Types of Network

 

 

 

 

There are three types of network. These are:

  • Local Area Network (LAN)
  • Wide Area Network (WAN)
  • Metropolitan Area Network (MAN)

Local Area Network

A local area network is a network system in which computers are interconnected in a limited geographical area, such as network of computers in college computer laboratory or network of computers in office building etc.

Wide Area Network

A wide area network is a network system that covers a large geographical area such as different cities of country or different countries of the world. In WAN telephone lines, satellites, microwave, fiber optic etc. are used as transmission media. If you enjoy learning about computer networks and want to become an IT professional, a CCNA Training Course might be right for you.”

Metropolitan Area Network

A Metropolitan area Network is a network system that covers area of a single city. Usually, MAN connects more than one LANS in a city or town and covers a smaller geographical area than a WAN. The cable television, telephone companies or local corporations use MAN.

Internet

The internet is a worldwide collection of networks that connects millions of computers, business, government agencies, educational institutions. It is a global network and source of information. Information can be communicated from one country to another through internet. More than two and half billion people around the world use the internet daily for different purposes. For example, to communicate with other people around the world, access information and latest news around the world. The computers are the main source through which information can be electronically sent and received from one location to another.

TCP/IP

In this protocol data transmission is managed by dividing the data into different pieces called packets. Each packet of data contains a part of actual data, source computer address, destination address, and information for reassembling data at destination computer. These packets of data travel along the fastest available path in the network. This type of data transmission is called packet switching and is used in internet communication.

IP Addressing

IP stands for internet protocol. Basically it is a unique identifier for a computer on the internet. It is numerical address with four numbers separated with dots and the format of an IP address is a 32-bit.Each number can be start from 0 and ends with 255. For example  IP address looks like this: 131.200.1.1

Every computer on the internet using unique IP address and this IP address can be static and dynamic. A server has a static IP address that does not change. When a computer dialing internet through modem then that computer has a IP address that is assigned by the local ISP. Your local ISP assigned you unique IP address for each session and it may be different each time.

What is Packets?

The transfer of data from one place to another place in the form of digital signals and this data is broken into small data segment before being transmitted from one computer to another computer. These small data segments are called packets. Each packets contains the information about its source and destination (sender’s and receiver’s information). A normal packets size between 1000 to 1500 bytes. The packets send to its destination using best possible path and this makes a network more efficient.

Network Devices

Digital Modem

The modem that is used to send and to receive data to and from a digital telephone line is called digital modem. ISDN modem and DSL modem are the best example of digital modem. These modem are used to send and receive the digital data to and from the digital line, So there is no need to convert to analog signals because digital lines already use the digital signals.

Bridge

It is an electronic device that connects two same type of networks and control the traffic between them. A bridge basically split an overloaded network into two separate network for reducing the amount of traffic on each segment and increase the performance of each network. A bridge looks at the information in each packet header and forwards data from one LAN to another.

Router

An electronic device that connects many computers together and transmits data to its correct destination using the available path on the network is called router. It stores the information of each node on the network and to use these information to transfer date between nodes.

Routable Protocols

There are not all protocols works with routers. Routable protocols are

  • IP
  • IPX
  • OSI
  • DECnet
  • XNS

Like a bridge, a router looks at each packet’s header to determine the destination of packet and thus reach its destination. Some routers also have built-in antivirus protection. Similarly, some routers also support wireless communication

Type of Routers

Static Router.

An administrator manually set up the router path and configuration of static router.

Dynamic Router.

A dynamic router automatically find the path to its destination with minimal configuration.

Gateway

A gateway is an electronic device that can perform logical function. A gateway is a device or system (because it may be a hardware or software) that connects two or more networks to share their information from one to the other. Packets from different networks have different type of information in their headers, and information in different format. The gateway can take a packet from one type of network, read the header, and then encapsulates the entire packet into a new one, adding a header that is understood by the network.

