Types and Characteristics of WANs
What is a WAN?
There are two prevailing definitions of a Wide Area Network (WAN). The book definition of a WAN is a network that spans large geographical locations, usually to interconnect multiple Local Area Networks (LANs). The practical definition of a WAN is a network that traverses a public network or commercial carrier, using one of several WAN technologies
What are its Main Components?
The main components for a WAN are routers, switches and modems. These components are described below in the hardware section.
CPE – Devices on the subscriber premises are called customer premises equipment (CPE).
The subscriber owns the CPE or leases the CPE from the service provider. A copper or fiber cable connects the CPE to the service provider’s nearest exchange or central office. This cabling is often called the local loop, or “last-mile”.
DTE/DCE – Devices that put data on the local loop are called data circuit-terminating equipment, or data communications equipment (DCE). The customer devices that pass the data to the DCE are called data terminal equipment (DTE). The DCE primarily provides an interface for the DTE into the communication link on the WAN cloud.
In a WAN you will need various types of hardware components for it to function. The typical items of hardware that you will need in a WAN are:
Router – An electronic device that connects a local area network (LAN) to a wide area network (WAN) and handles the task of routing messages between the two networks. Operates at layer 3, and makes decisions using IP addresses.
Switch – A switch is a network device that selects a path or circuit for sending a unit of data to its next destination. Operates at layer 2, and uses MAC addresses to send data to correct destination.
Modem – Short for modulator/demodulator, a modem enables a computer to communicate with other computers over telephone lines. Operates at layer 1, where signals are converted from digital to analogue and vice versa for transmission and receiving.
WANs operate within the OSI model using layer 1 and layer 2 levels. The data link layer and the physical layer. The physical layer protocols describe how to provide electrical, mechanical and functional connections to the services provided by the ISP. The data link layer defines how data is encapsulated for transmission to remote sites.
Encapsulation is the wrapping of data in a particular protocol header. Remember that WANs operate at the physical layer and the data link layer of the osi model and that higher layer protocols such as IP are encapsulated when sent across the WAN link. Serial interfaces support a wide range of WAN encapsulation types, which must be manually specified. These types include SDLC, PPP, Frame delay etc. Regardless of WAN encapsulation used it must be identical on both sides of the point to point link.
Packet and Circuit Switching
Circuit switching and packet switching are both used in high-capacity networks.
The majority of switched networks today get data across the network
through packet switching.
Circuit-switching is more reliable than packet-switching. Circuit switching is old and expensive, packet switching is more modern.
General Routing Issues
What is a Routing Protocol?
A routing protocol is a protocol that specifies how routers communicate and exchange information on a network. Each router has prior knowledge of its immediate neighbours and knows the structure of the network topology. The routers know this because the routing protocol shares this information.
RIP (Routing Information Protocol) was one of the most commonly uses protocols on internal networks. Routers use RIP to dynamically adapt changes to the network connections and communicate information about which networks routers can reach and the distance between them. RIP is sometimes said to stand for Rest in Pieces in reference to the reputation that RIP has for breaking unexpectedly and rendering a network unable to function.
This type of routing protocol requires that each router simply inform its neighbours of its routing table. The distance vector protocol is also known as the bellman-ford algorithm.
This type of routing protocol requires that each router maintain a partial map of the network. The link state algorithm is also know as Dijkstra’s algorithm.
IGRP is a type of distance vector routing protocol invented by cisco used to exchange routing data in a autonomous system. Distance vector protocols measure distances and compare routes. Routers that use distance vector must send all or a portion of their routing table in a routing update message at regular intervals to each neighbour router.
Addressing and Routing
What does routing mean?
Routing is the process of deciding how to move packets from one network to another.
The directions also known as routes can be learned by a router using a routing protocol then the information is passed from router to router along the route of the destination.
Every machine connected to the internet is assigned an IP address. An example of an IP address would be 192.168.0.1. IP addresses are displayed in decimal format to make it easier for humans to understand but computers communicate in binary form. The four numbers that separate an IP address are called Octets. Each position consists of eight bits. When added to together you get 32 bit address. The purpose of each octet in an IP address is to create classes of IP addresses that can be assigned within a network. There are three main classes that we deal with Class A, B and C. The octets of an IP address are split into two parts Network and Host. In a class A address the first octet is the network portion, this determines which network the computer belongs to, the last octets of the address are the hosts that belong to the network.
