WHAT ARE PROTOCOLS?

 

A protocol is a standard format for transmitting data between two devices.

 

The protocol determines the following:

 

1. The type of error checking to be used
2. Data  compression method, if any
3. The method by which the sending device will indicate that the transmission is complete

 

A protocol describes the format that a message must take and the way in which computers must exchange a message within the context of a particular activity. The activities include sending messages across networks, exchanging e-mail, establishing remote connections, and transferring files. Two networking models support open systems interconnection. The first model, TCP/IP, is based on a suite of protocols in which each protocol solves a particular network communications problem. The second model, OSI, is based on international standards.

 

WHAT IS TCP/IP?

 

The name TCP/IP refers to a suite of data communication protocols. The name is misleading because TCP and IP are only two of the several protocols that compose the suite. The name TCP/IP comes from two of the more important protocols in the suit: the Transmission Control Protocol (TCP) and the Internet Protocol (IP).

 

TCP/IP originated out of the investigative research into networking protocols that the Department of Defense (DOD) initiated in 1969. In 1968, the DOD Advanced Research Projects Agency (ARPA) began researching the network technology that is now called packet switching. The original focus of this research was to facilitate communication among the DOD community. The network that was initially constructed as a result of this research, and then called ARPANET, gradually became known as the Internet. The TCP/IP protocols played an important role in the development of the Internet. In the early 1980s, the TCP/IP protocols were developed. In 1983, they became standard protocols for the ARPANET. Owing to the history of the TCP/IP protocol suite, it is often referred to as the Internet protocol suite.

 

HOW TCP/IP WORKS?

The design of TCP/IP hides the function of this layer from users and is concerned with getting data across a specific type of physical network such as Ethernet and Token Ring. The TCP/IP design reduces the need to rewrite higher levels of a TCP/IP stack when new physical network technologies, such as ATM and Frame Relay, are introduced.

 

The functions performed at the TCP/IP level include encapsulating the IP data grams into frames that are transmitted by the network. TCP/IP also maps the IP addresses to the physical addresses used by the network. One of the strengths of TCP/IP is its addressing scheme, which uniquely identifies every computer on the network. This IP address must be converted into an address that is appropriate for the physical network over which the data gram is transmitted. Data to be transmitted is received from the inter network layer. The network access layer is responsible for routing and must add its routing information to the data. The network access layer information is added in the form of a header, which is appended to the beginning of the data. In Windows NT, the protocols in this layer appear as NDIS drivers and related programs. The modules that are identified with network device names usually encapsulate and deliver the data to the network, while separate programs perform related functions such as address mapping.

 

The first model, TCP/IP, is based on a suit of protocols in which each protocol solves a particular network communications problem. This model contains

 

1. Application Layer
2. Transport Layer
3. Internet Layer
4. Network Interface Layer
5. Physical Layer

 

APPLICATION LAYER

 

            A user invokes an application program the accesses a service available across a TCP/IP internet. The application passes data to and receives data from the transport layer.

 

TRANSPORT LAYER

 

This layer provides services that permit an application program on one host to communicate with an application program on a remote host. The transport layer divides the stream of data into packets, adds a destination address, and passes the packets to the next layer. The transport uses two protocols, TCP and UDP,

 

INTERNET LAYER

 

This layer ensures that data is routed to the correct destination. The internet layer encapsulates the packet received form the transport layer into a datagram, adds a header, and determines the routing requirement. For incoming datagram, it determines which transport protocol should handle the packet.

 

NETWORK INTERFACELAYER

 

            This layer controls access to network transmission mechanisms. The network interface is responsible for accepting IP datagram and transmitting them over a specific network. The interface can be a device driver (connected to a LAN) or a subsystem with its own data link protocol.

 

PHYSICALLAYER

 

The hardware connection provides the physical interconnection between the host and the network

 

INTERNET PROTOCOL

 

            IP is a connectionless protocol, which means that IP does not exchange control information, called a handshake, to establish an end-to-end connection before transmitting data. In contrast, a connection – oriented protocol exchanges control information with the remote computer to verify that it is ready to receive data before sending it. When the handshake is successful, the computers are said to have established a connection. IP relies on protocols in other layers to establish the connection if connection-oriented services are required. IP also relies on protocols in another layer to provide error detection and error recovery. IP is sometimes called an unreliable protocol because it contains no error detection or recovery code.