COMPUTER NETWORKING

COMPUTER NETWORKING.

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TCP/IP

 

TCP/IP uses the client/server model of communication in which a computer user (a client) requests and is provided a service (such as sending a Web page) by another computer (a server) in the network. TCP/IP communication is primarily point-to-point, meaning each communication is from one point (or host computer) in the network to another point or host computer. TCP/IP and the higher-level applications that use it are collectively said to be “stateless” because each client request is considered a new request unrelated to any previous one (unlike ordinary phone conversations that require a dedicated connection for the call duration). Being stateless frees network paths so that everyone can use them continuously. (Note that the TCP layer itself is not stateless as far as any one message is concerned. Its connection remains in place until all packets in a message have been received.)

Many Internet users are familiar with the even higher layer application protocols that use TCP/IP to get to the Internet. These include the World Wide Web’s Hypertext Transfer Protocol (HTTP), the File Transfer Protocol (FTP), Telnet (Telnet) which lets you logon to remote computers, and the Simple Mail Transfer Protocol (SMTP). These and other protocols are often packaged together with TCP/IP as a “suite.”

Personal computer users with an analog phone modem connection to the Internet usually get to the Internet through the Serial Line Internet Protocol (SLIP) or the Point-to-Point Protocol (PPP). These protocols encapsulate the IP packets so that they can be sent over the dial-up phone connection to an access provider’s modem.

Protocols related to TCP/IP include the User Datagram Protocol (UDP), which is used instead of TCP for special purposes. Other protocols are used by network host computers for exchanging router information. These include the Internet Control Message Protocol (ICMP), the Interior Gateway Protocol (IGP), the Exterior Gateway Protocol (EGP), and the Border Gateway Protocol (BGP).

 

Ethernet Cabling

CATERGORY 3 CABLE

Catergory 3 cable is used as telephone cable and has a 100mhz bandwidth, using an RJ11 connector. It is UTP cable, with a max distance of 100M (300 feet) before the signal starts to degrade and can use 10Base-T LAN applications with a max speed of 4mbps.

CATERGORY 4 CABLE

Catergory 4 cable is rarely used anymore, and has been wiped out by Cat5+ and has a 100mhz bandwidth. It is UTP cable, with a max distance of 100M (300 feet) before the signal starts to degrade and can use 10Base-T LAN applications with a max speed of 16mbps.

CATERGORY 5 CABLE

Catergory 5 cable is a little old nowadays, but it still used widely in networks and has a 100mhz bandwidth. Cat 5 is UTP cable, with a maximum length of 100M (300 feet) before the signal starts to degrade. Cat 5 can be used in 10BaseT, 100Base-Tx, ATM and CDDI LAN applications.

CATERGORY 5e CABLE

Catergory 5e cable is the most common type of cable used today in networks and has a 100mhz bandwidth. Cat 5e is UTP cable, with a maximum length of 100M (300 feet) before the signal starts to degrade, and can be used in 10Base-T and 100Base-T LAN applications.

CATERGORY 6 CABLE

Catergory 6 cable is the up and coming cable with a 250mhz bandwidth. It is full duplex cable which means that it can be used with gigabit routers. It has two 4 wires paths instead of 2 2-wire paths like the cabling before. Cat 6 is UTP cable, with a maximum length of 100M (300 feet) before the signal starts to degrade. Cat 6 can be used in 10BaseT, 100Base-T, and 1000Base-T LAN applications.

CATERGORY 7 CABLE

Catergory 7 is not being used yet. It is a hybrid cable with a 600mhz bandwidth. It is ScTP cable with a maximum length of 100M (300 feet) before the signal starts to degrade. Cat 7 can be used in 1000Base-T LAN applications.

BLOCK AMPLIFIER

A device attached to a long cable run, which works just like a repeater to re-boost the signal so that the signal can be carried over a longer distance.