Sub netting allows you to create multiple networks within a class A, B or C address. The subnet address is the address used by your LAN. In a Class C network address you would have a subnet mask of 255.255.255.0. A subnet mask identifies which portion is network and which is host. For example 192.168.6.15 the first octet three octets are the Network address and the last octet being the host(Workstation). It is important to subnet a network because gateways need to forward packets to other LANS. By giving each NIC on the gateway an IP address and a Subnet mask it allows the gateways to route packets from LAN to LAN. Once the packet arrives at its destination, the gateway then uses the bits of the subnet portion of the IP address to decide which LAN to send the packets.
Circuit Switched Leased Lines
A circuit switched network is one that establishes a dedicated circuit (or channel) between nodes and terminals before the users may communicate. Here are some terminologies associated with a Circuit switched network.
Frame relay is a telecommunication service designed for cost-efficient data transmission between local area networks (LANs)
Basic rate interference is a service used by small business for internet connectivity. An ISDN BRI provides two 64 Kbps digital channels to the user.
Primary rate interface (PRI) is a telecommunications standard for carrying voice and data transmissions between two locations
All data and voice channels are ISDN and operate at 64kbit/s
Packet switching refers to protocols in which messages are broken up into small packets before they are sent. Each packet is then transmitted over the Internet. At the destination the packets are reassembled into the original message. Packet switching main difference from Circuit Switching is that that the communication lines are not dedicated to passing messages from the source to the destination. In Packet Switching, different messages can use the same network resources within the same time period.
Asynchronous Transfer Mode (ATM) is a cell relay, packet switching network and protocolwhich encodes data into small fixed-sized cells.
ISDN is used to carry voice, data, video and images across a telephone network. ISDN stands for integrated services Digital Network. Isdn also provides users with a 128kbps bandwidth. This is done through frame relay. Frame relay complements and provides a service between ISDN, which offers bandwidth at 128 Kbps and Asynchronous Transfer Mode which operates in somewhat similar fashion to frame relay but at speeds from 155.520 Mbps or 622.080 Mbps. Frame relay is based on the older X.25 packet switching technology and is used to transmit analogue signals such as telephone conversations.
PSDN stands for packet switched data network and is a data communication network. Packet switched networks do not establish a physical communication signal like the public telephone does (circuit switched network) Packets are sent on a fixed length basis and assigned with a source and a destination address. The packets then rely on the routers to read the address and route the packets through the network.
Mobile and Broadband Services
Digital Subscriber line(DSL) is mainly used to bring high bandwidth connections to homes and small business’s over a copper wire telephone line. This is can only be achieved if you stay within the range of the telephone exchange. DSL offers download rates of up to 6mbps allowing continuous transmission of video, audio and 3D effects. DSL is set to replace ISDN and compete with the cable modem in providing multimedia to homes. DSL works by connecting your telephone line to the telephone office over copper wires that are twisted together.
Asymmetric Digital Subscribers Line is most commonly used for home users. It provides a high download speed but a lower upload speed. Using ADSL, up to 6.1 megabits per second of data can be sent downstream and up to 640 Kbps upstream.
Symmetric Digital Subscriber Line is a digital subcriber line which runs over one pair of copper wires. The main difference between ADSL and SDSL is the difference in upload and download speeds. SDSL allows the same upstream data rate and downstream data rate as ADSL upstream can be very slow.
HDSL High bit-rate Digital Subscriber Line, one of the earliest forms of DSL, is used for wideband digital transmission within a corporate site and between the telephone company and a customer. The main characteristic of HDSL is that provides equal bandwidth in both directions.
IDSL is a system in which data is transmitted at 128 Kbps on a regular copper telephone line from a user to a destination using digital transmission.
The Local Loop enables operators to connect directly to the consumer via copper local loops and then add their own equipment to offer broadband and other services. This process involves operators accessing local exchange buildings to connect to a network of copper lines which connect them to homes and businesses. BT is an Example of a Local Exchange. The local loop connecting the telephone exchange to most subscribers is capable of carrying frequencies well beyond the 3.4 kHz upper limit.
Benefits of using DSL
DSL can provide virtually instantaneous transmission of voice, data and video over ordinary copper phone lines. A DSL connection can eliminate delays when waiting to download information and graphics from the Internet. It provides users with a cost effective high speed Internet connection. Another benefit is that a DSL connection is always on-line (like a LAN connection) with no waiting time for dialling or connecting.
There are now more than 10 million broadband connections in the UK. By December 2005 there were 9.792 million broadband connections in the UK and the average broadband take up rate during the three months to December was more than 70,000 per week.