 

IP ADDRESSING

 

            As with any other network-layer protocol, the IP addressing scheme is integral to the process of routing IP datagrams though an internetwork. Each IP address has specific components and follows a basic format. These IP addresses can be subdivided and used to create addresses for subnetworks, as discussed in more detail later in this chapter.

 

Each host on a TCP/IP network is assigned a unique 32-bit logical address that is divided into two main parts: the network number and the host number. The network number identifies a network and must be assigned by the Internet Network Information Center (InterNIC) if the network is to be part of the Internet. An Internet Service Provider (ISP) can obtain blocks of network addresses form the InterNIC and can itself assign address space as necessary. The host number identifies a host on a network and is assigned by the local network administrator.

 

IP ADDRESS FORMAT

 

The 32-bit IP address is grouped eight bits at a time, separated by dots, and represented in decimal format (known as dotted decimal notation). Each bit in the octet has a binary weight (128, 64, 32, 16, 8, 4, 2, 1). The minimum value for an octet is 0, and the maximum value for an octet is 255. Figure 30-3 illustrates the basic format of an IP address.

 

IP FORMAT                                                                  

 

                                                                             32 Bits

                                  

                                

 

 

8 Bits                   8 Bits                   8 Bits                8 Bits

 

Dotted

Decimal

Notation                172                             16                        122          204

 

 

IP ADDRESS CLASSES

 

            IP addressing supports five different address classes: A, B, C, D, and E. Only classes A, B and C are available for commercial use.

 

IP Address Class	Purpose	Address Range	Max. Hosts
A	Few large organizations	1.0.0.0 to 126.0.0.0	16,777,2142 (224-2)
B	Medium-size organizations	128.1.0.0 to 191.254.0.0	65, 543 (216 – 2)
C	Relatively small organizations	192.0.1.0 to 223.255.254.0	245 (28 – 2)
D	Multicast groups (RFC 1112)	224.0.0.0 to 239.255.255.255	N/A
E	Experimental	240.0.0.0 to 254.255.255.255	N/A

 

 

SERIAL LINE IP (SLIP) AND COMPRESSED SERIAL LINE IP (CSLIP)

 

            SLIP is a minimal protocol used to send to send datagrams for transmission across a serial line such as a telephone circuit. (SLIP is not an Internet standard.) SLIP is used only when both hosts know each other’s address and only when IP datagrams are being transmitted. When a host makes a connection, the SLIP server behaves like a router for TCP/IP traffic. Once connected, the SLIP host sends all network traffic over the serial interface.

 

Because network traffic is exchanged over the telephone link, performance is an issue for applications that handle numerous graphics, file sharing, or hypermedia. Compressed SLIP (CSLIP) improves SLIP performance by compressing the TCP/IP headers.

 

POINT-TO-POINT PROTOCOL (PPP)

 

PPP was developed as an Internet standard to address the weaknesses of SLIP. Often used for dialup remote LAN access, PPP allows the remote host to connect to the network and use IP network protocols. PPP is defined as a three-layered protocol as follows: 

 

1. 1.                              Data Link Layer, a modified version of the High-Level Data Link Control (HDLC) that guarantees reliable delivery over any type of serial line.

 

1. 2.                              Link Control, used to establish the connection, negotiate configuration parameters (including compression), check link quality, and close the connection.

 

1. 3.                              Network Control Protocols, individual protocols that provide configuration and control information for the Network Layer protocols.

 

THE PORTS

 

            Many TCP/IP programs can be initiated over the Internet. Most of these are client/server oriented. As each connection request is received, inetd starts a server program, which then communicates with the requesting client machine.

 

To facilitate this process, each application (FTP or Telnet, for example) is assigned a unique address. This address is called a port. The application in question is bound to that particular port and, when any connection request is made to that port, the corresponding application is launched (inetd is the program that launches it).

 

There are thousands of ports on the average Internet server. For purposes of convenience and efficiency, a standard framework has been developed for port assignment. (In other words, although a system administrator can bind services to the port of his or her choice, services are generally bound to recognized ports. These are commonly referred to as well-known ports.)

 

COMMON PORTS AND THEIR CORRESPONDING SERVICES OR APPLICATIONS.