Hub And Bridges

In general, a hub is the central part of a wheel where the spokes come together. The term is familiar to frequent fliers who travel through airport “hubs” to make connecting flights from one point to another. In data communications, a hub is a place of convergence where data arrives from one or more directions and is forwarded out in one or more other directions. A hub usually includes a switch of some kind. (And a product that is called a “switch” could usually be considered a hub as well.) The distinction seems to be that the hub is the place where data comes together and the switch is what determines how and where data

is forwarded from the place where data comes together. Regarded in its switching aspects, a hub can also include a router.

1) In describing network topologies, a hub topology consists of a backbone (main circuit) to which a number of outgoing lines can be attached (“dropped”), each providing one or more connection port for device to attach to. For Internet users not connected to a local area network, this is the general topology used by your access provider. Other common network topologies are the bus network and the ring network. (Either of these could possibly feed into a hub network, using a bridge.)

2) As a network product, a hub may include a group of modem cards for dial-in users, a gateway card for connections to a local area network (for example, an Ethernet or a token ring), and a connection to a line (the main line in this example).

BRIDGES

A bridge is used to connect two networks together. Just like a bridge connects two roads, this bridge can join two different networks to extend the network. Say you have two home networks, one in the basement and one upstairs. You can put a bridge in the middle of the house, and then transfer files between networks while still having two seperate networks. The only disadvantage to doing is, is that the collision domain becomes larger (more chance of packets colliding) since the network is much larger.

REPEATERS

A repeater is like a router, but is used to re-strengthen a signal over a long distance. There are analog repeaters, which can only amplify the signal and there are digital repeaters that can restore a signal to near original quality. Some hubs can act as repeaters aswell. Repeaters cannot route internet like a router can though, they are strictly used to regenerate a signal. A repeater should be used when cat5e cabling is over 300feet (100metres) in length. A wireless repeater can be placed between the router and the computer, when length is an issue and the signal is degraded.

linksys bridge

Switch

In a telecommunications network, a switch is a device that channels incoming data from any of multiple input ports to the specific output port that will take the data toward its intended destination. In the traditional circuit-switched telephone network, one or more switches are used to set up a dedicated though temporary connection or circuit for an exchange between two or more parties. On an Ethernet local area network (LAN), a switch determines from the physical device (Media Access Control or MAC) address in each incoming message frame which output port to forward it to and out of. In a wide area packet-switched network such as the Internet, a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination. In the Open Systems Interconnection (OSI) communications model, a switch performs the Layer 2 or Data-link layer function. That is, it simply looks at each packet or data unit and determines from a physical address (the “MAC address”) which device a data unit is intended for and switches it out toward that device. However, in wide area networks such as the Internet, the destination address requires a look-up in a routing table by a device known as a router. Some newer switches also perform routing functions (Layer 3 or the Network layer functions in OSI) and are sometimes called IP switches. On larger networks, the trip from one switch point to another in the network is called a hop. The time a switch takes to figure out where to forward a data unit is called its latency. The price paid for having the flexibility that switches provide in a network is this latency. Switches are found at the backbone and gateway levels of a network where one network connects with another and at the subnetwork level where data is being forwarded close to its destination or origin. The former are often known as core switches and the latter as desktop switches. In the simplest networks, a switch is not required for messages that are sent and received within the network. For example, a local area network may be organized in a token ring or bus arrangement in which each possible destination inspects each message and reads any message with its address. Circuit-Switching version Packet-Switching A network’s paths can be used exclusively for a certain duration by two or more parties and then switched for use to another set of parties. This type of “switching” is known as circuit-switching and is really a dedicated and continuously connected path for its duration. Today, an ordinary voice phone call generally uses circuit-switching. Most data today is sent, using digital signals, over networks that use packet-switching. Using packet-switching, all network users can share the same paths at the same time and the particular route a data unit travels can be varied as conditions change. In packet-switching, a message is divided into packets, which are units of a certain number of bytes. The network addresses of the sender and of the destination are added to the packet. Each network point looks at the packet to see where to send it next. Packets in the same message may travel different routes and may not arrive in the same order that they were sent. At the destination, the packets in a message are collected and reassembled into the original message.

