Understanding LANs
A local-area network is a common type of network found in home offices, small businesses, and large enterprises. Understanding how a LAN functions, including network components, frames, Ethernet addresses, and operational characteristics, is important for an overall knowledge of networking technologies.
This section describes LANs and provides fundamental knowledge about LAN characteristics, components, and functions. It also describes the basic operations of an Ethernet LAN and how frames are transmitted over it.
The Definition of a LAN
A LAN is a network of computers and other components located relatively close together in a limited area. LANs can vary widely in their size. A LAN might consist of only two computers in a home office or small business, or it might include hundreds of computers in a large corporate office or multiple buildings. Figure 3-1 shows some examples of LANs.
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Figure 3-1.Examples of LANs
A small home business or a small office environment could use a small LAN to connect two or more computers and to connect the computers to one or more shared peripheral devices such as printers. A large corporate office could use multiple LANs to accommodate hundreds of computers and shared peripheral devices, for departments such as finance or operations, spanning many floors in an office complex.
Components of a LAN
Every LAN has specific components, including hardware, interconnections, and software. Figure 3-2 highlights some typical hardware components of a LAN.
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Figure 3-2.Typical Components of a LAN
Regardless of the size of the LAN, it requires these fundamental components for its operation:
- Computers: Computers serve as the endpoints in the network, sending and receiving data.
- Interconnections: Interconnections enable data to travel from one point to another in the network. Interconnections include these components:
- NICs: Network interface cards (NIC) translate the data produced by the computer into a format that can be transmitted over the LAN.
- Network media: Network media, such as cables or wireless media, transmit signals from one device on the LAN to another.
- Network devices: A LAN requires the following network devices:
- Hubs: Hubs provide aggregation devices operating at Layer 1 of the OSI reference model. However, hubs have been replaced in this function by switches, and it is very rare to see hubs in any LAN these days.
- Ethernet switches: Ethernet switches form the aggregation point for LANs. Ethernet switches operate at Layer 2 of the OSI reference model and provide intelligent distribution of frames within the LAN.
- Routers: Routers, sometimes called gateways, provide a means to connect LAN segments. Routers operate at Layer 3 of the OSI reference model.
- Protocols: Protocols govern the way data is transmitted over a LAN and include the following:
- Ethernet protocols
- Internet Protocol (IP)
- Internet Protocol version 6 (IPv6)
- Address Resolution Protocol (ARP) and Reverse Address Resolution Protocol (RARP)
- Dynamic Host Configuration Protocol (DHCP)
Functions of a LAN
LANs provide network users with communication and resource-sharing functions, including the following:
- Data and applications: When users are connected through a network, they can share files and even software application programs. This makes data more easily available and promotes more efficient collaboration on work projects.
- Resources: The resources that can be shared include both input devices, such as cameras, and output devices, such as printers.
- Communication path to other networks: If a resource is not available locally, the LAN, through a gateway, can provide connectivity to remote resources—for example, access to the web.
How Big Is a LAN?
A LAN can be configured in a variety of sizes, depending on the requirements of the environment in which it operates.
LANs can be of various sizes to fit different work requirements, including the following:
- Small office/home office (SOHO): The SOHO environment typically has only a few computers and some peripherals such as printers.
- Enterprise: The enterprise environment might include many separate LANs in a large office building or in different buildings on a corporate campus. In the enterprise environment, each LAN might contain hundreds of computers and peripherals..
Figure 3-3 demonstrates the dramatic differences that can exist with the size of LANs.
Figure 3-3.Different LAN Sizes
Ethernet
Ethernet is the most common type of LAN. It was originally developed in the 1970s by Digital Equipment Corporation (DEC), Intel, and Xerox (DIX) and was called DIX Ethernet. It later came to be called thick Ethernet (because of the thickness of the cable used in this type of network), and it transmitted data at 10 megabits per second (Mbps). The standard for Ethernet was updated in the 1980s to add more capability, and the new version of Ethernet was referred to as Ethernet Version 2 (also called Ethernet II).
