BASIC NETWORKING DEVICES

Repeaters and Hubs

Since a signal loses strength as it passes along a cable, it is often necessary to boost the signal with a device called a repeater. The repeater electrically amplifies the signal it receives and rebroadcasts it. Repeaters can be separate devices or they can be incorporated into a concentrator. They are used when the total length of your network cable exceeds the standards set for the type of cable being used.

A concentrator is a device that provides a central connection point for cables from workstations, servers, and peripherals. In a star topology, twisted-pair wire is run from each workstation to a central concentrator. Hubs are multislot concentrators into which can be plugged a number of multi-port cards to provide additional access as the network grows in size. Some concentrators are passive, that is they allow the signal to pass from one computer to another without any change. Most concentrators are active, that is they electrically amplify the signal as it moves from one device to another. Active concentrators are used like repeaters to extend the length of a network

Bridges and Switch A bridge is a device that allows you to segment a large network into two smaller, more efficient networks. If you

are adding to an older wiring scheme and want the new network to be up-to-date, a bridge can connect the two.

A bridge monitors the information traffic on both sides of the network so that it can pass packets of information to the correct location. Most bridges can "listen" to the network and automatically figure out the address of each computer on both sides of the bridge. The bridge can inspect each message and, if necessary, broadcast it on the other side of the network.

Routers A router translates information from one network to

another; it is similar to a super intelligent bridge. Routers select the best path to route a message, based on the destination address and origin. The router can direct traffic to prevent head-on collisions, and is smart enough to know when to direct traffic along back roads and shortcuts.

While bridges know the addresses of all computers on each side of the network, routers know the addresses of computers, bridges, and other routers on the network. Routers can even "listen" to the entire network to determine which sections are busiest -- they can then redirect data around those sections until they clear up.

THE OSIThe OSI Layers

Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer

Application LayerThe application layer (layer 7) of the OSI model supports the communication component on an application. The

term application in the sense does not refer to computer applications – such as word processing, presentation graphics, spreadsheets, databases – but rather to network applications. Examples of network applications include:

File transfer Electronic mail Remote access Client/Server process Information location Network management

Presentation Layer

The presentation layer (Layer 6) of the OSI reference model is responsible for presenting data in a form that a receiving device can understand. It serves as the translator - sometimes between different formats - for devices that need to communicate over a network, by providing code formatting and conversion. The presentation layer (Layer 6) formats and converts network application data into text, graphics, video, audio, or whatever format is necessary for the receiving device to understand it.

The presentation layer is not only concerned with the format and representation of data, but also with the data structure that the programs use. Layer 6 organizes the data for Layer 7. To understand how this works, imagine that you have two systems. One system uses EBCDIC, and the other uses ASCII to represent data. When the two systems need to communicate, Layer 6 converts and translates the two different formats.

Session Layer

The session layer (Layer 5) establishes, manages, and terminates sessions between applications. It coordinates the service requests and responses that occur when applications establish communications between different hosts.

Transport Layer

The transport layer (Layer 4) is responsible for transporting and regulating the flow of information from source to destination reliably and accurately. Its functions include:

Connection Synchronization Flow control

Error recovery Reliability through windowing

The transport layer (Layer 4) enables a user's device to segment several upper-layer applications for placement on the same Layer 4 data stream, and enables a receiving device to reassemble the upper-layer application segments.

As the transport layer sends its data segments, it also ensures the integrity of the data. This transport is a connection-oriented relationship between communicating end systems.

Segments upper layer dataEstablishes an end to end connectionSends segments from one host to anotherOptional, ensures data reliability

TCP Establish a connection

In order for data transfer to begin, one user of the transport layer must establish a connection-oriented session with its peer system. Then, both the sending and receiving application programs must inform their respective operating systems that a connection will be initiated. In concept, one device places a call to another device that the other device must accept. Protocol software modules in the two operating systems communicate by sending messages across the network to verify that the transfer is authorized and that both sides are ready. After all synchronization has occurred, a connection is established, and data transfer begins. During transfer, the two devices continue to communicate with their protocol software to verify that they are receiving the data correctly.

Establishing a connection

Layer 3 what they call as the network layer is the domain of routing. The network layer is responsible for navigating the data through the network. The function of the network layer is to find the best path through the network. The network layer's addressing scheme is used by devices to determine the destination of data as it moves through the network.

The network layer is responsible for moving data through a set of networks (internetwork). The network layer's addressing scheme is used by devices to determine the destination of data as it moves through the networks.

Protocols that have no network layer can only be used on small internal networks. These protocols usually use only a name (i.e. MAC address) to identify the computer on a network. The problem with this approach is that, as the network grows in size, it becomes increasingly difficult to organize all the names, such as making sure that two computers aren't using the same name.

Types of protocol in the Network Layer

Routed Protocols Routing Protocols

Routed Protocols

A routed protocol can be routed by a router, which means that it can be forwarded from one router to another.

A routed protocol contains the data elements required for a packet to be sent outside of its host network or network segment. In other words, a routed protocol can be routed.

Examples of routed protocol: TCP/IP, IPX/SPX, Apple Talk

Routing Protocols

Routing protocols (Note: Do not confuse with routed protocols.) determine the paths that routed protocols follow to their destinations.

Examples of routing protocols include the: Routing Information Protocol (RIP) Interior Gateway Routing Protocol (IGRP) Enhanced Interior Gateway Routing Protocol (EIGRP) Open Shortest Path First (OSPF).

Routing protocols enable routers that are connected, to create a map, internally, of other routers in the network or on the Internet. This allows routing (i.e. selecting the best path, and switching) to occur. Such maps become part of each router's routing table.

Data Link Layer

The Data-Link layer is the protocol layer in a program that handles the moving of data in and out across a physical link in a network. The Data-Link layer is layer 2 in the Open Systems Interconnect (OSI) model for a set of telecommunication protocols.

The Data-Link layer ensures that an initial connection has been set up, divides output data into data frames, and handles the acknowledgements from a receiver that the data arrived successfully. It also ensures that incoming data has been received successfully by analyzing bit patterns at special places in the frames.

Data Link Sub Layers

Logical Link Control (LLC) - The LLC works with the transport layer by providing connection-oriented and connectionless services. It manages and creates the communication link.

Media Access Control (MAC) - The MAC sub layer carries the physical address of each device on the network. This address is more commonly called a device's MAC address. The MAC address is a 48-bit address that's encoded on each network device by its manufacturer. It's the MAC address that the Physical layer uses to move data between nodes of the network.

Physical Layer

The physical layer is level one in the seven level OSI model. It performs services requested by the data link layer.

This level refers to network hardware, physical cabling or a wireless electromagnetic connection. It also deals with electrical specifications, collision control and other low-level functions.

The physical layer is the most basic network layer, providing only the means of transmitting raw bits. The shapes of the electrical connectors, which frequencies to broadcast on, and similar low-level things are specified here.