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About Earthlink IT Communication Ltd

Earthlink IT Communication Ltd is a trusted name for Internet Service, e-commerce Service, IT Infrastructure Development and Industry Standard Software. From the beginning Earthlink IT Communication Ltd envisaged ‘Total Excellence” as its principle for guiding light, around which revolves its entire spectrum of activities. With the unique vision, Earthlink IT Communication Ltd is the forerunner in the value centric service Market place and an architect of high value end-to-end ICT solutions and software.

Earthlink IT Communication Ltd was incorporated as a private limited company on 12th February 1997, under the Companies Act, 1994, and registered with the Registrar of Joint Stock Companies. Subsequently, the Company has converted into a public limited company in 2001 with a view to float its share to the public.

Earthlink IT Communication Ltd is providing internet service for all customers in home and office science its beginning. Beside this we are serving our corporate clients with our experienced expertise from building up their network to modify, upgrade and secure their network.

Earthlink IT Communication Ltd.

85/A, Chatteshwary road

Chawkbazar, Chittagong, Bangladesh.

Tel: 88-031-2852185

88-031-2852186

Fax: +88-031-2850470

Email: info@earthlinkitc.net

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ISP (Internet service provider)

An ISP is an organization that provides internet access for individuals, small business, and mid size organization that do not want to create an internet site and become involved in providing internet services for their employees. An ISP can provide these services. An ISP can be granted. The customers are connected via a dial-up modem, DSL, or cable modem to the ISP. However, each customer still needs an IP address to continue to internet connection.

ISP lease a fixed amount of bandwidth or they create a line, which is available to the clients. They divided the line as the basis of the needed. The corporate or small business or home user are the main clients of the ISPs. The main factor of the ISPs is the ensure the maximize the cost and give the clients the best service.

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Definition of network

The Elements of a Network

The diagram shows elements of a typical network, including devices, media, and services, tied together by rules, that work together to send messages. We use the word messages as a term that encompasses web pages, e-mail, instant messages, telephone calls, and other forms of communication enabled by the Internet. In this course, we will learn about a variety of messages, devices, media, and services that allow the communication of those messages. We will also learn about the rules, or protocols, that tie these network elements together.

Networking is a very graphically oriented subject, and icons are commonly used to represent networking devices. On the left side of the diagram are shown some common devices which often originate messages that comprise our communication. These include various types of computers (a PC and laptop icon are shown), servers, and IP phones. On local area networks these devices are typically connected by LAN media (wired or wireless).

The right side of the figure shows some of the most common intermediate devices, used to direct and manage messages across the network, as well as other common networking symbols. Generic symbols are shown for:

Switch - the most common device for interconnecting local area networks

Firewall -provides security to networks

Router - helps direct messages as they travel across a network

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Wireless Router - a specific type of router often found in home networks

Cloud-used to summarize a group of networking devices, the details of which may be unimportant to the discussion at hand

Serial Link - one form of WAN interconnection, represented by the lightning bolt-shaped line

Protocols are the rules that the networked devices use to communicate with each other. The industry standard in networking today is a set of protocols called TCP/IP (Transmission Control Protocol/Internet Protocol). TCP/IP is used in home and business networks, as well as being the primary protocol of the Internet. It is TCP/IP protocols that specify the formatting, addressing and routing mechanisms that ensure our messages are delivered to the correct recipient.

World wide web HTTP

E-mail SMTP (simple mail transfer protocol)

POP (post office protocol)

Internet message IMPP

OSCAR

IP telephony SIP

The Messages

In the first step of its journey from the computer to its destination, our instant message gets converted into a format that can be transmitted on the network. All types of messages must be converted to bits, binary coded digital signals, before being sent to their destinations. This is true no matter what the original message format was: text, video, voice, or computer data. Once our instant message is converted to bits, it is ready to be sent onto the network for delivery.

The Devices

To begin to understand the robustness and complexity of the interconnected networks that make up the Internet, it is necessary to start with the basics. Take the example of sending the text message using an instant messaging program on a computer. When we think of using network services, we usually think of using a computer to access them. But, a computer is only one type of device that can send and receive messages over a network. Many other types of devices can

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also be connected to the network to participate in network services. Among these devices are telephones, cameras, music systems, printers and game consoles.

