1. Introduction to Computer Networks


A network can be any connected devices. It can be as small as two computers or as complex as a multisite network such as a telecommunications infrastructure that contains 100s if not 1000s of computers that are connected together.

The common uses of a network are:

  • Communication video conferencing, email, chats, learning, etc…
  • Sharing Hardware – printers, scanners, storage…
  • Sharing data – files.
  • Sharing applications – Microsoft word, spread sheet, specialised software, etc.
  • Data Backup and retrieval

1.1     Types of Networks

1.1.1     LANs and WANs

They types of network used are dictated by the number of locations they span.

LANs – Local Area Networks are restricted to a single location (building, office, school).

WANs – Wide Area Networks spread over multiple geographic locations. They are slower than LANs and more expensive and they tend to use different technologies to connect LANs together to create an internetwork.

Between these networks are other types of networks that you might experience:

MANs – Metropolitan Area Networks spread over geographic locations; they’re not large enough to be called WANs and not too big to be called LANs, which is why on many occasions MANs are referred to as WANs because there is no rule or guideline to differentiate between the two.

CANs – Campus Area Networks are restricted to one location, similar to LANs but they’re too small to be called LANs.

PANs – Personal Area Networks are small networks that are used to connect personal devices together, such as connecting a laptop to a PDA device.

1.1.2     Peer-to-Peer and Client/Server Networks

Wired networks use two basic models, peer-to-peer and client/server:

Peer-to-Peer Networks

Peer-to-peer networks are made of a small number of computers (usually not more than 10 PCs) that are connected together and each one of these PCs is responsible for their own security and resource management. It is sometimes called decentralised network because there is no method to centralise the management of resources.

Advantages of Peer-to-Peer:

  • Low cost, no requirement for expensive servers.
  • Easy to implement.
  • Easy to maintain (the lower the number of devices the easier it is).

Disadvantages:

  • Security needs to be applied, managed and maintained separately on each device.
  • Data Backup needs to be performed separately on each device.
  • Limited number of PCs, although you can install more than 10 PCs or devices however managing these devices gets harder and unfeasible as the number of these devices increases.

1.1.3     Client/Server Network

Client/server networks on the other hand are the most common networks you’ll see installed in organisations even small businesses because although they’re more expensive the following advantages makes them the number one choice chosen by all businesses.

Advantages of client/server networks:

  • Scalable, very easy to expand and add as many devices as needed.
  • Centralised management.
  • Enhanced Security.
  • Simplified backup.

 

1.1.4     Hybrid Networks

A hybrid network describes a network that takes advantage of a number of other network types to operate. Like for example a network utilising a client/server technology and at the same time the network or part of it operates in peer-to-peer.

1.1.5     Distributed and centralised computing

Distributed computing is when the work or load of the network traffic is distributed or spread over to different systems. It also describes a network that uses client/server network but at the same time some of the load is distributed to the client. Like for example a hotel booking system; the booking system could be held on the mainframe, while the email system used to correspond to customers is held on the PC-based server.

1.1.6     VPN (Virtual Private Network)

VPNs are used to connect two networks of different locations by creating an encrypted point-to-point tunnel via public networks (internet). They provide a secure point-to-point dedicated link between the two networks over the internet. It can be thought of as a sort of a WAN connection. VPNs provide a cost effective way to expand the network, provide network connectivity over long distances, can be used to connect a private LAN to another (LAN-to-LAN internetworking), allow remote workers to connect to the company network as if they were inside and provides security; without the need of a leased line.

The following are the important elements that are involved when establishing a VPN connection:

  • VPN Client – The remote client that starts a connection.
  • VPN Server – Authenticate connections.
  • Access Method – internet or intranet.
  • VPN protocols – required to establish, manage and secure the data over the VPN connection. The VPN connection is managed by the PPTP (Point-to-Point Tunnelling Protocol) and the L2TP (Layer 2 Tunnelling Protocol). They enable authentication and encryption. The VPN connection supports analogue modems, ISDNs, Wireless and Broadband connections (cable or DSL).

 Advantages of VPNs:

  • Cost – no need for leased lines.
  • Network Scalability – allows expansion without the need to change the network infrastructure.
  • Simplified admin – Authentication server can easily add or remove clients.
  • Security – all data travels over the internet via an encrypted tunnel.

Disadvantages of VPNs:

  • Security and complexity – Admins need a good understanding of security protocols and the need to properly setup, configure and manage a VPN connection.
  • Reliability – Dependant on the ISP (how reliable the ISP is). The ISP is short for Internet Service Provider (Plusnet, BT Broadband, SkyNet).

1.1.7     VLANs (Virtual Local Area Networks)

VLANs are used for segmentation. This is a strategy to improve:

  • Network Performance.
  • Can increase security.
  • Removes Performance bottle neck.

A VLAN is a group of connected computers that act as if they’re on their own network segment even though they might not be. They’re a group of logically connected systems and configure through an interface on a switch or router. For example we have a multi-storey building that has employees from the same department but on different floors; the VLAN can be used to group these employees under one virtual segment where they can connect and use all the resource that are specialised for that department.

Advantages of VLANs:

  • Increase security and performance.
  • Organisation – network users and resources that are linked and communicate frequently can be grouped together in a VLAN.
  • Simplified Admin – easier to remove or reconfigure switches or routers, moving users between LAN segments and it is easier to re-cable and address new stations.

