LAN Technologies (Ethernet) Study Notes

By BYJU'S Exam Prep

Updated on: September 25th, 2023

Ethernet LANs consist of network nodes and interconnecting media. The network nodes fall into two major classes:

  • Data terminal equipment (DTE) : Devices that are either the source or the destination of data frames. DTEs are typically devices such as PCs, workstations, file servers, or print servers that,as a group, are all often referred to as end stations.
  • Data communication equipment (DCE) : Intermediate network devices that receive and forward frames across the network. DCEs may be either standalone devices such as repeaters, network switches, and routers, or communications interface units such as interface cards and modems.
Addressing: LAN data transmissions are classified into one of three categories: Unicast, Multicast, and Broadcast.
  • Unicast: Addressing is one-to-one, where one computer sends a frame to another computer. Even though many stations can receive the same data, they should ignore it since it is not addressed to them. With unicast transmissions, a single packet is sent from the source to a destination on a network. The source-node addresses the packet by using the network address of the destination node. The packet is then forwarded to the destination network and the network passes the packet to its final destination.
  • Multicast: Addressing is one-to-many, where one computer is sending a frame to many other computers. This can be done via a list of addresses, or some masking scheme that selects a subset of addresses.  With a multicast transmission, a single data packet is copied and forwarded to a specific subset of nodes on the network. The source node addresses the packet by using a multicast address.
  • Broadcast: Addressing is one-to-all, where one computer sends data to all computers connected to the LAN. Broadcasts are found in LAN environments. Broadcasts do not traverse a WAN unless the Layer 3 edge-routing device is configured with a helper address (or the like) to direct these broadcasts to a specified network address.

LAN Topologies: There are 4 types of LAN topologies are available. (i) Bus, (ii) Ring, (iii) Star, and (iv) Mesh. Bus: A bus topology consists of devices connected to a common, shared cable.

  • Failure of the medium disrupts communication.
  • Failure of devices doesn’t effect the communication (passive interface).
  • There is a limit on the length of the network (devices don’t amplify/repeat the signal).
  • The propagation delay isn’t effected by the number of devices.
  • The original Ethernet networks were implemented with a coaxial bus structure.
  • Segment lengths were limited to 500 meters with up to 100 stations connected to a single segment.
  • Individual segments could be interconnected with repeaters, as long as multiple paths did not exist between any two stations on the network and the number of DTEs did not exceed 1024.
  • Advantages of Bus:
    • Use of cable is economical.
    • Media is inexpensive and easy to work with.
    • System is simple and reliable.
    • Bus is easy to extend.
  • Disadvantages of Bus:
    • Network can slow down in heavy traffic.
    • Problems are difficult to isolate.
    • Cable break can affect many users.

Ring: Connecting computers to a cable that forms a loop is referred to as setting up a ring topology.

  • Failure of the medium seriously disrupts communication.
  • Malfunctioning of the interface can seriously effect communication (devices are active).
  • There is no limitation on the length of the network (devices repeat/retransmit the frame).
  • Advantages of Ring:
    • System provides equal access for all computers.
    • Performance is even despite many users.
  • Disadvantages of Ring:
    • Failure of one computer can impact the rest of the network.
    • Problems are hard to isolate.
    • Network reconfiguration disrupts operation.

Star: Connecting computers to cable segments that branch out from a single point, or hub, is referred to as setting up a star topology.

  • Failure of the medium does not seriously disrupt communication.
  • Malfunctioning of the station doesn’t seriously effect the communication.
  • The network can use guided or unguided media.
  • Failure of the hub disrupts the communication.
  • The network can use any medium, like twisted pair, coax, or fiber
  • The central network unit is either a multiport repeater (also known as a hub) or a network switch.
  • All connections in a star network are point-to-point links implemented with either twisted-pair or optical fiber cable.
  • Advantages of Star:
    • Modifying system and adding new computers is easy.
    • Centralized monitoring and management are possible.
  • Disadvantages of Star:
    • Failure of one computer does not affect the rest of the network.
    • If the centralized point fails, the network fails.

Mesh: A mesh topology connects all computers in a network to each other with separate cables.

  • Advantages of Mesh: System provides increased redundancy and reliability as well as ease of troubleshooting.
  • Disadvantages of Mesh: System is expensive to install because it uses a lot of cabling.