Service or Application Port

 

File Transfer Protocol (FTP): 21

Telnet: 23

Simple Mail Transfer Protocol (SMTP): 25

Gopher: 70

Finger: 79

Hypertext Transfer Protocol (HTTP): 80

Network News Transfer Protocol (NNTP): 119

 

OPEN SYSTEMS INTERCONNECTION (OSI)

 

Introduction to the ISO – OSI Model

 

            The ISO (International Standards Organization) has created a layered model, called the OSI (Open Systems Interconnect) mode, to describe defined layers in a network operating system. The purpose of the layers is to provide clearly defined functions that can improve Internetwork connectivity between “computer” manufacturing companies. Each layer has a standard defined input and a standard defined output.

 

OSI Model Explained

 

This is a top-down explanation of the OSI Model. It starts with the user’s PC and it follows what happens to the user’s file as it passes though the different OSI Model layers. The top-down approach was selected specifically (vs. starting at the Physical Layer and working up to the Application Layer) for ease of understanding. It is used here to show how the user’s files are transformed (through the layers) into a bit stream for transmission on the network.

 

These are the 7 Layers of the OSI model:

 

• 7. Application Layer (Top Layer)
• 6. Presentation Layer
• 5. Session Layer
• 4. Transport Layer
• 3. Network Layer
• 2. Data Link Layer
• 1. Physical Layer (Bottom Layer)

 

Layer 7 – Application Layer

 

 

                        Basic PC Logical Flowchart

 

 

A basic PC logic flowchart is shown in Fig. The Keyboard & Application are shown as inputs to the CPU (requesting access to the hard disk). The Keyboard requests accesses through user inquiries (such as “DIR” commands) and the Application seeks access through “File Openings” and “Saves”. The CPU, through the Disk Operating System, sends and receives data from the local hard disk (“C:” in this example).

 

 

Simple Network Redirection

A PC setup as a network workstation has a software “Network Redirector” (the actual name depends on the network – we will use a generic term here) placed between the CPU and DOS (as shown in Fig 2.). The Network Redirector is a TSR (Terminate and Stay Resident) program: it presents the network hard disk as another local hard disk (“G:” in this example) to the CPU. All CPU requests are intercepted by the “Network Redirector”. The Network Redirector checks to see if either a local or a network drive is requested; the request is passed on to DOS. However, if a network drive is requested, the request is then passed on to the network operating system (NOS).

 

Electronic mail (E-Mail), client-sever databases, games played over the network, print and file servers, remote logons, and network management programs (or any “network aware” applications) are all aware of the network redirector. They have the ability to communicate directly with other “network applications” on the network. The “Network Aware Applications” and the “Network Redirector” make up Layer 7 (the Application layer of the OSI Model, as shown in Fig).

 

 

 

 

PC Workstation with Network Aware Software               

 

Layer 6 – Presentation Layer

 

            The Network Redirector sends CPU operating system native code to the network operating system: the coding and format of the data is not recognizable by the network operating system. The data consists of file transfers and network calls by network aware programs.

 

For example, when a dumb terminal is used as a workstation (in a mainframe or minicomputer network), the network data is translated into (and from) the format that the terminal can use. The Presentation layer presents data to and from the terminal using special control characters to control the screen display (LF-line feed, CR-carriage return, cursor movement, etc…). The presentation of data on the screen would depend on the type of terminal that’s used: VT100, VT52, VT420, etc.

 

 

Similarly, The Presentation layer strips the pertinent file from the workstation operating system’s file envelope. The control characters, screen formatting, and workstation operating system envelope are all stripped or added to the file (if the workstation is receiving or transmitting data to the network). This could also include translating ASCII file characters from a PC world to EBCDIC in an IBN Mainframe world.

 

The Presentation Layer also controls security at the file level: this provides both file locking and user security. The DOS Share program is often used for file locking. When a file is in use, it is locked form other users to prevent 2 copies of the same file from being generated. If 2 users both modified the same file, and User A saved it, then User B saved it, then User A’s changes would be erased! At this point, the data is contiguous and complete (i.e. one large data file). See Fig.

 

Layer 5 – Session Layer

 

            The Session layer manages the communications between the workstation and the network. The Session layer directs the information to the correct destination, and identifies the source to the destination. The Session layer identifies the type of information as data or control. The Session layer manages the initial start-up of a session, and the orderly closing of a session. The Session layer also manages Log on procedures and Password recognition

 

Session Layer 

 

Layer 4 – Transport Layer

 

In order for the data to be sent across the network, the file must be broken up into usable small data segments (typically 512 – 18k bytes). The Transport layer breaks up the file into segments for transport to the network, and combines incoming segments into a contiguous file. The Transport layer does this logically, not physically, and it is done in software as opposed to hardware.

