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.

How To Expanding A Networks

Network Connectivity Devices:  

To expand a single network without breaking it into new parts or connecting it to other networks, we can use the following devices:

•  Hubs
•  Repeaters
•  Bridges
•  Multiplexers

Hub:

• All networks require a central location to bring media segments together
• The central locations are called hubs
•  A hub organizes the cables and relays signals to the other media segments.
• Important things about hub:
• There is a limit to the number of hubs that can be connected to each other to extend a network.
• When possible, connect each hub directly to a server network card rather than to another hub.
• Label the connection on the hub.
• The more hubs data passes through, the slower the connection.
• Passive hubs
• A passive hub simply combines the signals of network segments.
• There is no signal processing or regeneration.
• As it does not boost the signal and absorb some of the signal, it reduces by half of maximum cabling distance permitted.

Here, each computer receives the signals sent from all the other computers connected to the hub.

Active hub:

• It regenerates or amplify signals.
• The distance between devices can be increased.
• They also amplify the noise as well.
• They are more expensive than passive hub.
• Bcz some active hubs function as repeaters, they are sometimes called multi-port repeaters.

Intelligent hub:

Intelligent hubs can regenerate signals but it can perform some network management and intelligent path selection.

A switching hub chooses only the port of the device where the signal needs to go, rather than sending the signal along all paths.

Many switching hubs can choose which alternative path will be the quickest and send the signal that way.

Repeaters:

• All transmission media attenuate the electromagnetic waves that travel through them.
• Adding a device that amplifies the signal can allow it to travel farther, increasing the size of the network.
• Devices that amplify the signals in this way are called repeaters.
• Repeaters fall into two categories: amplifier and signal-regenerating repeaters.
• Amplifiers simply amplify the entire incoming signal and amplify both signal and noise.
• Signal regenerating repeaters create an exact duplicate of incoming data by identifying it amidst the noise, reconstructing it and retransmitting only the desired information.
• The original signal is duplicated, boosted to its original strength and sent.

Bridges:

Bridges connect network segments.

The use of bridge increases the maximum possible size of your network.

A bridge selectively determines the appropriate segment to which it should pass a signal.

It does this by reading the address of all the signals it receives.

The bridge reads the physical location of the source and destination computers from this address.

The process works like:

A bridge receives all the signals from both segment A and segment B.

The bridge reads the addresses and discards all signals from segment A that are addressed to segment A, bcz they do not need to cross the bridge.

Signals from segment A addressed to a computer on segment B are retransmitted to segment B.

The signals from segment B are treated in the same way

Through address filtering, bridges can divide busy networks into segments and reduce network traffic.

Network traffic will be reduced if most signals are addressed to the same segment and do not cross the bridge.

Two types of bridges:

• Transparent bridges keep a table of addresses in memory to determine where to send data
• Source-routing bridges require the entire route to be included in the transmission and do not route packets intelligently.
• Multiplexing allows you to use more bandwidth of the medium by combining two or more separate signals and transmitting them together.
• The original signals can then be extracted at the other end of the medium. This is demultiplexing.
• Multiplexing provides a way of sharing a single medium segment by combining several channels for transmission over that segment.

Three major methods of multiplexing are:

1. Frequency division 

2. Time division 

3. Statistical time division

Frequency-Division multiplexing:

FDM uses separate frequencies to combine multiple data channels onto a broadband medium.

You can use FDM to separate traffic traveling in different directions in a broadband LAN.

Time-division multiplexing:

• TDM divides channel into time slots.
• Each of the devices communicating over this multiplexed line is allocated a time slot in a round-robin fashion.
• If a device does not use its time slot, that slot is wasted.

Statistical Time-division multiplexing:

• TDM systems can be inefficient if many slot times are wasted.
• StatTDM provides an intelligent solution to this problem by dynamically allocating time slots to devices on a first-come, first-serve basis.
• The number of time slots allocated to a particular device depends on how busy it is.
• You can use priorities to allow on device greater access to time slots than another.
• For the multiplexer on the receiving end to determine which signal a particular time slot is carrying, there must be a control field tat identifies the owner attached to the data.