VLAN Membership

A membership is a term used to describe the assigning of a device to a particular VLAN. When a device is assigned to a VLAN, we say it is a member of that VLAN. Membership can be achieved using several methods:

  • Protocol-based – assigns a membership based on the protocol and IP address used.
  • Port-based – assigns membership based on ports.
  • MAC-address – membership assignment is based on the MAC address of the device. The MAC address (Media Access Control) is an address assigned by the manufacturer of the device, which is hardcoded into the device. This MAC address is made of a part that Identifies the manufacturer of the device and a part made of a sequence number of the device.

1.2     LAN Topologies

A network topology is a term used to describe the layout of the network. The topology affects the type of media, devices and network methods that are required.

1.2.1     Physical vs. Logical topology

Network topologies can be defined on a physical or logical level.

Physical topology – describes how a network is physically constructed.

Logical topology – How a network looks to the devices that use it or how the network functions.

1.2.2     Bus Topology

The bus is also called the trunk or backbone. Computers connect to these backbones via T connectors or drop cable. Each needs to be terminated and one end also needs to be grounded. This topology uses a single length cable and the devices connect directly to this cable.

Advantages of a Bus topology:

  • Inexpensive.
  • Easy to implement.
  • Requires less cable than other topologies.

Disadvantages of a Bus topology:

  • Cannot be expanded.
  • A break in a cable renders the entire segment.
  • Difficult to troubleshoot.

1.2.3     Star Topology

Each device in the network connects to a centralised device via separate cable, which creates a point-to-point connection between the two devices and gives the overall appearance of a star.

Hierarchical Star – Multiple stars connected together and rearranged into a tree like structure. This enables:

  • High levels of flexibility and expandability.
  • Manage traffic.
  • Isolate high traffic areas on a network.

Advantages of a Star Topology:

  • Devices connect to a central point.
  • Multiple star can be combined.
  • Easily expandable without disruption to existing systems.
  • A cable failure only affects one system.
  • Easy to troubleshoot.

Disadvantages of a Star Topology:

  • Requires additional equipment.
  • More cables
  • Single point of failure (if hub or switch fails than that whole segment fails).

1.2.4     Ring Topology

In this topology data travels in one direction until it reaches the destination. The devices can be connected as a physical ring with each node connecting to the ring, or it can be connected logically via a special central device called the MSAU (Multi-Station Access Unit). This topology is very rare and it is very unlikely to be encountered in real life.

The most popular topology is the physical star.

1.2.5     Wired Mesh Topology

Provides maximum reliability and redundancy; by connecting each device to every single device on the network. This type of topology is very expensive and it can be implemented in two ways partial mesh and full mesh.

Partial Mesh – some nodes in the network such as critical servers are connected in full mesh, while the rest of the devices are not.

Full Mesh – everything in the network is connected in full mesh. This type is usually reserved for backbone networks.

1.2.6     Wireless Network Topologies

The IEEE 802.11 Standard is a standard that describes wireless LANs. Under this standard there are different types of WLANs, all with its own speed, frequency and transmission range. There are also different topologies within WLANs:

  • Infrastructure (managed).
  • Ad-hoc (unmanaged).
  • Point-to-Point.
  • Point-to-Multipoint
  • Mesh.
  • Hybrid.

1.2.6.1          Infrastructure (managed) Topology

An infrastructure topology is used to extend a wired LAN to include wireless devices. In this topology the devices communicate with the wired LAN via base stations called an AP, which acts as a bridge between wired and wireless LANs (WLANs).

BSS and ESS

Basic Service Set (BSS) – this describes at least one AP connected to the wired LAN and a set of wireless end stations.

Extended Service Set (ESS) – describes two or more BSS (above) combined (interconnected) to make a sub-net.

1.2.6.2        Ad-Hoc WLANs

Ad-hoc topology is a form of peer-to-peer wireless network, where all devices are connected to each other directly without an AP.

1.2.6.3         Point-to-Point WLANs (PtP)

In a PtP WLAN setup the communication is established from one node directly to another node. These are usually found in wireless backbone or as a replacement for a single wired LAN cable.

1.2.6.4          Point-to-Multipoint (PtMP)

A point-to-multipoint (PtMP) wireless connection is designed to link multiple wired networks. Signals in point-to-multipoint networks travel from a central node, such as a base station of a cellular system, an access point of a WLAN, or a satellite. The function of the multipoint wireless topology is to interconnect multiple locations enabling them to access and share resources. Multipoint networks use a base station as the “hub” and client networks as the connection points communicating with the base station. These point-to-multipoint networks are used in wireless Internet service providers (WISP), large corporate campuses, interconnected branch offices, and more.

The reliability of the PtMP network topology relies on the quality of the central node and each connecting node. The location of the central node is important to ensure the range and strength of the wireless signal.

1.2.6.5           Wireless Mesh

In a wireless mesh topology several APs are interconnected to one another in a similar way to the wired mesh topology, where each node is connected to every single node in the network to provide redundancy, however because it’s wireless it doesn’t incur the expensive costs that would’ve been incurred if a wired mesh were to be implemented.

1.2.6.6           Hybrid WLAN topology

This is similar to the wired hybrid; in fact we can say that they’re the same. In here we might see a wireless LAN connected to a wired LAN or a star topology connected to a bus topology. In another words it’s a mixture of topologies.

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