OSI Model

         The OSI layer was introduced by the International Organization for Standardization (ISO) in 1984 in order to provide a reference model to make sure products of different vendors would interoperate in networks.

·         OSI is short for Open System Interconnection.

·         The OSI layer shows WHAT needs to be done to send data from an application on one computer, trough a network, to an application on another computer, not HOW it should be done.

·         A layer in the OSI model communicates with three other layers: the layer above it, the layer below it, and the same layer at its communication partner.

·         Data transmitted between software programs passes all 7 OSI layers.

·         The Application, Presentation and Session layers are also known as the Upper Layers.

·         The Data Link and Physical layers are often implemented together to define LAN and WAN specifications.

Data Encapsulation

·         Data Encapsulation is the process of adding a header to wrap the data that flows down the OSI model.

·         Each OSI layer may add it's own header to the data received from above. (from the layer above or from the software program 'above' the Application layer.)

·         The 5 Steps of Data Encapsulation are:

·         1. The Application, Presentation and Session layers create DATA from users' input.

·         2. The Transport layer converts the DATA to SEGMENTS

·         3. The Network layer converts the SEGMENTS to PACKETS (or datagrams)

·         4. The Data Link layer converts the PACKETS to FRAMES

·         5. The Physical layer converts the FRAMES to BITS.

·         At the sending computer the information goes from top to bottom while each layers divides the information received from upper layers in to smaller pieces and adds a header. At the receiving computer the information flows up the model discarding the corresponding header at each layer and putting the pieces back together.

(Although the data block shown in the animation below does not change size, it actually gets smaller as it passes down the OSI model until is goes in bits / electrical signals over the physical network cable.)

Application Layer (Layer 7)

·         Provides network services directly to applications. Type of software programs vary a lot: from groupware and web browser to Tactical Ops(video game). Software programs itself are not part of the OSI model.

·         Determines the identity and availability of communication partners, and determines if sufficient resources are available to start program-to-program communication.

·         This layer is closest to the user.

·         Examples of Application layer protocols are:

·         Telnet

·         SMTP

·         FTP

·         SNMP

·         NCP

·         SMB

·         Gateways operate at this layer.

·         Transmits Data.

Presentation Layer (Layer 6)

·         Defines coding and conversion functions.

·         Ensures that information sent from the application layer of one system is readable by the application layer of another system.

·         Includes common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes, common examples of these formats and schemes are:

·         MPEG, QuickTime

·         ASCII, EBCDIC

·         GIF, TIFF, JPEG

·         Gateways operate at this layer.

·         Transmits Data.

Session Layer (Layer 5)

·         The session layer establishes, manages, maintains and terminates communication channels between software programs on network nodes.

·         Provides error reporting for the Application and Presentation layer.

·         Examples of Session layer protocols are:

·         NFS

·         SQL

·         RPC

·         Zone Information Protocol (ZIP)

·         Gateways operate at this layer.

·         Transmits Data.

Transport Layer (Layer 4)

·         The main purpose of this layers is making sure that the data is delivered error-free and in the correct sequence.

·         Establishes, maintains and terminates virtual circuits.

·         Provides error detection and recovery.

·         Is concerned with reliable and unreliable transport. When using a connection-oriented, reliable transport protocol, such as TCP, acknowledgments are send back to the sender to confirm that the data has been received.

·         Provides Flow Control and Windowing.

·         Provides multiplexing; the support of different flows of data to different applications on the same host.

·         Examples of Transport layer protocols are:

·         TCP (connection-oriented, reliable, provides guaranteed delivery.)

·         UDP (connectionless, unreliable, less overhead, reliability can be provided by the Application layer)

·         Gateways operate at this layer.

·         Transmits Segments.

Network Layer (Layer 3)

·         Defines logical addressing for nodes and networks/segments.

·         Enables internetworking, passing data from one network to another.

·         Defines the logical network layout so routers can determine how to forward packets trough an internetwork.

·         Routing occurs at this layer, hence Routed and Routing protocols reside on this layer.

·         Routed protocols are used to encapsulate data into packets. The header added by the Network layer contains a network address so it can be routed trough an internetwork.

·         Examples of Network layer Routed protocols are:

·         IP

·         IPX

·         AppleTalk

·         Routing protocols are used to create routing tables; routing tables are used to determine the best path / route. Routing protocols provide periodic communication between routers in an internetwork to maintain information on network links in a routing table.

·         Examples of Network layer Routing protocols are:

·         OSPF

·         IGRP/EIGRP

·         RIP

·         BGP

·         NLSP

·         Transmits Packets.

·         Routers operate at this layer.

Data Link Layer (Layer 2)

·         Defines psychical addressing, network topology, and is also concerned with error notification, sequencing of frames and flow control.

·         Examples of network topologies are:

·         Star

·         Bus

·         Ring

·         Physical addresses are also known as hardware and BIA's (Burned In Addressess) but most commonly as MAC addresses.