 

 

The Transport layer provides error checking at the segment level (frame control sequence). This makes sure that the data grams are in the correct order: the Transport layer will correct out of order datagrams. The Transport layer guarantees an error-free host to host connection. It is not concerned with the path between machines.

 

Layer 3 – Network Layer

 

The Network layer is concerned with the path through the network. It is responsible for routing, switching, and controlling the flow of information between hosts. The Network layer converts the segments into smaller datagrams than the network can handle: network hardware source and destination addresses are also added. The Network layer does not guarantee that the datagram will reach its destination.

 

 

 

Network Layer

 

 

 

Layer 2 – Data Link Layer

 

The Data Link layer is a firmware layer of the network interface card. The Data Link layer puts the datagrams into packets (frames of bits: 1s & 0s) for transmission, and assembles received packets into datagrams. The Data Link layer works at the bit level, and adds start / stop flags and bit error checking (CRC or parity) to the packet frame. Error checking is at the bit level only: packets with errors are discarded and a request for re-transmission is sent out. The Data Link layer is primarily concerned with bit sequence.

 

 

 Data Link Layer

 

Layer 1 – Physical Layer

 

            The Physical layer concerns itself with the transmission of bits. It also manages the network card’s hardware interface to the network. The hardware interface involves the type of cabling (coax, twisted pair, etc.), frequency of operation (1 Mbps, 10Mbps, etc.), voltage levels, cable terminations, topography (star, bus, ting, etc.), etc. Examples of Physical layer protocols are as follows: 10Base5 – Thicknet, 10Base2 – Thinnet, 10BaseT – twisted pair, ArcNet, FDDI, etc. (see Fig. 9).

                                                             Physical Layer

 

 

LAYER-SPECIFIC COMMUNICATION

 

Each layer may add a Header and a Trailer to its Data (which consists of the next higher layer’s Header, Trailer and Data as it moves through the layers). The Headers contain information that specifically addresses layer-to-layer communication. For example, the Transport Header (TH) contains information that only the Transport layer sees. All other layers below the Transport layer pass the Transport Header as part of their Data.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PDU – Protocol Data Unit (a fancy name for Layer Frame

OSI Model Function Drawing

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1s & 0s

 

 

OSI Model Functional Layer Drawing

 

 

			Cables, Frequency, Voltages, Topography

 

 

 

 

ONE CLIENT                                            

10100110010101001

 

 

NETWORK

 

 

 

Other Workstations

File

Server

 

 

 

 

 

 

 

 

 

 

 

 

 

WHAT IS FDDI?

 

          Fiber Distributed Data Interface (FDDI) is a multi-vendor local area network standard developed by the American National Standards Institute (ANSI). It offers an industry-standard solution for organizations that need a high-speed LAN with high capacity and performance.

 

FDDI is based on dual counter-rotating token passing rings that are connected by optical fibers that transmit data at the rate of 100 Mbps. At this speed, the ring can support upto 500 nodes with a spacing of 2km between adjacent nodes. The basic operation of FDDI is similar to that of token ring. Since it is very difficult to tap into a fiber, a ring was the logical solution. FDDI groups stations, including workstations, bridges, and routers, into a ring. Each station has an input fiber form the previous station and an output to the

 

Due to the historical higher costs of its adapters and fiber optics, FDDI has been used primarily in backbone networks and for high-speed communications between host processors. The higher bandwidths offered by ATM, Fast Ethernet, and FDDI technologies complement each other, and are expected to coexist in virtually all organizations for many years to come.

 

FDDI was primarily designed to carry computer data. Beyond FDDI, FDDI-II is being developed to cater for a mix of voice, data and video traffic.

 

TECHNOLOGY OVERVIEW

 

          Token Ring and Ethernet are the two dominant LAN standards that are being widely utilized to connect users in a workgroup to each other and to common resources. The rapid increase in the speed and performance of personal computers and workstations has initiated a demand for a corresponding increase in LAN bandwidth. To meet this demand the ANSI (American National Standards Institute) committee had developed the FDDI standard that is being widely used.

