies like point-to-point, ring, bus, star, and mesh the physical topology has two different categories WAN and LAN topologies.

Common Physical WAN Topologies

WAN’s generally interconnected using the following physical topologies:

Point-to-Point

Point-to-point (PTP) network topology connects two nodes directly to one another. Only two devices involved in a point-to-point connection, with one wire (or air, in the case of wireless) sitting between them. This is the simplest topology in networking. For this reason, this is a very popular WAN topology. The figure below illustrates the physical point-to-point topology.

5 Types of Network Topology 7

Hub and Spoke

A hub and spoke network is a traditional and widely used network topology for all types of networks also known as a star topology. In this topology, a central site interconnects branch sites using point-to-point links. The Central site is known as hub and branch site known as spokes. Communication between two spokes always travels through the hub. The figure below illustrates the physical hub and spoke topology.

5 Types of Network Topology 8

Mesh Topology

A mesh network is a network topology where each node relays data for the network so this topology provides high availability but requires that every end system is interconnected to every other system. Therefore the administrative and physical costs can be very high. Each link is essentially a point-to-point link to the other node. Variations of this topology include a partial mesh where some but not all of the end devices inter-connected. The figure below illustrates the physical mesh topology.

5 Types of Network Topology 9

Physical LAN Topologies

Physical LAN topologies define how the end systems physically connected. We can connect, devices in LAN using the following physical topologies as illustrates in the figure below:

5 Types of Network Topology 10

Star topology

In Star topology, all end devices have connected to the central device. Early star topologies interconnected end devices using Ethernet hubs. However, star topologies now use Ethernet switches. The star topology is easy to install, very scalable (easy to add and remove end devices), and easy to troubleshoot.

All the data on the star topology passes through the central device before reaching the intended destination. Hub/Switch acts as a junction to connect different nodes present in Star Network, and at the same time, it manages and controls the whole of the network. Depending on which central device, “hub” and “Switch”  can act as repeater or signal booster. Central device can also communicate with other hubs and switch of a different network.

Extended Star Topology

In an extended star topology, additional Ethernet switches interconnect other star topologies.

Bus Topology

Bus Topology is the simplest of network topologies. All end systems (computers as well as servers) are connected to each other and terminated in some form on each end. Infrastructure devices such as switches not required to interconnect the end devices. Bus topologies using coax cables in legacy Ethernet networks because it was inexpensive and easy to set up.

Ring Topology

In the ring topology, end systems are connected to a single circle of cable. Unlike the bus topology, the ring does not need termination. The signals travel around the loop in one direction and pass through each computer, which acts as a repeater to boost the signal and send it to the next computer. Ring topologies use in legacy Fiber Distributed Data Interface (FDDI) and Token Ring networks.

The logical network topology

The logical topology is the way that the signals proceed on the network media or the way that the data passes through the network from one device to the next device without regard to the physical interconnection of the devices. In other words, it defines the way a network transfers frames from one node to the next.

This topology also consists of virtual links between the nodes of a network. These logical paths are defined by data link layer protocols.  The data link layer deals with the logical topology of a network when controlling data access to the media. It is the logical topology that influences the type of network framing and media access control used.

«‹ 102 103 104 105 ›»