The Medium

To send our instant message to its destination, the computer must be connected to a wired or wireless local network. Local networks can be installed in homes or businesses, where they enable computers and other devices to share information with each other and to use a common connection to the Internet.

Wireless networks allow the use of networked devices anywhere in an office or home, even outdoors. Outside the office or home, wireless networking is available in public hotspots, such as coffee shops, businesses, hotel rooms, and airports.

The Services

Network services are computer programs that support the human network. Distributed on devices throughout the network, these services facilitate online communication tools such as e-mail, bulletin/discussion boards, chat rooms, and instant messaging. In the case of instant messaging, for example, an instant messaging service, provided by devices in the cloud, must be accessible to both the sender and recipient.

The Rules

Important aspects of networks that are neither devices nor media are rules, or protocols. These rules are the standards and protocols that specify how the messages are sent, how they are directed through the network, and how they are interpreted at the destination devices. For example, in the case of Jabber instant messaging, the XMPP, TCP, and IP protocols are all important sets of rules that enable our communication to occur.

The Network Architecture

Networks must support a wide range of applications and services, as well as operate over many different types of physical infrastructures. The term network architecture, in this context, refers to both the technologies that support the infrastructure and the programmed services and protocols that move the messages across that infrastructure. As the Internet, and networks in general, evolve, we are discovering that there are four basic characteristics that the underlying architectures need to address in order to meet user expectations: fault tolerance, scalability, quality of service, and security.

Fault Tolerance

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The expectation that the Internet is always available to the millions of users who rely on it requires a network architecture that is designed and built to be fault tolerant. A fault tolerant network is one that limits the impact of a hardware or software failure and can recover quickly when such a failure occurs. These networks depend on redundant links, or paths, between the source and destination of a message. If one link or path fails, processes ensure that messages can be instantly routed over a different link transparent to the users on either end. Both the physical infrastructures and the logical processes that direct the messages through the network are designed to accommodate this redundancy. This is a basic premise of the architecture of current networks.

Scalability

A scalable network can expand quickly to support new users and applications without impacting the performance of the service being delivered to existing users. Thousands of new users and service providers connect to the Internet each week. The ability of the network to support these new interconnections depends on a hierarchical layered design for the underlying physical infrastructure and logical architecture.

Quality of Service (QoS)

The Internet is currently providing an acceptable level of fault tolerance and scalability for its users. But new applications available to users over internetworks create higher expectations for the quality of the delivered services. Voice and live video transmissions require a level of consistent quality and uninterrupted delivery that was not necessary for traditional computer applications. Quality of these services is measured against the quality of experiencing the same audio or video presentation in person.

Security

The security and privacy expectations that result from the use of internetworks to exchange confidential and business critical information exceed what the current architecture can deliver. Rapid expansion in communication areas that were not served by traditional data networks is increasing the need to embed security into the network architecture. As a result, much effort is being devoted to this area of research and development. In the meantime, many tools and procedures are being implemented to combat inherent security flaws in the network architecture.

OSI and TCP/IP Model

The Open Systems Interconnection (OSI) reference model is a layered, abstract representation created as a guideline for network protocol design. The OSI model divides the networking process into seven logical layers, each of which has unique functionality and to which are assigned specific services and protocols.

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In this model, information is passed from one layer to the next, starting at the Application layer on the transmitting host, proceeding down the hierarchy to the Physical layer, then passing over the communications channel to the destination host, where the information proceeds back up the hierarchy, ending at the Application layer. The figure depicts the steps in this process.

Although the TCP/IP protocol suite was developed prior to the definition of the OSI model, the functionality of the TCP/IP application layer protocols fit roughly into the framework of the top three layers of the OSI model: Application, Presentation and Session layers.

Most TCP/IP application layer protocols were developed before the emergence of personal computers, graphical user interfaces and multimedia objects. As a result, these protocols implement very little of the functionality that is specified in the OSI model Presentation and Session layers.