 

Why use FDDI?

 

            With growing demands on network systems, the search for a “high – bandwidth LAN” has gained importance in many organizations. The increased demands are coming from two areas: First, the number of network users has increased in recent years, creating congestion in system traffic. Second, the increasing use of peer-to-peer network architecture means that more information gets passed between users, instead of passing though a central server.

 

The need for networks to handle more data, more quickly than eve, is becoming urgent. As a result, several contenders are vying for throne of high-speed networking. While ATM and Fast Ethernet have gained the most attention, FDDI is the popular choice. FDDI is a full-fledged technology compared to the emerging high-speed networking technologies, such as ATM and Fast Ethernet. In addition, FDDI has established standard interoperability, and product availability.

 

WHAT IS ASYNCHRONOUS TRANSFER MODE (ATM)?

 

Asynchronous Transfer Mode (ATM) is a technology that arises from international standards. The standards relate to the transmission of data, voice, and video simultaneously over a network at speeds that are greater than the transmission rate without the standards. ATM can transport electronic communications from sources that range from phone calls through movies to targets such as email and files. The targets are contained on a gopher or a World Wide Web server.

 

ATM can transport electronic communication at a rate of hundreds of megabits per second. This transfer rate is faster than the ethernet technology that is available on local area networks. The rate of transfer b using the ATM technology allows the integration of speech, motion, and data into multimedia presentations in offices and schools. Although, ATM is still in its infancy, the technology is being deployed across the country. ATM and future enhancements will lead to a global information system over the next decade and will become a central feature of the Information Superhighway that is being developed across the world. The use of the ATM technology in public schools is being investigated. OWL ink will be among the first projects that implements the capabilities of the ATM.

ETHERNET

 

Ethernet is one of the standard’s of local area network (LAN) technology that transmits information between computers at speed of 10 & 100 million bits per second (mbps). Currently the most widely used version of ethernet technology is the 10 Mbps twisted pair variety. It includes the original thick coaxial system, as well as thin coaxial, twisted pair, and fiber optic systems. The most recent ethernet standard defines the new 100 mbps as the fast ethernet system which operates over twisted pair and fiber optic media.

 

HISTORY

 

Ethernet was invented at the Xerox Palo alto research centre in the 1970s by Dr. Robert M Metcalfe. It was designed to support research on the office of the future, which included one of the world first personal workstations, the Xerox alto. The first ethernet system ran at approximately 3Mbps and was known as “experimental ethernet.” There are several Lan technologies in use today, but ethernet is by far the most popular. Industry estimates indicates that as of 1994, over 40 million ethernet nodes had been installed worldwide. The widespread popularity of the ethernet ensures that there is a large market for ethernet equipment, which also helps to keep the technology competitively priced.

 

Elements of the ethernet system

 

The ethernet system consists of there basic elements

 

1. The physical medium used to carry ethernet signals between computers

 

1. A set of medium access control rules embedded in each ethernet interface that allows multiple computers to fairly arbitrate access to the shared ethernet channel

 

1. An ethernet frame that consists of a standardized set of bits used to carry data over the system

 

OPERATION OF ETHERNET

 

Each ethernet equipped computer, also known as a station operates independently of all stations on the network: there is no central controller. All stations attached to an ethernet are connected to a shared signaling system, also called the medium. Ethernet signals are transmitted serially, one bit at a time, over the shared signal channel is idle, the station transmits its data in the form of an ethernet frame or packet.

 

NETWORKS

 

It makes sense, most often for financial reasons but also for others, to network groups of computers where they share a common workload. All the computers in an administrative office, all the computers to do with a certain ward or discipline. Networking computers means that the people using them can share files easily, send each other messages and share each other’s printers. This idea has developed into Local Area Networks (LANs.) Nowadays most organizations have a local area network. LAN’s can be as small as just one shared office or as large as a whole city.

 

Wide Area Networks

 

In some cases an organization is spread over a large area, and you do not have the easy concentration of computing to provide a LAN for. In this instance computers may be connected by a Wide Area Network (WAN). The difference between a WAN and a LAN is partly one of scale (although this is relative) but also relates to the technology. With a LAN you will typically get a fast network that can network PC file servers. With a WAN the network will often be much slower and will usually involve some mainframe computer as the server rather than a PC.

