Exploring Wide Area Networks (WANs) and Local Area Networks (LANs)
Wide Area Networks (WANs)
WANs cover a large geographical area, often crossing public and private circuits provided by telecommunications service providers.
ISDN and Broadband ISDN
ISDN was designed to replace existing public telecommunication networks, offering a variety of services.
Local Area Networks (LANs)
A LAN is a reliable communication network connecting multiple devices and enabling data transmission between them. Key differences between LANs and WANs include:
- Coverage: LANs cover a small area, like a building or a group of nearby buildings, while WANs span larger areas.
- Ownership: LANs are typically owned by the same entity that owns the connected devices, unlike WANs where resources are often external.
LAN Technologies
Classification by Size
- Personal Area Networks (PANs): Small networks with no more than eight devices, such as a cafe offering internet access.
- Campus Area Networks (CANs): A collection of geographically dispersed LANs within a campus (university, government offices, or industrial plants) belonging to the same entity within a defined area, typically a few kilometers. CANs often use technologies like FDDI and Gigabit Ethernet over fiber optics and spread spectrum.
- Local Area Networks (LANs): Commonly used in offices or buildings, these small networks allow fast communication between connected devices. Due to their limited size, transmission times are predictable, simplifying network administration. They often use a single cable (coaxial or UTP) connecting all machines and operate at speeds between 10 and 100 Mbps.
Important Features
- Dedicated channels for users or companies.
- High-speed links.
- Close proximity of stations.
- Increased efficiency and productivity for office workers through information sharing.
- Lower error rates compared to WANs.
- Resource sharing capabilities.
LANs often use a single cable transmission technology, with various possible communication topologies.
Wide Area Networks (WANs)
WANs are point-to-point networks connecting countries and continents. Due to the distances involved, their speeds are generally lower than LANs, although they can handle larger amounts of data. WANs cover large geographic areas, such as cities or continents, and consist of interconnected computers (hosts) communicating through subnets. LANs often connect to WANs for services like internet access.
A WAN subnet consists of:
- Transmission lines: Carry bits between hosts.
- Switching elements (routers): Specialized computers connecting two or more transmission lines. Packets typically travel through multiple routers before reaching their destination.
Internet Works
An Internet Works is a collection of interconnected networks, each potentially using different software and hardware. A typical example is a group of LANs connected to WANs. Adding a subnet creates a host network. The global set of interconnected networks is known as the Internet.
Metropolitan Area Networks (MANs)
MANs cover a specific geographical location larger than 4 km and use a dual-bus system for data transfer. Essentially a larger LAN, MANs can cover a group of offices or an entire city and can be public or private. They use DQDB (Distributed Queue Dual Bus) for resolving data transmission conflicts.
Network Classifications
Point-to-Point Networks
In a peer-to-peer network, each computer acts as both client and server, facilitating data and peripheral sharing for small groups. Security can be challenging due to the lack of centralized administration.
Server-Based Networks
Server-based networks are better suited for sharing large amounts of resources and data. A manager oversees network operations and security. These networks can have multiple servers (e.g., print server, communication server, database server) depending on traffic volume and peripheral needs.
Classification According to Logical Distribution
All computers have both client and server aspects, with a machine potentially acting as a server for one service and a client for another.
- Server: Provides information or services to other network stations (e.g., print server, file server, web server, mail server).
- Client: Accesses information or services from servers (e.g., viewing a web page, using a remote printer).
All networks should be reliable, portable, and offer high processing power.
Network uses vary depending on the user’s needs, including:
- Data centralization for companies.
- Peripheral and file sharing.
- Reliable data transport.
- Increased information availability.
- Communication within departments.
- Cost savings.
- Home banking.
- Research contributions (video on demand, online TV, interactive gaming).
Network Topology
- Bus Topology: All stations receive transmitted information. One station transmits while others listen.
Advantages: Requires fewer cables than other topologies. A fault in one station doesn’t disable the entire network.
Disadvantages: A single communication channel means a channel or station failure can isolate other stations. Some manufacturers use parallel buses or algorithms to mitigate this.
Two mechanisms for resolving transmission conflicts:
- CSMA/CD (Carrier Sense Multiple Access with Collision Detection): Stations listen for collisions and compete for channel use. If the channel is busy, a station waits; otherwise, it transmits and listens for collisions, retrying after a delay if a collision occurs.
- Token Bus: A token (data frame) circulates among stations. Only the station with the token can transmit or receive for a specific time before passing it on. This prevents collisions.
- Star Networks: Stations connect to a central point (e.g., a wiring concentrator), usually with centralized control.
- Star-Bus Networks: Combines star wiring with bus topology for easier administration.
- Hierarchical Star Networks: Hubs connected in a cascade create a hierarchical structure, commonly used in LANs.
- Ring Networks: Stations connect in a circle using a common cable. Signals travel in one direction, regenerating at each node.
Advantages: Bottlenecks are rare.
Disadvantages: A single channel failure can disable the network. Some manufacturers use alternative channels or algorithms to address this. Administration is complex due to the need for a control station for the token.
Conflict resolution mechanism:
- Token Ring: Stations connect to the ring via an interface unit (RIU). The RIU monitors, regenerates, and relays data. When a data header indicates a specific destination station, the RIU passes the information to that station. The token circulates, and only the station holding the token can transmit. This method is used in LANs with or without priority.