Application layer

The application layer enables the user, whether human or software, to access the network. It provides user interface and support for services such as electronics mail, remote file access and transfer, access to the World Wide Web, and so on.

Presentation layer

External Data Representation (XDR) sits at the presentation level. It converts local representation of data to its canonical form and vice versa. The canonical uses a standard byte ordering and structure packing convention, independent of the host.

Session layer

The session protocol defines the format of the data sent over the connections. The NFS uses the Remote Procedure Call (RPC) for its session protocol. RPC may be built on either TCP or UDP. Login sessions uses TCP whereas NFS and broadcast use UDP.

Network layer

NFS uses Internetwork Protocol (IP) as its network layer interface. IP is responsible for routing, directing datagrams from one network to another. The network layer may have to break large datagrams, larger than MTU, into smaller packets and host receiving the packet will have to reassemble the fragmented datagram. The Internetwork Protocol identifies each host with a 32-bit IP address. IP addresses are written as four dot-separated decimal numbers between 0 and 255, e.g., 129.79.16.40.

Transport layer

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Transport layer subdivides user-buffer into network-buffer sized datagrams and enforces desired transmission control. Two transport protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), sits at the transport layer. Reliability and speed are the primary difference between these two protocols. TCP establishes connections between two hosts on the network through 'sockets' which are determined by the IP address and port number. TCP keeps track of the packet delivery order and the packets that must be resent. Maintaining this information for each connection makes TCP a stateful protocol. UDP on the other hand provides a low overhead transmission service, but with less error checking. NFS is built on top of UDP because of its speed and statelessness. Statelessness simplifies the crash recovery.

Data link layer

Data Link layer defines the format of data on the network. A network data frame, aka packet, includes checksum, source and destination address, and data. The largest packet that can be sent through a data link layer defines the Maximum Transmission Unit (MTU). The data link layer handles the physical and logical connections to the packet's destination, using a network interface. A host connected to an Ethernet would have an Ethernet interface to handle connections to the outside world, and a loopback interface to send packets to itself.

Ethernet addresses a host using a unique, 48-bit address called its Ethernet address or Media Access Control (MAC) address. MAC addresses are usually represented as six colon-separated pairs of hex digits, e.g., 8:0:20:11:ac:85. This number is unique and is associated with a particular Ethernet device. Hosts with multiple network interfaces should use the same MAC address on each. The data link layer's protocol-specific header specifies the MAC address of the packet's source and destination. When a packet is sent to all hosts (broadcast), a special MAC address (ff:ff:ff:ff:ff:ff) is used.

Physical layer

Physical layer defines the cable or physical medium itself, e.g., thinnet, thicknet, unshielded twisted pairs (UTP). All media are functionally equivalent. The main difference is in convenience and cost of installation and maintenance. Converters from one media to another operate at this level.

TCP/IP Network Model

Although the OSI model is widely used and often cited as the standard, TCP/IP protocol has been used by most Unix workstation vendors. TCP/IP is designed around a simple four-layer scheme. It does omit some features found under the OSI model. Also it combines the features of some adjacent OSI layers and splits other layers apart. The four network layers defined by TCP/IP model are as follows.

Layer 1 - Link

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This layer defines the network hardware and device drivers.

Layer 2 - Network

This layer is used for basic communication, addressing and routing. TCP/IP uses IP and ICMP protocols at the network layer.

Layer 3 - Transport

Handles communication among programs on a network. TCP and UDP falls within this layer.

Layer 4 - Application

End-user applications reside at this layer. Commonly used applications include NFS, DNS, arp, rlogin, talk, ftp, ntp and traceroute.

Firewall

We have a need for extra security, the Infinitum Managed Firewall service is just what you need. It will protect your corporate systems and servers from intruders and can also provide extra protection in the form of encryption for corporate data that traverses the Internet. The Infinitum Managed Firewall service will further enable our ISP to prevent improper usage of the Internet by our employees Firewall Configuration.