The Institute of Electrical and Electronics Engineers (IEEE) is a professional organization that defines network standards. IEEE standards are the predominant LAN standards in the world today. In the mid-1980s, an IEEE workgroup defined new standards for Ethernet-like networks. The set of standards they created was called Ethernet 802.3 and was based on the carrier sense multiple access with collision detection (CSMA/CD) process. Ethernet 802.3 specified the physical layer (Layer 1) and the MAC portion of the data link layer (Layer 2). Today, this set of standards is most often referred to as simply “Ethernet.”
Ethernet LAN Standards
Ethernet LAN standards specify cabling and signaling at both the physical and data link layers of the OSI reference model. This topic describes Ethernet LAN standards at the data link layer.
Figure 3-4 shows how LAN protocols map to the OSI reference model.
Figure 3-4.Ethernet and the OSI Model
The IEEE divides the OSI data link layer into two separate sublayers:
- Logical link control (LLC): Transitions up to the network layer
- MAC: Transitions down to the physical layer
LLC Sublayer
The IEEE created the LLC sublayer to allow part of the data link layer to function independently from existing technologies. This layer provides versatility in services to the network layer protocols that are above it, while communicating effectively with the variety of MAC and Layer 1 technologies below it. The LLC, as a sublayer, participates in the encapsulation process.
An LLC header tells the data link layer what to do with a packet when it receives a frame. For example, a host receives a frame and then looks in the LLC header to understand that the packet is destined for the IP protocol at the network layer.
The original Ethernet header (prior to IEEE 802.2 and 802.3) did not use an LLC header. Instead, it used a type field in the Ethernet header to identify the Layer 3 protocol being carried in the Ethernet frame.
MAC Sublayer
The MAC sublayer deals with physical media access. The IEEE 802.3 MAC specification defines MAC addresses, which uniquely identify multiple devices at the data link layer. The MAC sublayer maintains a table of MAC addresses (physical addresses) of devices. To participate on the network, each device must have a unique MAC address.
The Role of CSMA/CD in Ethernet
Ethernet signals are transmitted to every station connected to the LAN, using a special set of rules to determine which station can “talk” at any particular time. This topic describes that set of rules.
Ethernet LANs manage the signals on a network by CSMA/CD, which is an important aspect of Ethernet. Figure 3-5 illustrates the CSMA/CD process.
Figure 3-5.CSMA/CD Process
In an Ethernet LAN, before transmitting, a computer first listens to the network media. If the media is idle, the computer sends its data. After a transmission has been sent, the computers on the network compete for the next available idle time to send another frame. This competition for idle time means that no one station has an advantage over another on the network.
Stations on a CSMA/CD LAN can access the network at any time. Before sending data, CSMA/CD stations listen to the network to determine whether it is already in use. If it is, the CSMA/CD stations wait. If the network is not in use, the stations transmit. A collision occurs when two stations listen for network traffic, hear none, and transmit simultaneously (see Figure 3-5). In this case, both transmissions are damaged, and the stations must retransmit at some later time. CSMA/CD stations must be able to detect collisions to know that they must retransmit.
When a station transmits, the signal is referred to as a carrier. The NIC senses the carrier and consequently refrains from broadcasting a signal. If no carrier exists, a waiting station knows that it is free to transmit. This is the “carrier sense” part of the protocol.
The extent of the network segment over which collisions occur is referred to as the collision domain. The size of the collision domain has an impact on efficiency and therefore on data throughput. In today’s LANs, switches have replaced hubs. The reason this occurs is that switches create tiny collision domains containing just one device. This eliminates the potential for collisions. This process is often called “microsegmentation” of the network.
In the CSMA/CD process, priorities are not assigned to particular stations, so all stations on the network have equal access. This is the “multiple access” part of the protocol. If two or more stations attempt a transmission simultaneously, a collision occurs. The stations are alerted of the collision, and they execute a backoff algorithm that randomly schedules retransmission of the frame. This scenario prevents the machines from repeatedly attempting to transmit at the same time. Collisions are normally resolved in microseconds. This is the “collision detection” part of the protocol.
While collisions are resolved quickly, it is still advantageous to eliminate them entirely from the network. This allows much more efficient communications. This is accomplished through the use of switches as described earlier.
