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Ethernet LAN Fundamentals

The OSI Reference Model and Local-Area Networks (LANs)

The following layers of the OSI reference model provide data delivery over a wide variety of types of physical networks.

  • Physical layer (layer 1)

  • Data Link layer (layer 2)

The Physical layer (layer 1) of the OSI reference model deals with the sending of bits and the receiving of bits. The physical layer is hardware specific and deals with the physical connection between the computer and the network medium. The Physical layer identifies the interface between the data terminal equipment (DTE) and the data communication equipment (DCE).

The main Physical layer functions are summarized below:

  • Encoding; specifies the manner in which an endpoint device signals a binary 0 or 1 onto the transmit pin(s).

  • Cabling; specifies the number of wires and the type of shielding utilized.

  • Connectors; specifies the shape of the connectors and the number of pins allowed.

  • Pins; specifies the function of pins.

  • Voltage; specifies the endpoint devices’ electrical characteristics which are using the cable.

The protocols and standards operating at the Data-link layer format messages into data frames. Next, the Data-link protocols add a header that contains the specified hardware destination address and source address.

The main Data-link functions performed by the Data-link protocols are summarized here:

  • Arbitration: The Data Link layer only performs arbitration if there are instances when it is not appropriate to utilize the physical medium to transmit data. With Ethernet, arbitration uses the carrier sense multiple access collision detect (CSMA/CD) algorithm. Devices using CSMA/CD listen for an opening to transmit data. When an opening exists, the devices transmit data. By listening for openings on the line, the devices are able detect collisions. Collisions take place when a number of devices perceive that an opening exists on the line, and then each device proceeds to transmits data at the same time. When devices detect a collision, they wait for a random time period to expire before they start resending the data. The retransmitting of data is normally performed successfully.

The basic algorithm utilized when data is transmitted using Ethernet is explained here:

  1. Listens to determine when a frame is received.

  2. When no frames are being received, data is sent.

  3. When another frame exists on Ethernet, the algorithm pauses and listens.

  4. When collisions take place when data is sent, the algorithm stops, and then pauses and listens.

  • Packet addressing: One of the functions of the Data Link layer is packet addressing. The typical components that make up a packet are:

  • Source address

  • Destination address

  • Packet payload

  • Information which define the manner in which the data should send the data.

  • Reassembly information.

  • Error-checking information

Ethernet utilize Media Access Control (MAC) addresses. A MAC address is presented as a hexadecimal number and is 6 bytes in length.

The Media Access Control (MAC) sublayer of the Data Link layer controls the transmission of packets from one network interface card (NIC) to another, over a shared media channel. A network interface card has a unique MAC address, or physical address. This address identifies the particular NIC on the network. To ensure that these addresses are unique, the MAC addresses are usually permanently burned in the memory of the NIC. The MAC sublayer of the Data Link layer handles media access control which essentially prevents data collisions.

  • Error detection: With error detection, the frame check sequence (FCS) or cyclical redundancy check (CRC) field is used to check for bit errors in the frame. When errors are detected, the frame is simply dropped or discarded. The Data-link layer receives packets from the Network layer and structures these packets into frames. The frames are then moved to the Physical layer for sending.

A cyclic redundancy check (CRC) is added to the data frame. The CRC detects damaged frames. The computer at the receiving end can request the cyclic redundancy check so that it can verify that the frame is not damaged. The Data-link layer can determine when a frame is lost. It also requests any lost frames to be retransmitted. By performing these tasks, the Data-link layer makes it possible for data bits to be transmitted in an organized manner.

It is important to understand that error detection does not automatically mean error recovery as well. With error recovery, the protocol being used causes any lost data to be retransmitted. For this to occur, the data being sent is numbered. When data is received, the sender is informed that that specific data number was received.

  • Identification of encapsulated data: Within each Data Link layer header is a field that indicates which type of protocol header follows.

The Logical Link Control Sublayer (LLC) sublayer of the OSI Data Link layer utilizes source service access points (SSAPs) and destination service access points (DSAPs) to assist the lower levels with communicating to the Network layer protocols. These service access points, point to the various upper-layer protocols, such as IP or IPX.

The LLC sublayer is responsible for the following functions as well:

  • Flow control

  • Timing

  • Frame synchronization

  • Error checking

  • Connectionless and connection-oriented protocols for a few of the protocol stacks.