switch

Network Address Translation (NAT)

NAT (Network Address Translation or Network Address Translator) is the translation of an Internet Protocol address (IP address) used within one network to a different IP address known within another network. One network is designated the inside network and the other is the outside. Typically, a company maps its local inside network addresses to one or more global outside IP addresses and unmaps the global IP addresses on incoming packets back into local IP addresses. This helps ensure security since each outgoing or incoming request must go through a translation process that also offers the opportunity to qualify or authenticate the request or match it to a previous request. NAT also conserves on the number of global IP addresses that a company needs and it lets the company use a single IP addressin its communication with the world.

NAT is included as part of a router and is often part of a corporate firewall. Network administrators create a NAT table that does the global-to-local and local-to-global IP address mapping. NAT can also be used in conjunction with policy routing. NAT can be statically defined or it can be set up to dynamically translate from and to a pool of IP addresses. Cisco’s version of NAT lets an administrator create tables that map:

  • A local IP address to one global IP address statically
  • A local IP address to any of a rotating pool of global IP addresses that a company may have
  • A local IP address plus a particular TCP port to a global IP address or one in a pool of them
  • A global IP address to any of a pool of local IP addresses on a round-robin basis

NAT is described in general terms in RFC 1631. which discusses NAT’s relationship to Classless Interdomain Routing (CIDR) as a way to reduce the IP address depletion problem. NAT reduces the need for a large amount of publicly known IP addresses by creating a separation between publicly known and privately known IP addresses. CIDR aggregates publicly known IP addresses into blocks so that fewer IP addresses are wasted. In the end, both extend the use of IPv4 IP addresses for a few more years before IPv6 is generally supported.

 

VIRTUAL PRIVATE NETWORK(VPN)

Vrtual Private Networking, or VPN, is a technology that lets people access their office’s computer network over the Internet while at home or traveling. Accessing a network in this way is referred to as remote access. (For comparison, another common form of remote access is dialing in to the office network over a telephone line.)

But VPN is useful for more than just remote access. It can also be used to link two separate offices over a distance. This is sometimes called a “persistent VPN tunnel”, or “site-to-site VPN”.

VPN for Remote Access

So why would you want to use VPN for remote access? Let’s say you want users to be able to work from home. Or maybe someone needs to retrieve a file while traveling. Without VPN, in order to make resources on the office network available to users, the network administrator would have to weaken the security of your network by opening holes in your firewall — which isn’t usually a good idea. Or the remote user would have to dial in over a phone line, sometimes incurring long-distance charges.

With VPN, the integrity of your office network remains intact, but you can allow remote users to act as part of the office network. After connecting over VPN, remote users can access files, print to printers, and generally do anything with their computers that they would be able to do in the office.

Still, using VPN is not the same as being in the office. Most office networks are pretty fast. Most Internet connections are not. Even the fastest DSL and cable connections are around one-tenth the speed of your average office LAN. This means that accessing resources on the LAN will be much slower over VPN. It would also depend on the “upstream” or upload speed of your office’s network connection. As opposed to working on files directly over the VPN connection, it is often more time-efficient to to copy them to your computer over the VPN connection. When you are done working with them you would copy them back to the file server.

How It Works

In a small office network, VPN is most frequently implemented through a router. Just about every small office that shares an Internet connection with more than one computer already has a router of some kind, but most of them don’t include VPN. For example, small office/home office (SOHO) routers by Linksys, Netgear, or D-Link are popular choices, offering DHCP, NAT, and basic security features in a single device, but they don’t always include VPN support.