Ethernet Frames
Bits that are transmitted over an Ethernet LAN are organized into frames. In Ethernet terminology, the “container” into which data is placed for transmission is called a frame. The frame contains header information, trailer information, and the actual data that is being transmitted.
Figure 3-6 illustrates all the fields that are in a MAC layer of the Ethernet frame, which include the following:
- Preamble: This field consists of 7 bytes of alternating 1s and 0s, which synchronize the signals of the communicating computers.
- Start-of-frame (SOF) delimiter: This field contains bits that signal the receiving computer that the transmission of the actual frame is about to start and that any data following is part of the packet.
- Destination address: This field contains the address of the NIC on the local network to which the packet is being sent.
- Source address: This field contains the address of the NIC of the sending computer.
- Type/length: In Ethernet II, this field contains a code that identifies the network layer protocol. In 802.3, this field specifies the length of the data field. The protocol information is contained in 802.2 fields, which are at the LLC layer. The newer 802.3 specifications have allowed the use of Ethertype protocol identifiers when not using the 802.2 field.
- Data and pad: This field contains the data that is received from the network layer on the transmitting computer. This data is then sent to the same protocol on the destination computer. If the data is too short, an adapter adds a string of extraneous bits to “pad” the field to its minimum length of 46 bytes.
- Frame check sequence (FCS): This field includes a checking mechanism to ensure that the packet of data has been transmitted without corruption.
Figure 3-6.Ethernet Frames
Ethernet Frame Addressing
Communications in a network occur in three ways: unicast, broadcast, and multicast. Ethernet frames are addressed accordingly. Figure 3-7 shows forms of Ethernet communications.
Figure 3-7.Ethernet Communications
The three major types of network communications are as follows:
- Unicast: Communication in which a frame is sent from one host and addressed to one specific destination. In a unicast transmission, you have just one sender and one receiver. Unicast transmission is the predominant form of transmission on LANs and within the Internet.
- Broadcast: Communication in which a frame is sent from one address to all other addresses. In this case, you have just one sender, but the information is sent to all connected receivers. Broadcast transmission is essential when sending the same message to all devices on the LAN.
- Multicast: Communication in which information is sent to a specific group of devices or clients. Unlike broadcast transmission, in multicast transmission, clients must be members of a multicast group to receive the information.
Ethernet Addresses
The address used in an Ethernet LAN, which is associated with the network adapter, is the means by which data is directed to the proper receiving location. Figure 3-8 shows the format of an Ethernet MAC address.
Figure 3-8.Ethernet MAC Address
The address that is on the NIC is the MAC address, often referred to as the burned-in address (BIA), and some vendors allow the modification of this address to meet local needs. A 48-bit Ethernet MAC address has two components:
24-bit Organizational Unique Identifier (OUI): The letter O identifies the manufacturer of the NIC. The IEEE regulates the assignment of OUI numbers. Within the OUI, the two following bits have meaning only when used in the destination address:
- Broadcast or multicast bit: This indicates to the receiving interface that the frame is destined for all or a group of end stations on the LAN segment.
- Locally administered address bit: Normally the combination of OUI and a 24-bit station address is universally unique; however, if the address is modified locally, this bit should be set.
- 24-bit vendor-assigned end station address: This uniquely identifies the Ethernet hardware.
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MAC Addresses and Binary-Hexadecimal Numbers
The MAC address plays a specific role in the function of an Ethernet LAN. The MAC sublayer of the OSI data link layer handles physical addressing issues, and the physical address is a number in hexadecimal format that is actually burned into the NIC. This address is referred to as the MAC address, and it is expressed as groups of hexadecimal digits that are organized in pairs or quads, such as the following: 00:00:0c:43:2e:08 or 0000:0c43:2e08. Figure 3-9 shows the MAC address format compared to the MAC frame.
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Figure 3-9.Hexadecimal MAC Address
Each device on a LAN must have a unique MAC address to participate in the network. The MAC address identifies the location of a specific computer on a LAN. Unlike other kinds of addresses used in networks, the MAC address should not be changed unless you have some specific need.