Understanding the Initial Ethernet Standards

Digital, Intel, and Xerox (DIX) defined and created Ethernet version 1 in the 1980s. Following the release of Ethernet version1 is Ethernet version 2, which included the specification of Carrier Sense, Multiple Access with Collision Detect (CSMA/CD).

Ethernet is a baseband LAN specification. Ethernet is in fact very similar to the IEEE 802.3 series of standards.

IEEE Ethernet specifications which match the OSI Layer 2 functions are separated into the following functions:

  • Media Access Control (MAC) sublayer

  • Logical Link Control (LLC) sublayer

The different protocol specifications for the original three IEEE LAN standards and the original version of Ethernet are shown in Table 1:

TABLE 2: The different protocol specifications for the original three IEEE LAN standards and the original version of Ethernet.

IEEE LAN standards/

Ethernet version

Media Access Control (MAC) sublayer specification

Logical Link Control (LLC) sublayer specification

Ethernet Version 2



IEEE Ethernet

IEEE 802.3

IEEE 802.2

IEEE Token Ring

IEEE 802.5

IEEE 802.2



IEEE 802.2

The two earliest Ethernet specifications that specified the Physical layer of the very early Ethernet networks were:

  • 10BASE2: A 10-Mbps baseband Ethernet specification which uses 50-ohm thin coaxial cable. 10Base2 has a distance limit of 606.8 feet per segment.

  • 10BASE5: A 10-Mbps baseband Ethernet specification which uses thick ohm baseband coaxial cable. 10Base5 has a distance limit of 1640 feet per segment.

While the two Ethernet specifications differ somewhat when it comes to cabling; they are regarded as behaving the same.

With the 10BASE2 and 10BASE5 specifications, the following applies to Ethernet networks:

  • The coaxial cables are installed, and each device is connected to the Ethernet network

  • No hub, switch, or wiring panel exists.

  • Ethernet comprises only of the following components:

  • Ethernet cards in the computers

  • Cabling

The cables create an electrical bus which is shared between all devices on the Ethernet. A computer that wants to send data to a different computer on the bus transmits an electrical signal. Then, the electrical signal propagates to all devices on the Ethernet.

Collisions would typically occur when multiple stations send data at the same time. A collision initially takes place on the wire. Following this, the sending stations detect the collision – some time passes before the sending stations detect that a collision has occurred.

The factors here can result in Ethernet congestion:

  • Collisions would occur when stations simultaneously send frames. Because collided frames are not received, each station has to resend the frames.

  • When a frame is being received at the same time that a device is ready to send a frame, that device has to wait before the frame can be sent.

  • The limit specified for the number of bits that can be sent might have been reached. The maximum throughput for a LAN segment is 10 Mbps.

As mentioned early, the carrier sense multiple access collision detect (CSMA/CD) algorithm is used to assist in preventing collisions. The CSMA/CD algorithm also defines what should occur when collisions are detected.

The CSMA/CD algorithm performs as follows.

  • A device which has a frame to send first listens to determine whether Ethernet is free or busy.

  • The sending device starts sending the frame if it has determined that Ethernet is not busy.

  • The sending device listens to check whether any collisions have occurred.

  • When the sender detects a collision, it sends a jamming signal. This signal notifies all other stations of the collision.

  • Once jamming has completed, the senders randomizes a timer. The senders then wait for the timer to expire.

  • After the timer expires, the data sending process starts again.

Comparing 10Base2 and 10Base5 Ethernet

The similarities and differences between 10Base2 and 10Base5 Ethernet are shown in Table 2:

TABLE 2: Comparing 10Base2 and 10Base5 Ethernet

10Base2 Ethernet


Baseband network technology

Baseband network technology



Can run up to 185 meters

Can run up to 2500 meters

Can support up to 30 workstations on a single segment.

Can support up to 1024 users

To connect to the network, Ethernet cards utilize BNC and T-connectors

A physical and logical bus with AUI connectors is used

10BASE-T Ethernet Overview

With 10BaseT/UTP; hosts connect to a hub or switch using unshielded twisted-pair (UTP) cable:

  • Category 3 UTP is defined to 10Mbps.

  • Category 5 UTP is defined to 100Mbps Carrier Sense Multiple Access with Collision Detection (CSMA/CD).

The initial IEEE 802.3 standards defined for10BASE-T Ethernet are summarized here:

  • 10Mbps.