In a typical firewall configuration, a firewall is placed between the Internet and the organization's servers or office computers. The firewall will block all unauthorized inbound and/or outbound traffic.

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IPv4 Addresses

Anatomy of an IPv4 Addresses

Each device on a network must be uniquely defined. At the Network layer, the packets of the communication need to be identified with the source and destination addresses of the two end systems. With IPv4, this means that each packet has a 32-bit source address and a 32-bit destination address in the Layer 3 header.

These addresses are used in the data network as binary patterns. Inside the devices, digital logic is applied for their interpretation. For us in the human network, a string of 32 bits is difficult to interpret and even more difficult to remember. Therefore, we represent IPv4 addresses using dotted decimal format.

Types of Addresses in an IPv4 Network

Within the address range of each IPv4 network, we have three types of addresses:

Network address - The address by which we refer to the network

Broadcast address - A special address used to send data to all hosts in the network

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Host addresses - The addresses assigned to the end devices in the network

Network Address

The network address is a standard way to refer to a network. For example, we could refer to the network shown in the figure as "the 10.0.0.0 network." This is a much more convenient and descriptive way to refer to the network than using a term like "the first network." All hosts in the 10.0.0.0 network will have the same network bits.

Within the IPv4 address range of a network, the lowest address is reserved for the network address. This address has a 0 for each host bit in the host portion of the address.

Broadcast Address

The IPv4 broadcast address is a special address for each network that allows communication to all the hosts in that network. To send data to all hosts in a network, a host can send a single packet that is addressed to the broadcast address of the network.

The broadcast address uses the highest address in the network range. This is the address in which the bits in the host portion are all 1s. For the network 10.0.0.0 with 24 network bits, the broadcast address would be 10.0.0.255. This address is also referred to as the directed broadcast.

Host Addresses

As described previously, every end device requires a unique address to deliver a packet to that host. In IPv4 addresses, we assign the values between the network address and the broadcast address to the devices in that network.

Assigning Addresses within a Network

As you have already learned, hosts are associated with an IPv4 network by a common network portion of the address. Within a network, there are different types of hosts.

Some examples of different types of hosts are:

End devices for users

Servers and peripherals

Hosts that are accessible from the Internet

Intermediary devices

Each of these different device types should be allocated to a logical block of addresses within the address range of the network.

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An important part of planning an IPv4 addressing scheme is deciding when private addresses are to be used and where they are to be applied.

Different types of connection: There are different types of connection in this institution, these are a. fiber optic connection and b. drop wire connection

Fiber Optic Connection:

Pres cord

Fiber

Drop Wire Connection:

UTP

Main

Server

Proxy Server

MAC

Address

Media

Converter

Joint

Box

Joint

Box

Media

Converter

Client

PC

TDSL/ADSL

Switch /Connector

Connector Fuse Drop

Wire

Connector ADSL/TDSL

Modem

Client

PC

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Addresses for User Devices

In most data networks, the largest population of hosts includes the end devices such as PCs, IP phones, printers, and PDAs. Because this population represents the largest number of devices within a network, the largest number of addresses should be allocated to these hosts.

IP addresses can be assigned either statically or dynamically.

Static Assignment of Addresses

With a static assignment, the network administrator must manually configure the network information for a host, as shown in the figure. At a minimum, this includes entering the host IP address, subnet mask, and default gateway.

Static addresses have some advantages over dynamic addresses. For instance, they are useful for printers, servers, and other networking devices that need to be accessible to clients on the network. If hosts normally access a server at a particular IP address, it would cause problems if that address changed. Additionally, static assignment of addressing information can provide increased control of network resources. However, it can be time-consuming to enter the information on each host.

When using static IP addressing, it is necessary to maintain an accurate list of the IP address assigned to each device. These are permanent addresses and are not normally reused.

Dynamic Assignment of Addresses

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Because of the challenges associated with static address management, end user devices often have addresses dynamically assigned, using Dynamic Host Configuration Protocol (DHCP), as shown in the figure.

