Introduction to Computer Network – Part 2

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Local Area Network (LAN)

The term local area network (LAN) is used to describe a network of devices inside a limited area (in the same room, building…). A typical SOHO (small office/home office) LAN  consist of PCs, printers, switches, routers, and cabling that connects all these devices together. The following figure shows a typical LAN:

typical lan+

This type of network is usually capable of achieving a high data transfer rate of (up to 10 Gbps) at relatively low cost. The twisted-pair cabling is commonly used in LANs for connections between end user devices and switches, while fiber-optic cabling is used for links between network devices, such as switches and routers.

Some of the LAN technologies are Ethernet, Token Ring and FDDI. Ethernet is by far the most popular wired LAN technology. It defines wiring, signaling, connectors, frame formats, protocol rules, etc. Most modern LANs also support the wireless LAN (WLAN) technology, defined by the IEEE 802.11 standards. WLANs use radio waves instead of wires or cables for links between devices.

What is Ethernet?

The term Ethernet refers to an entire family of standards that define wiring, signaling, connectors, frame formats, protocol rules, etc. Ethernet is standardized by the Institute of Electrical and Electronics Engineers (IEEE) as the 802.3 standard. The standard defines several wiring variants, such as coaxial, twisted pair and fiber optic cabling. Coaxial cables are rarely used anymore, while twisted pair cables are usually used in SOHO environments. Optical fibers are the most expensive option, but they allow longer cabling distances and greater speeds.

Ethernet uses the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) access method and supports speeds up to 100 Gbps. It is by far the most popular LAN technology today.

The term Ethernet LAN refers to a combination of computers, switches, and different kinds of cables that use the Ethernet standard to communicate over the network.

Ethernet frame

We have already learned that encapsulated data defined by the Network Access layer is called an Ethernet frame. An Ethernet frame starts with a header, which contains the source and destination MAC addresses, among other data. The middle part of the frame is the actual data. The frame ends with a field called Frame Check Sequence (FCS).

The Ethernet frame structure is defined in the IEEE 802.3 standard. Here is a graphical representation of an Ethernet frame and a description of each field in the frame:

ethernet frame

  • Preamble – informs the receiving system that a frame is starting and enables synchronisation.
  • SFD (Start Frame Delimiter) – signifies that the Destination MAC Address field begins with the next byte.
  • Destination MAC – identifies the receiving system.
  • Source MAC – identifies the sending system.
  • Type – defines the type of protocol inside the frame, for example IPv4 or IPv6.
  • Data and Pad – contains the payload data. Padding data is added to meet the minimum length requirement for this field (46 bytes).
  • FCS (Frame Check Sequence) – contains a 32-bit Cyclic Redundancy Check (CRC) which allows detection of corrupted data

Unicast, multicast, broadcast addresses

Three types of Ethernet addresses exist:

  • unicast addresses – represents a single LAN interface. A unicast frame will be sent to a specific device, not to a group of devices on the LAN.
  • multicast addresses – represents a group of devices in a LAN. A frame sent to a multicast address will be forwarded to a group of devices on the LAN.
  • broadcast addresses – represents all device on the LAN. Frames sent to a broadcast address will be delivered to all devices on the LAN.


The broadcast address has the value of FFFF.FFFF.FFFF (all binary ones). The switch will flood broadcast frames out all ports except the port that it was received on.

Multicast frames have a value of 1 in the least-significant bit of the first octet of the destination address. This helps a network switch to distinguish between unicast and multicast addresses. One example of an Ethernet multicast address would be 01:00:0C:CC:CC:CC, which is an address used by CDP (Cisco Discovery Protocol).

Half and full duplex

In telecommunication, a duplex communication system is a point-to-point system of two devices that can communicate with each other in both direction. Two types of duplex communication systems exist in Ethernet environments:

half-duplex – a port can send data only when it is not receiving data. In other words, it cannot send and receive data at the same time. Network hubs run in half-duplex mode in order to prevent collisions. Since hubs are rare in modern LANs, the half-duplex system is not widely used in Ethernet networks anymore.

full-duplex – all nodes can send and receive on their port at the same time. There are no collisions in full-duplex mode, but the host NIC and the switch port must support  the full-duplex mode. Full-duplex Ethernet uses two pairs of wires at the same time instead of a single wire pair like half-duplex.

Each NIC and switch port has a duplex setting. For all links between hosts and switches, or between switches, the full-duplex mode should be used. However, for all links connected to a LAN hub, the half-duplex mode should be used in order to prevent a duplex mismatch that could decrease network performance.



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