  • Baseband network technology – term used to refer to a network technology where only one carrier frequency is utilized.

  • Hosts connect to a hub or switch using unshielded twisted-pair (UTP) cable:

  • Category 3 UTP is defined to 10Mbps.

  • Category 5 UTP is defined to 100Mbps Carrier Sense Multiple Access with Collision Detection (CSMA/CD).

  • One host per segment or wire is allowed.

  • A RJ-45 connector with a physical star topology and a logical bus is utilized.

10BASE-T addresses a number of the issues experienced in the initial Ethernet specifications. With 10BASE-T, networks use devices called hubs.

The physical 10BASE-T Ethernet uses:

  • Ethernet cards in the computers

  • Cabling

  • A hub

Multiport repeaters were used to create the 10BASE-T Ethernet. The hub in this case regenerates the electrical signal which arrives in one port and then transmits the electrical signal from all other ports. In this way, 10BASE-T actually creates an electrical bus, which is not much different from what 10BASE2 and 10BASE5 does. This basically means that collisions can also occur with10BASE-T Ethernet. This is the reason why the carrier sense multiple access collision detect (CSMA/CD) algorithm is continued to be used to prevent collisions.

One main advantage of 10BASE-T is that the 10BASE-T hubs provide a higher degree of availability than what was provided by 10BASE2 and 10BASE5. This is mainly due to a cable being run from each device to the central hub. One cable problem cannot therefore cripple the entire LAN. In fact, the main design enhancement of 10BASE-T Ethernet is the cabling of each specific device to the hub. This concept is known as shared Ethernet, purely because all devices share a 10-Mbps bus.

In an Ethernet a 10BASE-T network, the hub and PCs usually utilize Category 5 UTP cables with RJ-45 connectors. Both the Ethernet cards in a PC and the hub have an RJ-45 connector. The term used to describe the cable that connects the PCs to the hub is called a straight-through cable.

Ethernet specifies the following:

  • Pins 1 and 2 are used to send data

  • Pins 3 and 4 are used to receive data

With straight-through cable, the following occurs:

  • Wire connected to pin 1 on one end of the cable is connected to pin 1 on the other end of the cable.

  • Wire connected to pin 2 on one end of the cable is connected to pin 2 on the other end of the cable.

  • When data is sent on the pair on pins 1 and 2, the central hub receives the electrical signal over the straight-through cable on pin 1 and pin 2.

  • Hubs receive data on pin 1 and pin 2. Hubs send data on pin 3 and pin 4.

Crossover cabling can be used to achieve the following:

  • Cable two devices directly together with Ethernet, where each device utilizes the same pair for sending data.

  • Create a small Ethernet between two PCs by cabling the two PCs together, where each PC utilizes pin 1 and pin 2 for sending data.

With crossover cabling, the following occurs:

  • Wire connected to pin 1 on one end of the cable is no longer connected to pin 1 on the other end of the cable.

  • Pin 1 on one end of the cable is connected pin 3 on the other end of the cable.

  • Pin 2 on one end of the cable is connected pin 6 on the other end of the cable.

10BASE-T hubs address the following problems, typically of 10BASE2 and 10BASE5

  • Cabling issues

  • Availability issues.

The operation of half-duplex 10BASE-T with hubs is outlined below:

  • A frame is transmitted by the network interface card (NIC).

  • The network interface card then loops the transmitted frame onto its receive pair internally on the NIC.

  • The frame is received by the hub.

  • The internal wiring of the hub propagates the signal to all other ports, excluding the port on which the signal was received.

  • The signal is repeated to all receive pair to all the other devices.

Using LAN Switching to Reduce Collisions

In Ethernet, the term collision describes the situation where two nodes transmit simultaneously. The term collision domain refers to the network area wherein frames that have collided are propagated.

Collisions are propagated by:

  • Hubs

  • Repeaters

Collisions are not propagated by:

  • LAN switches

  • Bridges

  • Routers

Devices on the following types of Ethernet networks use a hub, and are therefore susceptible to collisions between frames which are transmitted:

  • 10BASE2 Ethernet network

  • 10BASE5 Ethernet network

  • 10BASE-T Ethernet network

Switches basically remove the likelihood of collisions occurring. Unlike hubs, Layer 2 switches do not create a single shared bus. These switches handle each specific physical port as a separate bus. In addition to this, switches use memory buffers to store incoming frames. This means that when two attached devices simultaneously transmit a frame, the switch can store one frame in its memory buffer while it sends the other frame.