DHCP enables the automatic assignment of addressing information such as IP address, subnet mask, default gateway, and other configuration information. The configuration of the DHCP server requires that a block of addresses, called an address pool, be defined to be assigned to the DHCP clients on a network. Addresses assigned to this pool should be planned so that they exclude any addresses used for the other types of devices.

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DHCP is generally the preferred method of assigning IP addresses to hosts on large networks because it reduces the burden on network support staff and virtually eliminates entry errors.

Another benefit of DHCP is that an address is not permanently assigned to a host but is only "leased" for a period of time. If the host is powered down or taken off the network, the address is returned to the pool for reuse. This feature is especially helpful for mobile users that come and go on a network.

Types of Physical Media

The Physical layer is concerned with network media and signaling. This layer produces the representation and groupings of bits as voltages, radio frequencies, or light pulses. Various standards organizations have contributed to the definition of the physical, electrical, and mechanical properties of the media available for different data communications. These specifications guarantee that cables and connectors will function as anticipated with different Data Link layer implementations.

Unshielded twisted pair (UTP)

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Unshielded twisted pair cable

Most of the billions of conductor feet (millions of kilometres) of twisted pairs in the world are outdoors, owned by telephone companies, used for voice service, and only handled or even seen by telephone workers. The majority of data or Internet connections use those wires.

UTP cables are not shielded. This lack of shielding results in a high degree of flexibility as well as rugged durability. UTP cables are found in many ethernet networks.

Wiring schemes

As mentioned previously, this article is related to UTP cables, with RJ45 connectors. Before explaining the various sniffer types, it is fundamental to understand the standard wiring schemes. Figure 2 shows the pinouts of a RJ45 connector.

Figure 1: Front view of a RJ45 connector

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Figure 2: EIA/TIA 568A and 568B norms

Figure 3 describes the wiring schemes of straight through and crossover cables. Straight through cables can be used to connect a host or a router to a switch or hub. Crossover cables may be used to connect the following: host to host, hub to hub, switch to switch, switch to hub, router to router. The models of sniffing cables in this paper are based on straight through cables, but the crossover cables can be also used if the sniffer's transmit signal is modified in the same manner.

There are eight wires grouped into four coloured pairs. The pairs are twisted to reduce the effects of noise and interference. Each pair has a different twist ratio that can affect the signalling at higher speeds, so it becomes important to follow the colour codes. Note that pairs 1 (blue) and 4 (brown) are not used in 10Base-T or 100Base-TX Ethernet. All eight wires are used for 1000Base-T Ethernet. [1]

Figure 3: Straight through and crossover wiring schemes (10/100Base-T)

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Coding

Ethernet LANs use digital signals to share data among network devices. 10Base-T uses Manchester encoding to transmit the signal: transition occurs in the middle of each bit period. Two levels represent one bit. A low to high transition in the middle of the bit represents a `1'. A high to low transition in the middle of the bit represents a `0'. There is no DC component. It uses positive/negative voltages.

100-BaseTX uses 4B/5B encoding, where each 4-bit nibbles is being transferred encoded as 5-bit symbols. The signalling model is a three level multi-level technique called MLT-3.

Table 1: Ethernet encoding and signalling

10Base-T 100Base-TX

Data rate 10 Mbps 100 Mpbs

Encoding Manchester 4B/5B

Signalling 5v. differential MLT-3

Cable Cat. 3 UTP Cat. 5 UTP

UTP Cable Types

UTP cabling, terminated with RJ-45 connectors, is a common copper-based medium for interconnecting network devices, such as computers, with intermediate devices, such as routers and network switches.

Different situations may require UTP cables to be wired according to different wiring conventions. This means that the individual wires in the cable have to be connected in different orders to different sets of pins in the RJ-45 connectors. The following are main cable types that are obtained by using specific wiring conventions:

Ethernet Straight-through

Ethernet Crossover

Rollover

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The figure shows the typical application of these cables as well as a comparison of these three cable types.

Using a crossover or straight-through cable incorrectly between devices may not damage the devices, but connectivity and communication between the devices will not take place. This is a common error in the lab and checking that the device connections are correct should be the first troubleshooting action if connectivity is not achieved.

