Understanding Computer Networks: From LANs to the OSI Model
Local Area Networks (LANs)
Local Area Networks (LANs) are structured as a set of communication protocols operating on a defined topology. This topology dictates how computers connect within the network.
Hosts and Nodes
For our purposes, a host or node refers to a computer capable of network interaction or hosting network services. While technically synonymous, “host” is more common in telecommunications.
Clients and Servers
A client is a network computer that utilizes services provided by another computer, known as a server.
Isolated Systems
An isolated system is a computer incapable of external communication.
Distributed Systems
Distributed systems also consist of networked computers but offer a key distinction: the presence of multiple computers is transparent to the user. Operations can be executed without the user knowing which specific computer is responding.
Network Architectures
Client-Server Networks
In client-server networks, servers are often dedicated to providing specific services. This specialization makes them less suitable for running user applications directly. An example of a client-server network operating system is Novell NetWare.
Peer-to-Peer Networks
In peer-to-peer networks, the operating system can function as both a client and a server. Each node can provide and consume services, allowing for resource sharing across the network.
The OSI Model
Transport Layer
The transport layer acts as a bridge between the network and user layers, providing an abstract service for software and hardware information transport.
The first three layers of the OSI model (physical, link, and network) are network-oriented, dealing with the physical environment and signal transport. The last three (session, presentation, and application) are user-oriented. The transport layer facilitates the transition between these two sets of layers.
OSI Protocol Groups
- Group 1: System management protocols (manage the network itself).
- Group 2: Application management protocols (control, implementation, security, resource allocation).
- Group 3: System protocols (file access, remote task execution).
- Group 4 and 5: Application-specific protocols (defined by programmers for inter-application communication).
IP Address Classes
- Class A: Uses one byte for network identification and three for host addresses.
- Class B: Uses two bytes for network and two for host addresses.
- Class C: Uses three bytes for network and one for host addresses. (Currently the only class with available addresses).
Physical Layer
The physical layer defines the mechanical, electrical, and functional characteristics for establishing connections between computers. Transmission occurs at the bit level.
Link Layer
The link layer ensures error-free communication for the network layer. It operates with frames (blocks of bits) and handles error detection, correction, and flow control.
Network Layer
The network layer’s primary function is routing, determining the optimal path for data packets to reach their destination.
The ARPANET Model and the Internet
The ARPANET model, developed by the US Department of Defense, emerged as a Wide Area Network (WAN). Its protocols are analogous to the OSI network and transport layers. Today, it forms the foundation of the Internet.
Key Internet Protocols
- IP (Internet Protocol): A connectionless protocol designed for interconnecting networks.
- TCP (Transmission Control Protocol): A connection-oriented protocol, providing reliable data transmission for applications like email and client-server interactions.