While an Ethernet switch utilizes the same logic as a transparent bridge, switches use hardware to learn addresses to make filtering decisions and forwarding decisions. Switches utilize application specific integrated circuits (ASICs) to create filter tables and to maintain the content of these tables. Because Layer 2 switches do not utilize and reference Network layer header information, they are faster than both bridges and routers. The Media Access Control (MAC) address of the network interface cards (NICs) of the host is utilized to filter the network.

The benefits of Layer 2 switching include:

  • Low cost

  • Hardware-based bridging

  • High speed

  • Wire speed

  • Low latency

  • Increases bandwidth for each user

The main features of switching that improve performance for Ethernet are summarized here:

  • When there is only a single device cabled to each port of a switch, no collisions can occur. This basically means that the CSMA/CD algorithm is no longer needed.

  • When the switch ports do not share bandwidth, but has its own specific bandwidth, Ethernet performance is improved. Basically, a switch with 10-Mbps ports would have 10 Mbps of bandwidth per port.

Network loops can typically occur when there are numerous connections between switches. Multiple connections between switches are usually created to allow redundancy. To prevent network loops from occurring, and to still maintain redundant links between switches, the Spanning-Tree Protocol (STP) can be used.

In the FragmentFree or Modified cut-through switching mode, the switch first checks the initial 64 bytes of a frame for fragmentation before the frame is forwarded. This is done to avoid possible collisions. FragmentFree or Modified cut-through mode is the default LAN switch type mode used for the Catalyst 1900 switch. FragmentFree switching can be considered a modified version of the cut-through switching mode. The 64 byte collision window is waited for, because at this point, it will be known whether a packet has errors. When compared with the cut-through switching mode, FragmentFree switching mode provides enhanced error checking while at the same time not increasing latency.

Full-Duplex Ethernet Overview

Full-duplex Ethernet refers to data transmission between a sending station and a receiving station where actual data sending occurs simultaneously. Full-duplex Ethernet uses two pairs of wires. Full-duplex Ethernet is said to provide 100 percent efficiency in each direction.

Full-duplex Ethernet can be used to lessen congestion. Using full-duplex Ethernet leads to about two times more bandwidth than that available with half-duplex Ethernet networks. In full-duplex mode, flow control is utilized for the communication session. The nodes can transmit and receive data at the same time. Remember to ensure that the switch port and the network interface cards can actually operate in full-duplex mode.

With full-duplex Ethernet, frames that are sent do not collide with frames which are currently being received.

Full-duplex Ethernet is typically used in the following circumstances:

  • A connection from a host to a switch crossover cable.

  • A connection from a switch to a host.

  • A connection from a switch to a switch.

The advantages of using full-duplex Ethernet are:

  • Ethernet congestion is alleviated.

  • Collisions no longer take place. This means that frames do not need to be resent.

  • Bandwidth is increased because 10 Mbps exist in either direction.

  • The wait time for sending frames is reduced.

Ethernet Data-link Protocols Overview

The Ethernet Data-link protocols provide the following functions:

  • Ethernet addressing. Other terms used to signify Ethernet addressing include MAC addressing and hardware addressing.

  • Ethernet framing of packets received from the Network layer.

  • Identifying the data within the Ethernet frame:

Unicast addresses are also called:

  • Ethernet addresses

  • LAN address

  • MAC addresses

The format of LAN addresses and assignment of LAN addresses are defined and controlled by the Institute of Electrical and Electronics Engineers (IEEE). One requirement defined by the IEEE is the existence of unique MAC addresses on all LAN interface cards.

A MAC address is a 6 byte long (48-bit) hardware address, which is expressed in the hexadecimal format. The MAC address is encoded on the Ethernet card by the Ethernet card manufacturers to guarantee uniqueness.

The MAC address contains the following:

  • The first part of the MAC address identifies the Ethernet card manufacturer. This is called the Organizational Unique Identifier (OUI) of the Ethernet address, which consists of 24 bits. The OUI is assigned to an organization by the IEEE.

  • The organization then assigns a globally administered address to all devices that they manufacture, as the second part of the MAC address. This is a number that was not used on any of the previous devices.