 

Combined Networks

 

LAN’s and WAN’s are not mutually exclusive. In fact they combine very well together. Many organizations now have both, where a WAN has been created by connecting up a series of LAN’s. A good example of this is the Academic Community. Every University has it’s own LAN providing email, printing, file sharing and other facilities. Then every University is connected to the joint Academic Network (JANET) which is a WAN running the length of the country.

 

LAN TOPOLOGIES

 

The application in use, such as multimedia, database updates, e-mail, or file and print sharing, generally determines the type of data transmission.

 

LAN transmissions fit into one of three categories:

 

• Unicast
  • Multicast
  • Broadcast

 

UNICAST

 

With unicast transmissions, a single packet is sent form the source to a destination on a network. The source-node addresses the packet by using the network address of the destination node. The packet is then forwarded to the destination network and the network passes the packet to its final destination. Figure 2-1 is an example of a unicast network.

 

 

 

 

 

 

 

 

 

 

Client                 Client            Client

 

Unicast Network

 

 

 

MULTICAST

 

                 With a multicast transmission, a single data packet is copied and forwarded to a specific subset of nodes on the network. The source node addresses the packet by using a multicast address. For example, the TCP/IP suite uses 224.0.0.0 to 239.255.255.255. The packet is then sent to the network, which makes copies of the packet and sends a copy to each segment with a node that is part of the multicast address. Figure 2-2 is an example of a multicast network.

 

 

Clint                    Client                      Client

 

Multicast Network

 

 

 

 

 

BROADCAST

 

                 Broadcasts are found in LAN environments. Broadcasts do not traverse a WAN unless the Layer 3 edge-routing device is configured with a helper address (or the like) to direct these broadcasts to a specified network address. This Layer 3 routing device acts as an interface between the local-area network (LAN) and the wide-area network (WAN).

 

Server

 

 

 

 

 

 

Client                 Client                  Client

 

Broadcast Network

 

 

 

 

Multimedia broadcast traffic is a much more bandwidth-intensive broadcast traffic type. Multimedia broadcasts, unlike data broadcasts, typically are several megabits in size; therefore, they can quickly consume network and bandwidth resources. Broadcast-based protocols are not preferred because every network device on the network must expend CPU cycles to process each data frame and packet to determine if that device is the intended recipient. Data broadcasts are necessary in a LAN environment, but they have minimal impact because the data broadcast frames that are traversing the network are typically small. Broadcast storms can cripple a network in no time because the broadcasting device uses whatever available bandwidth is on the network.

 

An example of a data broadcast on a LAN could be a host searching for server resources, such as Novell’s IPX GNS (Get Nearest Server) or Apple Talk’s Chooser application. Unlike data broadcasts, which are usually made up of small frames, multimedia broadcasts are typically several megabits in size. As a result, multimedia broadcasts can quickly consume all available band – width on a network, bringing a network and its attached devices to a crawl, if not render them inoperable.

 

Four LAN topologies exist:

 

• Star (Hub-and-Spoke)
• Ring
• Bus
• Tree

 

STAR (HUB-AND-SPOKE) TOPOLOGY

 

All stations are attached by cable to a central point, usually a wiring hub or other device operating in a similar function. Several different cable types can be used for this point-to-point link, such as shielded twisted-pair (STP), unshielded twisted-pair (UTP), and fiber-optic cabling. Wireless media can also be used for communications links.

 

The advantage of the star topology is that no cable segment is a single point of failure impacting the entire network. This allows for better management of the LAN. If one of the cables develops a problem, only that LAN-attached station is affected; all other stations remain operational. The disadvantage of a star (hub-and-spoke) topology is the central hub device. This central hub is a single point-of –failure in that if it fails, every attached station is out of service. These central hubs, or concentrators, have changed over the years. Today, it is common to deploy hubs with built-in redundancy. Such redundancy is designed to isolate a faulty or failed component, such as the backplane or power supply. Figure 2-4 is an example of a star (hub-and-spoke) topology.

 

 

This example demonstrates a star topology with a file server, printer, and two workstations. If a cable to one of the workstation fails, the rest of the devices are unaffected unless they need to access resources from the “disconnected” device.