Once the VPN router is in place, individual computers can be set up to connect to it from outside the network. Depending on the router and the computers involved, you might need to install software on the computers that will use VPN. Sometimes computers have the ability to connect built-in. Either way, once the hardware and software has been set up, the remote user can initiate a VPN connection.

How a VPN session is initiated depends on how the computer is connected to the Internet. Usually it works something like this: the user double-clicks on a shortcut and the VPN connection window appears. The user enters a username and password and hits “connect.” If the computer has an always-on connection like DSL or cable, the VPN connection is immediately established. If the computer dials in to an ISP in order to access the Internet, that connection is established first and then the VPN connection is established on top of that. Once users are connected to the office network over VPN, they can access files and other resources.

When users are done working, they simply disconnect the VPN connection.

VPN As a Persistent Tunnel

VPN technology can also be used to link two separate networks over the Internet so they operate as a single network. This is useful for organizations that have two physical sites. Rather than set up VPN connections on every person’s computer, the connection between the two sites can be handled by routers, one at each location. Once configured, the routers maintain a constant tunnel between them that links the two sites. In this scenario, users don’t have to do anything to initiate the VPN session because it is always on.

Security and Encryption

There are mainly two kinds of VPN: Point to Point Tunneling Protocol (PPTP) and Layer 2 Tunneling Protocol (L2TP). Both can link a remote computer to a network, but only L2TP offers strong security. If you must transmit sensitive information, do not use PPTP. Remember that when you set up VPN, you’re offering a way into your office network. To minimize the risk of unauthorized parties poking around your network, choose and enforce a strong password policy.

If you allow home users to connect to the office network via VPN, you have to consider viruses or other security threats that could come from the user’s home. One way to address this risk is by giving home users a computer that is owned and maintained by the organization, so is certified as up-to-date and virus-free.

Implementing VPN

Before you implement VPN, evaluate the benefits to your organization and weigh it against the costs of equipment, installation time, and staff training. Maybe you’re considering VPN because your executive director wants to be able to access files on the server while traveling. Maybe VPN would be a good solution. Or perhaps it would work just as well for your executive director to call the office and ask the receptionist to e-mail the file. Given the plethora of online collaboration tools and web-based technologies available now, VPN may not be the only method to access documents off site. However, VPN remains to be the industry standard that is established, scaleable, and secure. Before deciding on any of these technologies, determine the many risks and rewards first.

Once you have decided to implement VPN, determine whether you need help or not. If someone on your staff understands TCP/ IP networking well and can set up the new router, you might be set. If not, consider finding a trusted consultant to help set it up.

In order to use VPN, your Internet connection should have a static IP address. Most types of Internet connections — dial-up, DSL, and cable — provide you with a numerical address on the Internet that changes from time to time. This is called a dynamic IP address. In order to provide VPN access to remote users it is preferable to have an address that doesn’t change, a static IP. Alternately, you can use a dynamic DNS (DDNS) service that can map a domain name to a dynamic IP. There are free services that can map a fixed domain to an account, which your router can update as it obtains different IP addresses. Consult your router or firewall documentation if DDNS is supported

To obtain a static IP address for your Internet connection, talk to your Internet service provider. It may require an additional monthly fee of a few dollars. If you have a friendly ISP, sometimes you can talk it into just giving you a static IP. Occasionally, an ISP will try to sell you much more expensive DSL service, possibly bundled with equipment, when you ask about a static IP. The company might call it a “business class” of service. If the upgrade is too expensive, test the VPN functionality in a pilot phase if DDNS is supported, only then should you decide to pay for the upgrade if necessary.

 

Computer Operating Systems

The most important program that runs on a computer. Every general-purpose computer must have an operating system to run other programs. Operating systems perform basic tasks, such as recognizing input from the keyboard, sending output to the display screen, keeping track of files and directories on the disk, and controlling peripheral devices such as disk drives and printers.