The IEEE defines the following types of group addresses for Ethernet:

  • Broadcast addresses: Broadcast addresses are 32-bit numbers used to specify all hosts in a specific network. The broadcast address is used by both hosts and applications to send data to all hosts residing on the particular network. Broadcast addresses mean that the frame should be processed by all devices on the LAN.

  • Multicast addresses: Multicast addresses support IP multicasting. IP multicasting allows a single address to be utilized to transmit information to multiple destinations. Multicast addresses are used to enable a group of devices residing on the LAN to communicate. Multicast addresses can be used on Ethernet and FDDI. Multicast addresses are not burned into the LAN card. The software configures the NIC to listen for a specific multicast address.

Ethernet Framing specifies how a string of binary numbers or bits are interpreted when they are received.

The IEEE 802.3 standard specifies where the Destination Address (DA) field resides within the string of bits sent across the Ethernet. The Destination Address field is used by receiving stations to determine whether an incoming packet is addressed to a specific node. The destination address field sends a 48-bit value and uses the least significant bit (LSB) to begin with.

The destination address can be either of these:

  • A single specific MAC address

  • A broadcast address

  • A multicast MAC address

The specific fields in an Ethernet frame that identify the type of data contained within the frame are listed below:

  • Length field or Type field; 802.3 Ethernet uses a Length field, while Ethernet_II frames utilizes a Type field. Ethernet_II frames use the Type field to determine the Network layer protocol. 802.3 Ethernet frames on the other hand cannot determine this information and therefore have to be utilized with a proprietary LAN. The Ethernet Type field is 2 bytes in length and is used in DIX Ethernet.

  • Destination service access point (DSAP): Name used by the IEEE to indicate the Type field. The DSAP field is 1 byte, and is used in:

  • IEEE Ethernet

  • IEEE Token Ring


  • Subnetwork Access Protocol (SNAP) header: An additional header that provides for the storing of additional protocols in its 2-byte Protocol Type field. The SNAP header follows the 802.2 header. The field is used in

  • IEEE Ethernet

  • IEEE Token Ring


Understanding the Recent Ethernet Standards

The recent Ethernet standards provide faster Ethernet options than what was provided in the earlier Ethernet standards. Ethernet can easily integrate with other new technologies, such as Fast Ethernet and Gigabit Ethernet.

The IEEE extended the 802.3 Committee to the following committees:

  • 802.3u Committee – Fast Ethernet.

  • 802.3ab Committee – Gigabit Ethernet on Category 5.

  • 802.3ae – 10Gbps over fiber and coax

Fast Ethernet and Gigabit Ethernet are discussed here:

  • Fast Ethernet: Fast Ethernet takes about 90 percent less time than what 10 Mbps Ethernet requires to send the same frame. Through faster speed, Fast Ethernet increases capacity.

Fast Ethernet is defined in IEEE 802.3u standard:

  • Defines 100BTX

    • Cable type; CAT 5 UTP – 2 pair

    • Max distance between devices: 412m

    • Max length to hub; 100m

  • Defines 100BFX

    • Cable type; MM Fiber – 2 strands

    • Max distance between devices: 412m – 2km w/ FDX

    • Max length to hub; 100m

  • Defines 100BT4

    • Cable type; CAT 3,4,5 UTP – 4 pair

    • Max distance between devices: 412m

The main advantages of Fast Ethernet are listed here:

  • Collisions are reduced with Fast Ethernet.

  • Wait time for sending frames is also reduced. When sending a specific frame, Fast Ethernet takes about 90 percent less time than what 10 Mbps Ethernet requires to send the identical frame.

  • Fast Ethernet alleviates congestion.

  • Through its faster speed, Fast Ethernet increases capacity.

  • Support for half-duplex mode and full-duplex is negotiated.

  • Gigabit Ethernet: The IEEE defines Gigabit Ethernet in the following IEEE standards:

  • IEEE 802.3ab: Electrical cabling.

  • IEEE 802.3z: Optical cabling.

Both of these IEEE standards define IEEE 802.3 MAC and 802.2 LLC framing for LAN headers and LAN trailers. Gigabit Ethernet includes a few of the features of the slower initial Ethernet standards. Gigabit Ethernet is though fast –1000 Mbps.

Gigabit Ethernet is typically used:

  • Between switches.

  • Between switches and a router.

  • Between a switch and a server.

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