·         Examples of Data Link LAN specifications are:

·         Ethernet

·         FastEthernet

·         Token Ring

·         FDDI

·         Examples of Data Link WAN specifications are:

·         Frame Relay (operates also on the Physical layer)

·         PPP (operates also on the Physical layer)

·         X.25 (operates also on the Physical and Network layer)

·         Transmits Frames.

·         Bridges and Switches operate at this layer.


The Data Link layer consists of two sublayers:

·         LCC (Logical Link Control) Layer

·         Manages communication between devices over a single link of a network.

·         Enables multiple higher-layer protocols to share a single physical data link.

·         MAC Layer

·         Manages protocol access to the physical network medium.

·         Determines hardware addresses.

Physical Layer (Layer 1)

·         The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems.

·         Physical layer specifications define characteristics such as:

·         voltage levels

·         timing of voltage changes

·         physical data rates

·         maximum transmission distances

·         physical connectors

·         Physical layer implementations can be categorized as either LAN or WAN specifications.

·         Examples of LAN specifications are:

·         Ethernet

·         FastEthernet

·         Token Ring

·         FDDI

·         Examples of WAN specifications are:

·         HSSI

·         V.24

·         V.35

·         BRI

·         SLIP

·         RS-232

·         Transmits bits. (bitstream)

·         Repeaters operate at this layer.

Modem

A modem is like a telephone for a computer to communicate with other computers over telephone lines. Broadly, a modem’s performance depends on the characteristics of the modem and those of the telephone line it is working with. The condition of telephone lines varies widely across the country. So how your modem works will depend to a large extent on the telephone line.

There’re several different types of noise that can happen either individually or in combination in a telephone line. Different modems react differently to these. And there’s no way you can identify which of these conditions affect your line. So nobody can correctly predict which modem will work the best on your line.
Test it on your phone line
There’s no other way out. You’ll have to convince your vendor to test the modem at your place, and to take it back if it does not work properly.

Internal or external?
If you’re buying a modem separately, go for an external fax-modem. External ones are easier to install. They cost Rs 2,000 upward. Buy the cheapest one that works okay with your line, giving you a steady high-speed connect for at least an hour.
If you get an internal modem card pre-installed when you buy a new PC, that’s okay, if it’s a 56 kbps fax-modem. An internal card does have one advantage: it reduces clutter. Again, it helps to check that it’s working okay on your line, else you may need to change it
Other modem features
True plug-and-play with USB
If you’re looking for swift installs and convenience, go for a USB modem. They cost about Rs 4,000.
The answering machine modem
If you’re looking for a modem that’s also a digital answering machine and a fax machine, go for a message modem. Message modems can store voice messages and faxes even while the PC is switched off. Once the computer is switched on, these messages are transferred to it and can then be heard or viewed. Some also let you retrieve the messages from a remote location—all you have to do is call up your message modem.

Ethernet

Cables

Cable is the medium through which information usually moves from one network device to another. There are several types of cable which are commonly used with LANs. In some cases, a network will utilize only one type of cable; other networks will use a variety of cable types. The type of cable chosen for a network is related to the network's topology, protocol, and size. Understanding the characteristics of different types of cable and how they relate to other aspects of a network is necessary for the development of a successful network. The following sections discuss the

types of cables used in networks and other related topics.

Unshielded Twisted Pair (UTP) Cable

Twisted pair cabling comes in two varieties: shielded and unshielded. Unshielded twisted pair (UTP) is the most popular and is generally the best option for school networks (See fig. 1).

Fig.1. Unshielded twisted pair

The quality of UTP may vary from telephone-grade wire to extremely high-speed cable. The cable has four pairs of wires inside the jacket. Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs and other electrical devices. The tighter the twisting, the higher the supported transmission rate and the greater the cost per foot. The EIA/TIA (Electronic Industry Association/Telecommunication Industry Association) has established standards of UTP and rated five categories of wire.

Categories of Unshielded Twisted Pair

Type Use Category 1 Voice Only (Telephone Wire) Category 2 Data to 4 Mbps (LocalTalk) Category 3 Data to 10 Mbps (Ethernet) Category 4 Data to 20 Mbps (16 Mbps Token Ring) Category 5 Data to 100 Mbps (Fast Ethernet)

Buy the best cable you can afford; most schools purmyoung Category 3 or Category 5. If you are designing a 10 Mbps Ethernet network and are considering the cost savings of buying Category 3 wire instead of Category 5, remember that the Category 5 cable will provide more "room to grow" as transmission technologies increase. Both Category 3 and Category 5 UTP have a maximum segment length of 100 meters. In Florida, Category 5 cable is required for retrofit grants. 10BaseT refers to the specifications for unshielded twisted pair cable (Category 3, 4, or 5) carrying Ethernet signals. Category 6 is relatively new and is used for gigabit connections.

Unshielded Twisted Pair Connector

The standard connector for unshielded twisted pair cabling is an RJ-45 connector. This is a plastic connector that looks like a large telephone-style connector (See fig. 2). A slot allows the RJ-45 to be inserted only one way. RJ stands for Registered Jack, implying that the connector follows a standard borrowed from the telephone industry. This standard designates which wire goes with each pin inside the connector.