 

Ring Topology

 

All stations in a ring topology are considered repeaters and are enclosed in a loop. Unlike the star (hub-and-spoke) topology has no end points. The repeater in this case is a function of the LAN-attached station’s network interface card (NIC). Because each NIC in a LAN-attached station is a repeater, each LAN station will repeat any signal that is on the network, regardless of whether it is destined for that particular station. If a LAN-attached station’s NIC fails to perform this repeater function, the entire network could come down. The NIC controller is capable of recognizing and handling the defective repeater and can pull itself off the ring, allowing the ring to stabilize and continue operating. Token Ring (IEEE 802.5) best represents a ring topology. Although the physical cabling is considered to be a star topology, Token Ring is a ring in logical topology, as demonstrated by the following figures. Although physical topology is a physical layer attribute, the media access method used at the data link layer determines the logical topology. Token Ring defines a logical ring and contention, as Ethernet defines a logical bus. Even when attached to a hub, when one Ethernet device transmits, everyone hears the transmission, just as though on a bus.

 

 

 

 

Fiber Data Distributed Interface (FDDI) is another example of a ring topology implementation Like Token Ring, FDDI rings are physically cabled in a star topology. FDDI stations can be configured either as a single attachment station (SAS) or as a dual attachment station (DAS). SASs are connected to one of the two FDDI rings, whereas DASs are connected to both rings via an A and B port on the FDDI stations and concentrator. Token Ring and FDDI LANs will be discussed in greater detail in Chapters 6, “Token Ring/IEEE 802.5,” and 7, “FDDI.”

 

Bus Topology

 

Sometimes referred to as linear-bus topology, Bus is a simple design that utilizes a single length of cable, also known as the medium, with directly attached LAN stations. All stations share this cable segment. Every station on this segment sees transmissions form every other stations on the cable segment; this is known as a broadcast medium. The LAN attachment stations are definite endpoints to the cable segment and are known as bus network termination points. This single cable segment lends itself to being a single point of failure. If the cable is broken, no LAN station will have connectivity or the ability to transmit and receive. Ethernet (IEEE 802.3) best represents this topology. Ethernet has the ability to utilize many different cable schemes.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

LAN Node                LAN Node

 

Bus Topology

TREE TOPOLOGY

 

The tree topology is a logical extension of the bus topology and could be described as multiple interconnected bus networks. The physical (cable) plant is known as a branching tree with all stations attached to it. The tree begins at the root, the pinnacle point, and expands to the network endpoints. This topology allows a network to expand dynamically with only one active data path between any two network endpoints.

 

A tree topology network is one that does not employ loops in its topology.  An example of a tree topology network is a bridged or switched network running the spanning tree algorithm, usually found with Ethernet (IEEE 802.3) networks. The spanning tree algorithm disables loops in what would otherwise be a looped topology. Spanning tree expands though the network and ensures that only one active path exists between any two LAN-attached stations.

 

 

 

DNS (Domain Name System)

 

DNS (Domain Name Server) is the system used in Internet to be able to assign and universally use unequivocal names to refer to systems connected to the net. Thus both human users and applications can used DNS names instead of the numeric addresses of the IP net. This represents great advantages, among others the fact that it is easier and less burdensome for us to use names and not numbers and it allows an organization to make the name of machines, services, email addresses etc independent from the specific number addresses which working systems may have at a given time in function of changing elements such as the topology of the net and the provider of access to Internet.

 

Technically DNS is an immense data base distributed hierarchically throughout Internet: there are innumerable servers which interact among themselves to find and provide for the customer applications which consult them the translation of a name to its associated PI address, to which the desired connection may be made. Each part of the data base has a replica in at least two servers and this ensures the proper redundancy factor. The reason which motivated the development and implementation of DNS in Internet was the great growth in the number of machines connected. Previously, the link between numbers and IP addresses was made via lists kept centrally is a single file (Host.txt) which had to be constantly updated with each new system connected u[ and to be present in all the computers connected to Internet.

 

Name Servers:

 

In large installations, there are a number of different collections of names that have to be managed. This includes users and their passwords, names and network addresses for computers, and accounts. It becomes very tedious to keep this data up to date on all of the computers. Thus the databases are kept on a small number of systems.

 

The objective of DNS is to allow the scaling down, from both the administrative and technical points of view, of the system of Internet names by means of a hierarchical distribution of delegated domains. The domains are administrative entities the object of which is to divide up the management load of central administrative entities the object of which is to divide up the management load of a central administrator and share it out among the sub administrators. The latter in their turn may repeat the process if the volume of the domain to be administered makes this advisable.

