Internetwork Connectivity Devices for Networking

Internetwork Connectivity Devices

An internetwork connectivity consists of two or more independent networks that are connected and yet maintain independent identities. An internetwork may include different types of networks. To connect independent networks, we use internetwork connectivity devices. The devices are: routers, brouters, gateways, CSUs/DSUs etc. 

Some of the benefits of internetworking are:

• Reduces network traffic:With internetwork connectivity devices, most traffic stays on the local network and only packets destined for other networks cross internetwork connectivity devices.
• Optimizes performance:The benefit of reduced traffic is optimized performance.
• Simplifies management:Network problems can be more easily identified and isolated in smaller networks, as opposed to one large network.
• Efficiently spans long geographical distances: Because WAN links are many times slower and more expensive than LAN links, having a single large network spanning long distances can complicate network management and slow network performance. We can more efficiently span long distance by connecting multiple smaller networks.
• Interconnection: To connect our network with those of other organizations is also a good reason.

Routers:

• Practically Routers are devices that connect two or more networks. 
• Router consist of a combination of hardware and software. 
• In Router the hardware can be a network server, a separate computer or a special black box device.
• For Router the two main pieces of software is the OS and the routing protocol. 
• Management software is another component of router. 
•  For routing the Routers use logical and physical addressing to connect two or more logically separate networks. 
• Different Router accomplish this connection by organizing the large network into logical network segments (subnet).
• In each of these sub network it has given a logical address. 
• For packet transferring each packet in addition to having a physical device address, has logical network address. 
• The network address allows routers to more accurately and efficiently calculate the optimal path to a workstation or computer.
•  Routers keep the networks separate and router processing is generally slower than bridge processing.
• Routers are more intelligent than bridges because they use algorithms to determine the best path to send a packet to a network. 
• By passing packets only according to network addresses, routers can help prevent a broadcast storm.
• Routers list network addresses in routing tables.
• These tables contain all known network addresses and possible paths. 

Terms to quantify the routing cost:

• Hop count describes the number of routers a message must pass through to reach its destination.
• Tick count describes the amount of time required for a message to reach its destination. A tick is 1/18 second.
• Relative expense is a number you can assign based on the actual monetary cost or some other relevant criteria required to use a given link. 
• Route discovery is the process of finding the possible routes through the internetwork and then building routing tables to store that information.

Two methods of route discovery are:

• Distance-vector  
• Link-state

Distance-Vector Routing:

• In distance-vector routing, each router advertises its presence to other routers on the network.
• Periodically each router on the network broadcasts the information contained in its routing table.
• The other routers then update their routing tables with the broadcast information they receive.
• These periodic broadcasts of routing table information by the routers performing distance-vector route discovery add up to a noticeable amount of traffic.
• This traffic is not a problem in LANs, because plenty of bandwidth is available and the number of routers is usually low.
• However, it seriously affect performance in a WAN.
• In a large internetwork, distance-vector routing tends to be quite inefficient as route changes must be broadcast through the network from router to router and because changes are contained within complete routing tables, it can take a long time before all the routers on the network know of a change.

Link-State Routing:

• Link-state routers broadcast their complete routing tables only at startup and at certain intervals-much less frequently than distance-vector broadcasts.
• This type of routing generates less network traffic than the distance-vector method.
• The major difference between the link-state and distance-vector methods is that once the initial routing-table exchange has occurred, a link-state router will generally broadcast routing updates only when it detects a change in its routing table. When it does broadcast, it sends only information about the change, it doesn't send its complete routing table.
• Selection of the optimum route can be dynamic or static.
• Dynamic route selection permits routers to constantly adjust to changing network conditions.
• With static route selection, packets must always follow a predetermined path.
• Dynamic route selection uses the cost information that is continually being generated by routing algorithms and placed in routing tables to select the best for each packet.
• Other routers that receive broadcast messages regarding changes in the state of network links use this information to update their own routing tables.
• As only the changes are sent, these updates can be done in less time.
• Once a router has created its routing table, it can use the cost information contained within that table to calculate the best path through the internetwork.
• Routing protocol can select the best path based on the minimum number of hops, number of ticks or relative expense.
• As network conditions change, the router can select different paths to maintain the lowest possible costs.
• The router can even select new paths “on the fly” as it is transmitting packets.
• If changes occur during a transmission that make one route suddenly less attractive than another, the router can send the remaining packets of the transmission along a different path from the packets in the first part of the transmission.
• With static route selection, the data path is not selected on the fly by the routers involved.
• Instead, the data path is designed in advance.
• Either the network administrator or a computer on the network selects a route for the data from a predefined table.
• All packets are then forced along that route and intermediate routers are not allowed to make route selection decisions.
• Static route selection tends to be less efficient than dynamic route selection because it cannot adapt to changing network conditions.

Brouter:

• A brouter is a router that can also bride.
• A brouter first tries to deliver the packet based on network protocol information.
• If the brouter does not support the protocol the packet is using or cannot deliver the packet based on protocol information, it bridges the packet using the physical address.
• True routers simply discard a packet if it doesn’t have a correct logical address.
• A brouter can be a more affordable option to having both a router and a bridge.
• Keep the following in mind when working with routers:
• Some routers may not follow standards. This can cause problems when we use different vendors’ routers on the same network.
• Be sure the router is rated to handle the speed of the network connection.
• Routers slow down network communications to a small extent, so don’t use them unnecessarily.
•  Routable protocol: DECnet, DDP, TCP/IP, NWLink IPX, OSI, XNS
• Non-routable protocol: LAT, NetBEUI
• A gateway is a device that can interpret and translate the different protocols that are used on two distinct networks.
• Gateway can be comprised of software, dedicated hardware or a combination of both.
• Gateways can function at network layer.
• When you need to have different environments communicating, you may wish to consider a gateway.
• A gateway can actually convert data so that it works with an application on a computer on the other side of the gateway.
• You can connect systems with different communication protocols, languages and architecture using a gateway.
• Gateway can be slow because they need to perform intensive conversion and they can be expensive.

CSUs/DSUs:

• Sometimes, when expanding your network, it is less costly and easier to use existing public networks, such as the public telephone network in your area.
• Connecting to some of these networks requires the use of CSUs/DSUs (channel service units/digital service units).
• Network service providers may require you to use a CSU/DSU to translate in signals of your LAN into a different signal format and strength for use on their transmission media.
• CSUs/DSUs are also useful for shielding your network from both noise and voltage currents that can come through the public network.