Wide Area Networks

A Wide Area Network (WAN) is a network that covers a broad area (metropolita regional, or national boundaries) using private or public network transports. Using WANS, data can be transmitted over very long distances.

There are a variety of WAN technologies available offering temporary (pay as per usage) and permanent (24/7 availability) connectivity. Some of the commen connectivity includes:

  • Dial-Up
  • ISDN
  • DSL
  • Cable
  • Satellite
  • Wireless

Dial-up

A Dial-up connection is a form of network connectivity using telephone networks. in the initial stages of networking, dial-up connections were used to connect to the ISP for Internet connectivity. In this, computers are connected over telephone networks using a device called the MODEM (a device used for modulating and demodulating signals from analog signals to digital signals and vice versa).

Dial­up

Dial-up connections have a maximum theoretical speed of 56 Kbps through the practical speeds are considerably less. Major disadvantages of dial-up inability to use telephone lines for calls during Internet or network connectivity and painfully slow speeds. Dial-up connectivity usage has become limited due to the popularity, speed and wide availability of broadband technologies such as DSL B Cable Modem. However, you may still find people using dial-up connections for Internet Access or for connecting to offices in remote location.

To set up a dial-up connection, two parts are involved: RAS & Dial-Up Clients. One computer needs to be setup for accepting connections and the other, for connecting to a remote computer. Computers that are setup for accepting remote connections are referred to as Remote Access Servers (RAS) and computers that connect to remote computers are referred to as Dial-up clients (DUN).

Protocols used in dial-up networking are SLIP & PPP.

SLIP or Serial Line Internet Protocol is a protocol designed to work over serial ports and modem connections. SLIP needs static IP addresses before the hosts can be connected and supports only TCP/IP Protocol. SLIP has been largely replaced by PPP.

PPP or Point-to-Point Protocol supports TCP/IP and also multiple other protocols such as NETBEUI, IPX/SPX. PPP also supports dynamic IP addressing and is widely used for dial-up access to Internet. PPP supports multi-links (use of more than one modem simultaneously to increase bandwidth), authentication, encryption and compression.

RRAS (Routing & Remote Access Server)

RRAS (Routing & Remote Access Server) is a feature that is used for managing remote connections. RRAS supports a variety of connections including Dial-up & VPN (Virtual Private network you will read about this later) and usually support tens to hundreds of incoming connections. This is usually used by large organizations and ISPs.

On packet switching networks, routers use static or dynamic methods:

  • In Static routing, packets are transmitted through a fixed route. Manual route entries of different routes are added to the routing table by the administrator to define the routing path. Packets are transmitted by the router only through the path set in the routing table. If there is a problem with a particular route, packets will never reach the destination through other routes. In such cases, administrators need to alter the routes to ensure packets are delivered to the destination. It is not fault-tolerant as it can lead to a single point failure; however, as the route path is known to the administrator, static route is considered to be secure. For example if the route is fixed to travel only through B & D from A to E, packet will never be delivered if there is a broken link between B & D (refer figure below) even though connectivity is still available between A to E through B & C or D.
  • Dynamic routing refers to the capability where the routes are determined by the router automatically based on conditions. Packets may take alternate routes in case of a change in network condition. For example, if the packet has to travel between A to E (image below), packets may be transmitted typically through D as it is the shortest path, if there is a problem through D packet will be delivered through alternate route B & C even though if it is lengthier.

When you specify an IP address and a subnet mask, you also need to specify an IP address in the default gateway column. Routing takes place when a packet is identified for delivery to a remote network. Once the packet is determined not a part of the local network, it is forwarded to the default gateway. Such packets forwarded to the default gateway, are routed and sent to the destination network.

Routing Protocols

In dynamic routing, routing tables are created and managed by routing protocols that automatically run on a router. Routing protocols enable routers to exchange the routing table between them periodically or when there is a change in network condition. Two categories of dynamic routing protocols are Distance-vector protocols and Link-state protocols.

  • Distance-vector routing protocols such as RIPv1, RIPv2 & IGRP use some form of distance to calculate the route metric; for example the number of hops required between the source and the destination. Complete routing tables are periodically (for example, every 30 seconds) exchanged between neighboring routers and use algorithms such as Bellman-Ford to determine shortest path. Routing Information Protocol (RIP) uses hop count as its metric and Interior Gateway Routing Protocol (IGRP) uses a combination of bandwidth & delay as its metric.
  • Link-state routing protocols such as OSPF & IS-IS operate by building topology table based on links and its status from neighboring routers. In case of any change in a link or its status, an advertisement about the change is sent to all routers within the area to adjust the topology table which in turn is used to determine the new best route.

RIP (Routing Information Protocol)

RIP is a distance-vector routing protocol that uses hop count as a routing metric. RIP is limited to 15 hops beyond which it is considered infinite distance and inoperable.

IGRP(Interior Gateway Routing Protocol)

IGRP is a proprietary distance-vector routing protocol invented by Cisco. IGRP overcomes the limitation of RIP by supporting up to a maximum of 255 hops (default 100).

OSPF (Open Shortest Path First)

OSPF is a link-state routing protocol is the most widely used IGP (Interior Gateway Protocol) in large enterprise networks. OSPF was designed to support variable length subnet masking (VLSM) or Classless Inter-Domain Routing (CIDR) addressing models.

IS-IS (Intermediate System to Intermediate System)

15-15 is a Interior Gateway protocol used within an administrative domain or network and is the de facto standard for large service provider network backbones.

IGP (Interior Gateway Protocol)

IGP is a routing protocol that is used to exchange routing information within an autonomous system (AS).

EGP (Exterior Gateway Protocol)

EGP is a routing protocol used to exchange routing information between autonomous systems. This exchange is crucial for communications across the Internet.

BGP (Border Gateway Protocol)

BGP is the protocol used for making routing decisions on the Internet today. ISP’s must establish communication with each other router using BGP and it serves as one of the most important protocols on the Internet.

Routing Commands

TRACERT

TRACERT is a command line utility used for displaying path taken by a packet and measuring transmit delays across an IP network. If you want to know the routes taken by a packet from your computer to the destination, you can use the TRACERT command. TRACERT uses ICMP.

ROUTE

Route is a command line utility that is used for viewing and manipulating routing tables. Routing table on computers is automatically built based on the IP configuration of your computer. Route command is also used by administrators to make manual entries in the routing table to define static routes.

PATHPING

PATHPING is a command line utility that combines the power of both PING and TRACERT PATHPING can provide PING-like statistics between each host traced through TRACERT.

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