Fig. 2. RJ-45 connector

Shielded Twisted Pair (STP) Cable

A disadvantage of UTP is that it may be susceptible to radio and electrical frequency interference. Shielded twisted pair (STP) is suitable for environments with electrical interference; however, the extra shielding can make the cables quite bulky. Shielded twisted pair is often used on networks using Token Ring topology.

Coaxial Cable

Coaxial cabling has a single copper conductor at its center. A plastic layer provides insulation between the center conductor and a braided metal shield (See fig. 3). The metal shield helps to block any outside interference from fluorescent lights, motors, and other computers.

Although coaxial cabling is difficult to install, it is highly resistant to signal interference. In addition, it can support greater cable lengths between network devices than twisted pair cable. The two types of coaxial cabling are thick coaxial and thin coaxial.

Thin coaxial cable is also referred to as thinnet. 10Base2 refers to the specifications for thin coaxial cable carrying Ethernet signals. The 2 refers to the approximate maximum segment length being 200 meters. In actual fact the maximum segment length is 185 meters. Thin coaxial cable is popular in school networks, especially linear bus networks.

Thick coaxial cable is also referred to as thicknet. 10Base5 refers to the specifications for thick coaxial cable carrying Ethernet signals. The 5 refers to the maximum segment length being 500 meters. Thick coaxial cable has an extra protective plastic cover that helps keep moisture away from the center conductor. This makes thick coaxial a great choice when running longer lengths in a linear bus network. One disadvantage of thick coaxial is that it does not bend easily and is difficult to install.

Coaxial Cable Connectors

The most common type of connector used with coaxial cables is the Bayone-Neill-Concelman (BNC) connector (See fig. 4). Different types of adapters are available for BNC connectors, including a T-connector, barrel connector, and terminator. Connectors on the cable are the weakest points in any network. To help avoid problems with your network, always use the BNC connectors that crimp, rather than screw, onto the cable.

Fig. 4. BNC connector

Fiber Optic Cable

Fiber optic cabling consists of a center glass core surrounded by several layers of protective materials (See fig. 5). It transmits light rather than electronic signals eliminating the problem of electrical interference. This makes it ideal for certain environments that contain a large amount of electrical interference. It has also made it the standard for connecting networks between buildings, due to its immunity to the effects of moisture and lighting.

• Outer insulating jacket is made of Teflon or PVC.
• Kevlar fiber helps to strengthen the cable and prevent breakage.
• A plastic coating is used to cushion the fiber center.
• Center (core) is made of glass or plastic fibers.

Fiber Optic Connector

The most common connector used with fiber optic cable is an ST connector. It is barrel shaped, similar to a BNC connector. A newer connector, the SC, is becoming more popular. It has a squared face and is easier to connect in a confined space.

Ethernet Cable Summary

Specification	Cable Type	Maximum length
10BaseT	Unshielded Twisted Pair	100 meters
10Base2	Thin Coaxial	185 meters
10Base5	Thick Coaxial	500 meters
10BaseF	Fiber Optic	2000 meters
100BaseT	Unshielded Twisted Pair	100 meters
100BaseTX	Unshielded Twisted Pair	220 meters

Wireless LANs

Not all networks are connected with cabling; some networks are wireless. Wireless LANs use high frequency radio signals, infrared light beams, or lasers to communicate between the workstations and the file server or hubs. Each workstation and file server on a wireless network has some sort of transceiver/antenna to send and receive the data. Information is relayed between transceivers as if they were physically connected. For longer distance, wireless communications can also take place through cellular telephone technology, microwave transmission, or by satellite.

Wireless networks are great for allowing laptop computers or remote computers to connect to the LAN. Wireless networks are also beneficial in older buildings where it may be difficult or impossible to install cables.

The two most common types of infrared communications used in schools are line-of-sight and scattered broadcast. Line-of-sight communication means that there must be an unblocked direct line between the workstation and the transceiver. If a person walks within the line-of-sight while there is a transmission, the information would need to be sent again. This kind of obstruction can slow down the wireless network.

Scattered infrared communication is a broadcast of infrared transmissions sent out in multiple directions that bounces off walls and ceilings until it eventually hits the receiver. Networking communications with laser are virtually the same as line-of-sight infrared networks.

Wireless LANs have several disadvantages. They provide poor security, and are susceptible to interference from lights and electronic devices. They are also slower than LANs using cabling.

Installing Cable - Some Guidelines

When running cable, it is best to follow a few simple rules:

• Always use more cable than you need. Leave plenty of slack.
• Test every part of a network as you install it. Even if it is brand new, it may have problems that will be difficult to isolate later.
• Stay at least 3 feet away from fluorescent light boxes and other sources of electrical interference.
• If it is necessary to run cable across the floor, cover the cable with cable protectors.
• Label both ends of each cable.
• Use cable ties (not tape) to keep cables in the same location together.