Core 3: Communication Systems

On any network there are two types of computers present – servers and clients. By definition Client-Server architecture refers to… Client server architecture is servers providing requests for clients. Clients request a service and wait for a response.

For example when a web server retrieves and transmits web pages OR a database server retrieving and transmitting records.  A client is: a computer that requests a service.  A Server is: a computer on a network dedicated to providing clients with a service. Eg. Web, or Print.

This definition seems to imply that servers are a special type of dedicated computer on a network and on a large LAN this might be the case. For a home network this is not the case. A computer in a small office that connects to the internet could act as a web server or similarly a print server.

Regarding client machines there are two types – Fat clients and Thin clients.  A Fat client is a client that has applications installed that are executed by the client CPU.  A thin client is a basic computer terminal that is involved in simple tasks such as displaying data.

The topology of a network describes the way in which the devices (nodes) are connected. A node is any device connected to a network including computers, printers, hubs, switches and routers. All nodes must be able to communicate using the suite of protocols defined for the particular networks. Nodes are connected to each other via transmission media, either wired or wireless.

The topology of a network describes these connections in terms of their physical layout and also in terms of how data is transferred between nodes. We call this Physical and Logical Topologies. The Physical Topology determines how the devices are physically connected. The Logical Topology describes how nodes communicate with each other rather than how they are physically connected.

There are three basic topologies – the bus, the star and the ring. In addition there are two other topologies common on larger networks – hybrid and mesh. We will look at these in terms of the Physical and Logical Topologies which are often different.

Physical Bus Topology All nodes are connected to a single backbone – also known as a trunk or bus. The backbone is a single cable that carries data packets to all nodes. Each node attaches and listens for data present on the backbone. A single break in the cable deactivates the entire network.

Physical Bus Topology

Physical Star Topology All nodes connect to a central node via their own dedicated cable. Mostly used for LANs. Central node is usually a switch containing multiple ports. Advantages: if one node goes down, others don’t and nodes can be added or removed without the network being deactivated. Wired routers use star.

Physical Star Topology

Physical Ring Topology Similar to BUS except the cable makes a complete ring. Nodes are connected in series which means that if one is deactivated, all others suffer because the series is broken. Similarly adding or removing a node requires the network to be stopped.

Physical Ring Topology

Physical Hybrid Topology Hybrid networks use a combination of bus, star and ring topologies. Commonly a bus is the start, or backbone, of the network with a series of physical star networks branching off. All hybrid topologies have a single transmission between each node which is why they can often be called a ‘tree’.

Physical Hybrid Topology

Physical Mesh Topology A physical mesh topology means that there are more than one physical path between each pair of nodes. This is the primary topology of the internet where IP datagrams can travel different paths from the transmitter to the reciever. Commonly the nodes on a network are all routers and each router connects to further routers or a LAN.

Physical Mesh Topology

As a reminder… The logical topology of a network describes how data is transmitted and received on a network – regardless of the physical connections.

Logical Bus Topology  Logical Topology (or Flow of data) in BUS makes use of CSMA/CD – “Carrier Sense Multiple Access with Collision Detection”.  This means a node will send data if transmission medium is available (BUS is clear). IF a collision occurs, the node waits and sends again.

Logical Ring Topology  Data, in packets or frames, circulate the ring one at a time through each node in the network. As a result there cannot be data collisions. Ring networks use either Ethernet or token ring protocols. Token ring is a token that continually circles around the network.

Logical Star Topology  The logical flow of data matches the physical star layout. Nodes are attached to a central switch which sends information back and forth to each node along its own network segment.