 

At the first hierarchical level are the “Top Level Domains” or TLD’s which are one per country (two letter domains which correspond to the ISO-3166 code for each territory) plus the special 3 letter domains: “edu”, “com”, “gov”, “mil”, “org”, “int” and “net”.

 

 

Generic Top Level Domains (g TLD)

 

                 One of the conveniences of computer technology is that it often masks complex interactions behind a simple human experience. The Internet is a perfect example. Everyday, millions of users move from Web site to Web site by simply typing domain names (e.g., www.networksolutions.com) into a computer’s Internet browser. Yet, unseen and often unknown to the user, each entry actually triggers a critical, time-sensitive process before the Web site can be accessed.

 

In order for Internet users to reach a Web site, their computer must find the address of the Web server that hosts the desired site. Computers locate one another across the Internet using numbers, not letters. For each Web site on the Internet there is a unique domain name and numeric address, known as an Internet Protocol   (IP) address (e.g.,205.139.94.60). This number, while quite convenient for the computer to use, is difficult for Internet users to remember; thus the need for domain names.

 

 

WHAT IS CLIENT/SERVER TECHNOLOGY

 

                     A client/server system consists of two different types of computers, client computers and server computers that are connected by a network. Users work at the client computers and pass requests to the server computers. Server computer controls the information, files, or programs that the client computers require. Both these types of computers have software that meet the specific role they play in the client/server environment. Although it is common to refer to a server computer as the server, in reality the server refers t o the software application that servers files, not the computer on which it runs. Similarly a client is a computer that has client software.

 

Client/server computing is important because it centralizes the control of data. Consider a business that takes orders for products over the phone. The company has a server computer, connected by a network to a variety of client computers. A database on the server computer contains information about all the products the company sells, including each item’s price and the number of units in stock. The server software gives client applications access to this database.

 

Some of the client computers are used by employees who take orders over the phone. The client software gives these employees access to the database on the server computer. When an order comes in, an employee uses their client software to se if the item the customer wants is in stock. If it is, the order taker hen uses the client software to place the order. The server software receives the order, accesses the database, subtracts the appropriate number of units from the total number of units in stock, and sends a message to a client computer in another part of the company asking for the order to be fulfilled.

 

 

ADVANTAGES OF CLINET/SERVER TECHNOLOGY

Consolidate data in one place-Users know the source of data, because data is always accessed by requesting it from the server, Easy administration-Administrators can create reports on the data that is available, and can track or  monitor the use of that data. In addition administrators can backup and restore data. Prudent use of resources – IN client/server computing, processor-intensive tasks is concentrated on a powerful server computer, freeing up the processors of slower client computers. Easy connectivity- The network on which a client/server system resides does not have to be a standard LAN (local area network). You can use a dial-up modem connection to access a server computer from a client computer on the other side of the planet.

 

WHAT DO I NEED TO GET ON THE INTERNET?

 

To get connected to the Internet you need a computer, a modem or other access device, an Internet service provider (ISP) and software.

 

Modem: Modems are devices which use phone lines to connect your computer to your Internet service provider’s (ISP) host computer (also called a server) which is connected to the Internet. The speed of a modem determines how fast information is transmitted to and from your computer and is measured in bits per second (bps)(it takes about 10 bits to transmit a single character such as the letter A). There are faster ways of connecting to the Internet using ISDN lines, cable modems and Digital Subscriber Lines (DSL). An ISDN line transfers information at a rate of between 57,600 bps and 128,000 bps. ISDN adapter instead of a modem, and some specialized equipment that the phone company must install. A T-1 line is very fast connection used by web servers or other computers than need to be connected to the Internet all the time. You can get internet access through some cable TV networks. Speeds can be as great as 36 Mbps. Cable access requires a special cable modem. Because it works with your TV cable, is doesn’t tie up a telephone line. Cable modems are always on, so there is no need to connect.

DSL (Digital Subscriber Line0 is another high-speed technology that is becoming increasingly popular, DSL lines are always connected to the Internet, so you don’t need to dial-up. Information can be transferred at rates up to 1.54 Mbps (around 30 times faster than a 56-kbps modem) over ordinary telephone lines using your existing line. Not all types of services are available in all parts of the country. You need to know types of connections your ISP supports.