LAN Technologies

  • LAN Protocols function at the lowest two layers of the OSI reference model between the physical layer and the data link layer.
  • The IEEE 802.3 standard defines Ethernet protocols for (Open Systems Interconnect) OSI’s Media Access Control (MAC) sublayer and physical layer network characteristics.
  • The IEEE 802.2 standard defines protocols for the Logical Link Control (LLC) sublayer.
  • Media contention occurs when more than one network device has data to send at the same time. The following two methods are used to access the network media where multiple devices cannot talk on the network simultaneously.
    • CSMA/CD : This network uses Ethernet technology.
    • Token Passing : It uses Token Ring technology.

Carrier Sense Multiple Access with Collision Detect (CSMA/CD):

  • The CSMA/CD method is internationally standardized in IEEE 802.3 and ISO 8802.3
  • CSMA/CD is a type of contention protocol.
  • Standard Ethernet networks use CSMA/CD to physically monitor the traffic on the line at participating stations.
  • It is a set of rules determining how network devices respond when two devices attempt to use a data channel simultaneously (called a collision).
  • If no transmission is taking place at the time, the particular station can transmit.
  • If two stations attempt to transmit simultaneously, this causes a collision, which is detected by all participating stations.
    • The stations that collided attempt to transmit again after a random time interval.
    • If another collision occurs, the time intervals from which the random waiting time is selected are increased step by step. This is known as Binary exponential back off.

IEEE Standards

  • IEEE 802.1: Standards related to network management.
  • IEEE 802.2: Standard for the data link layer in the OSI Reference Model
  • IEEE 802.3: Standard for the MAC layer for bus networks that use CSMA/CD. (Ethernet standard)
  • IEEE 802.4: Standard for the MAC layer for bus networks that use a token-passing mechanism (token bus networks).
  • IEEE 802.5: Standard for the MAC layer for token-ring networks.
  • IEEE 802.6: Standard for Metropolitan Area Networks (MANs).

Ethernet (IEEE 802.3) :

  • The term Ethernet refers to the family of local-area network (LAN) products covered by the IEEE 802.3 standard that defines what is commonly known as the CSMA/CD protocol.
  • A standard for a 1-persistent CSMA/CD LAN.
  • It operates at 10 Mbps using carrier sense multiple access collision detect (CSMA/CD) to run over coaxial cable.
  • It covers the physical layer and MAC sublayer protocol.
  • 3 Ethernet uses Manchester Phase Encoding (MPE) for coding the data bits on the outgoing signal.
  • In Ethernet, both the data link and the physical layers are involved in the creation and transmission of frames.
  • The physical layer is related to the type of LAN cabling and how the bits are transmitted and received on the cable.
  • The hardware address, or MAC address is transmitted and stored in Ethernet network devices in Canonical format i.e. Least significant Bit (LSB) first called as little endian.
  • The data link layer is divided into sublayers, the Logical Link Control (LLC) and the Media Access Control layers (MAC).
  • The frames created by these layers contain several fields that are processed by Network Interface Cards (NICs) in the sending and receiving devices.

Three data rates are currently defined for operation over optical fiber and twisted-pair cables:

  • 10 Mbps – 10Base-T Ethernet
  • 100 Mbps – Fast Ethernet
  • 1000 Mbps – Gigabit Ethernet

Ethernet Specifications:


Advantages of Ethernet over other technologies:

  • It is easy to understand, implement, manage, and maintain.
  • It allows low-cost network implementations.
  • It provides extensive topological flexibility for network installation.
  • It guarantees successful interconnection and operation of standards-compliant products, regardless of manufacturer.

IEEE 802.3 Frame Format: Maximum 802.3 frame size is 1518 bytes and the minimum size is 64 bytes. 1

  • Preamble field: Establishes bit synchronization and transceiver conditions so that the PLS circuitry synchs in with the received frame timing.
  • Start Frame Delimiter: Sequence 10101011 in a separate field..
  • Destination address: Hardware address (MAC address) of the destination station (usually 48 bits i.e. 6 bytes).
  • Source address: Hardware address of the source station (must be of the same length as the destination address, the 802.3 standard allows for 2 or 6 byte addresses).
  • Length: Specifies the length of the data segment, actually the number of LLC data bytes.
  • Pad: Zeros added to the data field to ‘Pad out’ a short data field to 46 bytes.
  • Data: Actual data which is allowed anywhere between 46 to 1500 bytes within one frame.
  • FCS: Frame Check Sequence to detect errors that occur during transmission.