For large systems, the operating system has even greater responsibilities and powers. It is like a traffic cop — it makes sure that different programs and users running at the same time do not interfere with each other. The operating system is also responsible for security, ensuring that unauthorized users do not access the system.

Operating systems can be classified as follows:

  • multi-user : Allows two or more users to run programs at the same time. Some operating systems permit hundreds or even thousands of concurrent users.
  • multiprocessing : Supports running a program on more than one CPU.
  • multitasking : Allows more than one program to run concurrently.
  • multithreading : Allows different parts of a single program to run concurrently.
  • real time: Responds to input instantly. General-purpose operating systems, such as DOS and UNIX, are not real-time.

Operating systems provide a software platform on top of which other programs, called application programs, can run. The application programs must be written to run on top of a particular operating system. Your choice of operating system, therefore, determines to a great extent the applications you can run. For PCs, the most popular operating systems are DOS, OS/2, and Windows, but others are available, such as Linux.

As a user, you normally interact with the operating system through a set of commands. For example, the DOS operating system contains commands such as COPY and RENAME for copying files and changing the names of files, respectively. The commands are accepted and executed by a part of the operating system called the command processor or command line interpreter. Graphical user interfaces allow you to enter commands by pointing and clicking at objects that appear on the screen.

     network operating system

Abbreviated as NOS, an operating system that includes special functions for connecting computers and devices into a local-area network (LAN). Some operating systems, such as UNIX and the Mac OS, have networking functions built in. The term network operating system, however, is generally reserved for software that enhances a basic operating system by adding networking features. Novell Netware, Artisoft’s LANtastic, Microsoft Windows Server, and Windows NT are examples of an NOS.

mobile operating system

An operating system for mobile devices. It is the software platform on top of which other programs, called application programs, can run on mobile devices such as mobile phones, smartphones, PDAs, and handheld computers. Abbreviated as mobile OS.

unix

unix

 

Mac-OS-X-Lion

The OSI Model

       Compatible interconnection of network devices is fundamental to reliable network communications. Developing a set of standards that equipment manufacturers could adhere to went a long way towards providing an open environment for network communications.

 

In the late 1970s the International Organization for Standardization (ISO) worked on a seven layer model for LAN architectures by defining the Open Systems Interconnection Basic Reference Model (OSI). Alongside this The ISO developed a set of protocols that fit within this model. Since then, other models such as the 5 layer TCP/IP model were developed, however the OSI model is still used to map and categorise protocols because of its concise and clear way of representing network functions.

 

The IEEE formed the 802 committee in February 1980 with the aim of standardising LAN protocols. This resulted in the IEEE 802 series of committees that sit to develop worldwide standards for communications. Within the OSI model, the Data Link layer was split into two, the Media Access Control (MAC) sub-layer and the 802.2 Logical Link Control (LLC) sub-layer.

 

 

You can make up expressions to remember the order of the 7 layers, for example, ‘Angus Prefers Sausages To Nibbling Dried Pork’ or ‘A Pretty Silly Trick Never Does Please’. I remember it best using the natty expression ‘Application, Presentation, Session, Transport, Network, Datalink, Physical’. It just rolls off the tongue!

 

The OSI protocol set is rarely used today, however the model that was developed serves as a useful guide when referencing other protocol stacks such as ATM, TCP/IP and SPX/IPX.

 

Application Layer 7

 

It is employed in software packages which implement client-server software. When an application on one computer starts communicating with another computer, then the Application layer is used. The header contains parameters that are agreed between applications. This header is often only sent at the beginning of an application operation. Examples of services within the application layer include:

  • FTP
  • DNS
  • SNMP
  • SMTP gateways
  • Web browser
  • Network File System (NFS)
  • Telnet and Remote Login (rlogin)
  • X.400
  • FTAM
  • Database software
  • Print Server Software

 

Presentation Layer 6

 

This provides function call exchange between host operating systems and software layers. It defines the format of data being sent and any encryption that may be used, and makes it presentable to the Application layer. Examples of services used are listed below:

  • MIDI
  • HTML
  • GIF
  • TIFF
  • JPEG
  • ASCII
  • EBCDIC

 

Session Layer 5

 

The Session layer defines how data conversations are started, controlled and finished. The Session layer manages the transaction sequencing and in some cases authorisation. The messages may be bidirectional and there may be many of them, the session layer manages these conversations and creates notifications if some messages fail. Indications show whether a packet is in the middle of a conversation flow or at the end. Only after a completed conversation will the data be passed up to layer 6. Examples of Session layer protocols are listed below:

  • RPC
  • SQL
  • NetBIOS names
  • Appletalk ASP
  • DECnet SCP

 

Transport Layer 4

 

This layer is resonsible for the ordering and reassembly of packets that may have been broken up to travel across certain media. Some protocols in this layer also perform error recovery. After error recovery and reordering the data part is passed up to layer 5. Examples are:

  • TCP
  • UDP
  • SPX

 

Network Layer 3

 

This layer is responsible for the delivery of packets end to end and implements a logical addressing scheme to help accomplish this. This can be connectionless or connection-oriented and is independent of the topology or path that the data packets travel. Routing packets through a network is also defined at this layer plus a method to fragment large packets into smaller ones depending on MTUs for different media (Packet Switching). Once the data from layer 2 has been received, layer 3 examines the destination address and if it is the address of its own end station, it passes the data after the layer 3 header to layer 4. Examples of Layer 3 protocols include:

  • Appletalk DDP
  • IP
  • IPX
  • DECnet

 

Data Link Layer 2

 

This layer deals with getting data across a specific medium and individual links by providing one or more data link connections between two network entities. End points are specifically identified, if required by the Network layer Sequencing. The frames are maintained in the correct sequence and there are facilities for Flow control and Quality of Service parameters such as Throughput, Service Availability and Transit Delay.

 

Examples include:

  • IEEE 802.2
  • IEEE 802.3
  • 802.5 – Token Ring
  • HDLC
  • Frame Relay
  • FDDI
  • ATM
  • PPP

The Data link layer performs the error check using the Frame Check Sequence (FCS) in the trailer and discards the frame if an error is detected. It then looks at the addresses to see if it needs to process the rest of the frame itself or whether to pass it on to another host. The data between the header and the trailer is passed to layer 3. The MAC layer concerns itself with the access control method and determines how use of the physical transmission is controlled and provides the token ring protocols that define how a token ring operates. The LLC shields the higher level layers from concerns with the specific LAN implementation.

 

Physical Layer 1

 

This layer deals with the physical aspects of the media being used to transmit the data. The electrical, mechanical, procedural and functional means This defines things like pinouts, electrical characteristics, modulation and encoding of data bits on carrier signals. It ensures bit synchronisation and places the binary pattern that it receives into a receive buffer. Once it decodes the bit stream, the physical layer notifies the data link layer that a frame has been received and passes it up. Examples of specifications include:

  • V.24
  • V.35
  • EIA/TIA-232
  • EIA/TIA-449
  • FDDI
  • 802.3
  • 802.5
  • Ethernet
  • RJ45
  • NRZ
  • NRZI

You will notice that some protocols span a number of layers (e.g. NFS, 802.3 etc.). A benefit of the seven layer model is that software can be written in a modular way to deal specifically with one or two layers only, this is often called Modular Engineering.

 

Each layer has its own header containing information relevant to its role. This header is passed down to the layer below which in turn adds its own header (encapsulates) until eventually the Physical layer adds the layer 2 information for passage to the next device which understands the layer 2 information and can then strip each of the layers’ headers in turn to get at the data in the right location. Each layer within an end station communicates at the same layer within another end station.

