Local Area Network (LAN)

Computer Networks and Internet Basics (9hrs) · Computer Fundamentals & Emerging Technologies

1. Definition

A Local Area Network (LAN) is a network that connects computers and devices within a limited geographic area — a room, an office, a building or a campus (typically up to about 1 km). It is privately owned and administered, offers high data rates (100 Mbps to 100 Gbps) and has very low error rates because the medium is well controlled and short. Standards for LANs come from IEEE 802 (802.3 Ethernet, 802.11 Wi-Fi, 802.5 Token Ring).

2. Characteristics

Feature	Typical value
Coverage	A room to a campus, ≤ 1 km
Speed	100 Mbps / 1 Gbps / 10 Gbps common, up to 100 Gbps
Ownership	Single organisation (private)
Administration	One administrator, one policy domain
Error rate	Very low (10⁻⁹ to 10⁻¹²) — short, shielded paths
Latency	Microseconds to a few milliseconds
Media	UTP Cat5e/6, fibre for backbones, Wi-Fi
Devices	NIC, hub, switch, access point, router (for internet), server
Cost	Low per node; cable/switch installation is the main expense
Topology	Star (dominant), bus and ring legacy

3. LAN Technologies and Their Speeds

Ethernet (IEEE 802.3) — the dominant wired LAN standard:

Name	Speed	Medium	Notes
10Base-T	10 Mbps	UTP Cat3, 100 m	Classic star LAN, CSMA/CD
100Base-TX (Fast Ethernet)	100 Mbps	UTP Cat5, 100 m	Two pairs, full duplex
1000Base-T (Gigabit Ethernet)	1 Gbps	UTP Cat5e/Cat6, 100 m	Four pairs used simultaneously
10GBase-T / 10GBase-SR	10 Gbps	Cat6a (100 m) / fibre	Server and backbone links
40/100 Gigabit Ethernet	40-100 Gbps	Fibre / twinax	Data-centre core

Wi-Fi (IEEE 802.11 a/b/g/n/ac/ax/be) — wireless LAN: 802.11n/ac/ax give hundreds of Mbps to multi-Gbps in the 2.4, 5 and 6 GHz bands, covering 30-100 m per access point.
Token Ring (802.5) and FDDI — deterministic token-passing LANs, now largely obsolete but historically important for guaranteeing access time.
VLAN (Virtual LAN) — logical segmentation of one physical switch fabric, so different departments are isolated without separate hardware; it also shrinks broadcast domains.

4. Access Methods

Method	Used by	Idea
CSMA/CD (Carrier Sense Multiple Access with Collision Detection)	Wired Ethernet (half duplex)	Listen before sending; if a collision is detected, stop, send a jam signal, wait a random back-off, retry
CSMA/CA (Collision Avoidance)	Wi-Fi	Avoid collisions with ACK, inter-frame spacing, optional RTS/CTS; wireless stations cannot detect collisions easily
Token passing	Token Ring, FDDI	Only the station holding the token may transmit — deterministic and collision-free

Collision domain vs broadcast domain: a switch gives each port its own collision domain (full-duplex ports have none) but the whole switch is one broadcast domain unless VLANs are used; a router separates broadcast domains; a hub puts every port in one collision domain.

5. Ethernet Frame and MAC Address

 ┌──────────┬────┬───────────┬───────────┬──────┬────────────────┬──────┐
 │ Preamble │SFD │ Dest MAC  │ Src MAC   │ Type │ Payload        │ FCS  │
 │ 7 bytes  │1 B │ 6 bytes   │ 6 bytes   │2 B   │ 46-1500 bytes  │4 B   │
 └──────────┴────┴───────────┴───────────┴──────┴────────────────┴──────┘
   synchronisation      │            │      EtherType  data      CRC-32
   for the receiver     │            └── sender's hardware address
                        └── receiver's hardware address
 
 Minimum frame = 64 bytes, maximum payload (MTU) = 1500 bytes

MAC address — a 48-bit hardware address written as 12 hex digits, e.g. 00:1A:2B:3C:4D:5E. The first 24 bits are the OUI (manufacturer's ID) and the last 24 bits are the serial number, so addresses are globally unique. Special forms: FF:FF:FF:FF:FF:FF is the broadcast address, and the least-significant bit of the first byte marks multicast/unicast. MAC addresses work only inside one LAN; the IP address is what carries data between LANs.

6. Advantages and Limitations

Advantages	Limitations
High speed at low cost per node	Limited distance (~1 km); cannot link distant offices
Simple, well-understood technology; easy to install and expand	Performance degrades with heavy traffic or many broadcast frames
Resource sharing (printers, files, internet) among nearby users	Security risk on a shared medium — sniffing, rogue devices; needs VLANs/802.1X
Low error rate, easy maintenance, one administrative domain	A loop without STP, or a failed core switch, can disrupt the whole LAN
Supports both wired (Ethernet) and wireless (Wi-Fi) access	Wi-Fi is susceptible to interference, and bandwidth is shared per AP

7. LAN vs WAN (short contrast)

Basis	LAN	WAN
Area	Building/campus, ≤ 1 km	City, country, global
Ownership	Private, single organisation	Multiple operators; leased/public links
Speed	100 Mbps-100 Gbps	2 Mbps-10 Gbps typical per link
Latency & error rate	Very low	Higher latency and error rate
Devices	Switches, hubs, APs	Routers, modems, WAN switches
Cost	Low setup, no recurring	High recurring cost (leased lines/MPLS/ISP)

8. Exam Points

Define a LAN and list its characteristics — 5 marks, almost certain.
Differentiate LAN, MAN and WAN with a comparison table — 5 marks, very frequent.
Explain Ethernet and its speed variants — 5 marks.
Draw the Ethernet frame format and explain each field — 5 marks, frequent.
Explain MAC addressing (48-bit structure, OUI, broadcast address) — 5 marks.
Differentiate CSMA/CD and CSMA/CA; explain collision and broadcast domains — 5 marks.
State advantages and limitations of a LAN — 3-5 marks.

Local Area Network (LAN)