Propagation Delay: Time taken for a signal to travel from the transmitter to the receiver

  • Speed of light is the fastest a signal will propagate
    • 3 × 108 m/sec through space
    • 2 × 108 m/sec through copper

Transmission Delay (Time): Time taken to put the bits on the transmission media. Transmission speed of 2Mbps means 2 × 106 bits can be transmitted in 1 second

Processing Delay: Time taken to execute protocols. (check for errors and send Acks etc.)

Queuing Delay: Only in packet switched networks.

  • Time spent waiting in buffer for transmission
  • Increases as load on network increases

Round Trip Delay: Round trip delay is defined as the time between the first bit of the message being put onto the transmission medium, and the last bit the acknowledgement being received back by the transmitter. It is the sum of the all the delays detailed above. The round trip delay is a critical factor in the performance of packet switched protocols and networks. Indeed, it has been stated that a good algorithm for estimating the round trip delay is at the heart of a good packet switch protocol.

Ethernet Efficiency

  • Let tprop denote the maximum time it takes signal energy to propagate between any two adapters.
  • Let ttrans be the time to transmit a maximum-size Ethernet frame (approximately 1.2 msecs for a 10 Mbps Ethernet).


Token Ring (IEEE 802.5):

  • Token Ring is formed by the nodes connected in ring.
  • The principle used in the token ring network is that a token is circulating in the ring and whichever node grabs that token will have right to transmit the data.
  • Whenever a station wants to transmit a frame it inverts a single bit of the 3-byte token which instantaneously changes it into a normal data packet. Because there is only one token, there can atmost be one transmission at a time.
  • Since the token rotates in the ring it is guaranteed that every node gets the token with in some specified time. So there is an upper bound on the time of waiting to grab the token so that starvation is avoided.
  • There is also an upper limit of 250 on the number of nodes in the network.
  • To distinguish the normal data packets from token (control packet) a special sequence is assigned to the token packet.
  • When any node gets the token it first sends the data it wants to send, then recirculates the token.
  • If a node transmits the token and nobody wants to send the data the token comes back to the sender.
  • Token Ring and FDDI, on the other hand, transmit the MAC address with the Most Significant Bit (MSB) first, orBig-Endian, This is known as Non-Canonical

If the first bit of the token reaches the sender before the transmission of the last bit, then error situation araises. To avoid this situation, the following condition should hold.

  • Propagation delay + transmission of n-bits (1-bit delay in each node) > transmission of the token time

Modes of Operation

  1. Listen Mode: In this mode the node listens to the data and transmits the data to the next node. In this mode there is a one-bit delay associated with the transmission.
  2. Transmit Mode: In this mode the node just discards the any data and puts the data onto the network.
  3. By-pass Mode: In this mode reached when the node is down. Any data is just bypassed. There is no one-bit delay in this mode.

A Token Ring network includes the following features:

  • Star-wired ring topology
  • Token-passing access method
  • Shielded and unshielded twisted-pair (IBM Types 1, 2, and 3) cabling
  • Transfer rates of 4 and 16 Mbps
  • Baseband transmission
  • 802.5 specifications

IEEE 802.5 Token Ring Frame Format:

  • Start delimiter (1 octet): Indicates start of the frame.
  • Access control (1 octet): Indicates the frame’s priority and whether it is a token or a data frame
  • Frame control (1 octet):  Contains either Media Access Control information for all computers or end station information for only one computer
  • Destination address (2 or 6 octets ): Indicates the address of the computer to receive the frame
  • Source address (2 or 6 octets ): Indicates the computer that sent the frame
  • Information or data (0 or more octets up to 4027): Contains the data being sent. No upper limit on amount of data as such, but it is limited by the token holding time.
  • Frame check sequence (4 Octets): Contains CRC error-checking information. The source computes and sets this value. Destination too calculates this value. If the two are different, it indicates an error, otherwise the data may be correct.
  • End delimiter (1 Octet): Indicates the end of the frame
  • Frame status (1 Octet): Tells whether the frame was recognized, copied, or whether the destination address was available.


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