 

 

More Networking Protocols

Ethernet

The Ethernet protocol is by far the most widely used. Ethernet uses an access method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection). This is a system where each computer listens to the cable before sending anything through the network. If the network is clear, the computer will transmit. If some other node is already transmitting on the cable, the computer will wait and try again when the line is clear. Sometimes, two computers attempt to transmit at the same instant. When this happens a collision occurs. Each computer then backs off and waits a random amount of time before attempting to retransmit. With this access method, it is normal to have collisions. However, the delay caused by collisions and retransmitting is very small and does not normally effect the speed of transmission on the network.

The Ethernet protocol allows for linear bus, star, or tree topologies. Data can be transmitted over wireless access points, twisted pair, coaxial, or fiber optic cable at a speed of 10 Mbps up to 1000 Mbps.

Fast Ethernet

To allow for an increased speed of transmission, the Ethernet protocol has developed a new standard that supports 100 Mbps. This is commonly called Fast Ethernet. Fast Ethernet requires the use of different, more expensive network concentrators/hubs and network interface cards. In addition, category 5 twisted pair or fiber optic cable is necessary. Fast Ethernet is becoming common in schools that have been recently wired.

Local Talk

Local Talk is a network protocol that was developed by Apple Computer, Inc. for Macintosh computers. The method used by Local Talk is called CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). It is similar to CSMA/CD except that a computer signals its intent to transmit before it actually does so. Local Talk adapters and special twisted pair cable can be used to connect a series of computers through the serial port. The Macintosh operating system allows the establishment of a peer-to-peer network without the need for additional software. With the addition of the server version of AppleShare software, a client/server network can be established.

The Local Talk protocol allows for linear bus, star, or tree topologies using twisted pair cable. A primary disadvantage of Local Talk is speed. Its speed of transmission is only 230 Kbps.

Token Ring

The Token Ring protocol was developed by IBM in the mid-1980s. The access method used involves token-passing. In Token Ring, the computers are connected so that the signal travels around the network from one computer to another in a logical ring. A single electronic token moves around the ring from one computer to the next. If a computer does not have information to transmit, it simply passes the token on to the next workstation. If a computer wishes to transmit and receives an empty token, it attaches data to the token. The token then proceeds around the ring until it comes to the computer for which the data is meant. At this point, the data is captured by the receiving computer. The Token Ring protocol requires a star-wired ring using twisted pair or fiber optic cable. It can operate at transmission speeds of 4 Mbps or 16 Mbps. Due to the increasing popularity of Ethernet, the use of Token Ring in school environments has decreased.

FDDI

Fiber Distributed Data Interface (FDDI) is a network protocol that is used primarily to interconnect two or more local area networks, often over large distances. The access method used by FDDI involves token-passing. FDDI uses a dual ring physical topology. Transmission normally occurs on one of the rings; however, if a break occurs, the system keeps information moving by automatically using portions of the second ring to create a new complete ring. A major advantage of FDDI is speed. It operates over fiber optic cable at 100 Mbps.

ATM

Asynchronous Transfer Mode (ATM) is a network protocol that transmits data at a speed of 155 Mbps and higher. ATM works by transmitting all data in small packets of a fixed size; whereas, other protocols transfer variable length packets. ATM supports a variety of media such as video, CD-quality audio, and imaging. ATM employs a star topology, which can work with fiber optic as well as twisted pair cable.

ATM is most often used to interconnect two or more local area networks. It is also frequently used by Internet Service Providers to utilize high-speed access to the Internet for their clients. As ATM technology becomes more cost-effective, it will provide another solution for constructing faster local area networks.

Gigabit Ethernet

The most recent development in the Ethernet standard is a protocol that has a transmission speed of 1 Gbps. Gigabit Ethernet is primarily used for backbones on a network at this time. In the future, it will probably be used for workstation and server connections also. It can be used with both fiber optic cabling and copper. The 1000BaseTX, the copper cable used for Gigabit Ethernet, is expected to become the formal standard in 1999.