Fiber Media

Fiber-optic cabling uses either glass or plastic fibers to guide light impulses from source to destination. The bits are encoded on the fiber as light impulses. Optical fiber cabling is capable of very large raw data bandwidth rates. Most current transmission standards have yet to approach the potential bandwidth of this media.

Fiber Compared to Copper Cabling

Given that the fibers used in fiber-optic media are not electrical conductors, the media is immune to electromagnetic interference and will not conduct unwanted electrical currents due to grounding issues. Because optical fibers are thin and have relatively low signal loss, they can be

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operated at much greater lengths than copper media, without the need for signal regeneration. Some optical fiber Physical layer specifications allow lengths that can reach multiple kilometers.

Optical fiber media implementation issues include:

More expensive (usually) than copper media over the same distance (but for a higher capacity)

Different skills and equipment required to terminate and splice the cable infrastructure

More careful handling than copper media

At present, in most enterprise environments, optical fiber is primarily used as backbone cabling for high-traffic point-to-point connections between data distribution facilities and for the interconnection of buildings in multi-building campuses. Because optical fiber does not conducts electricity and has low signal loss, it is well suited for these uses.

Single-mode and Multimode Fiber

Fiber optic cables can be broadly classified into two types: single-mode and multimode.

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The most common type of single-mode fiber has a core diameter of 8 to 10 µm and is designed for use in the near infrared. The mode structure depends on the wavelength of the light used, so that this fiber actually supports a small number of additional modes at visible wavelengths. Multi-mode fiber, by comparison, is manufactured with core diameters as small as 50 micrometres and as large as hundreds of micrometres.

Fiber with large (greater than 10 µm) core diameter may be analyzed by geometric optics. Such fiber is called multimode fiber, from the electromagnetic analysis (see below). In a step-index multimode fiber, rays of light are guided along the fiber core by total internal reflection. Rays that meet the core-cladding boundary at a high angle (measured relative to a line normal to the boundary), greater than the critical angle for this boundary, are completely reflected. The critical angle (minimum angle for total internal reflection) is determined by the difference in index of refraction between the core and cladding materials. Rays that meet the boundary at a low angle are refracted from the core into the cladding, and do not convey light and hence information along the fiber. The critical angle determines the acceptance angle of the fiber, often reported as a numerical aperture. A high numerical aperture allows light to propagate down the fiber in rays both close to the axis and at various angles, allowing efficient coupling of light into the fiber. However, this high numerical aperture increases the amount of dispersion as rays at different angles have different path lengths and therefore take different times to traverse the fiber. A low numerical aperture may therefore be desirable.

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Media Connectors

Common Copper Media Connectors

Different Physical layer standards specify the use of different connectors. These standards specify the mechanical dimensions of the connectors and the acceptable electrical properties of each type for the different implementations in which they are employed.

Although some connectors may look the same, they may be wired differently according to the Physical layer specification for which they were designed. The ISO 8877 specified RJ-45 connector is used for a range of Physical layer specifications, one of which is Ethernet. Another specification, EIA-TIA 568, describes the wire color codes to pin assignments (pinouts) for Ethernet straight-through and crossover cables.

Although many types of copper cables can be purchased pre-made, in some situations, especially in LAN installations, the termination of copper media may be performed onsite. These terminations include crimped connections to terminate Cat5 media with RJ-45 plugs to make patch cables, and the use of punched down connections on 110 patch panels and RJ-45 jacks. The figure shows some of the Ethernet wiring components.

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Common Optical Fiber Connectors

Fiber-optic connectors come in a variety of types. The figure shows some of the most common:

Straight-Tip (ST) a very common bayonet style connector widely used with multimode fiber.

Subscriber Connector (SC) - A connector that uses a push-pull mechanism to ensure positive insertion. This connector type is widely used with single-mode fiber.

Lucent Connector (LC) - A small connector becoming popular for use with single-mode fiber and also supports multi-mode fiber.

Terminating and splicing fiber-optic cabling requires special training and equipment. Incorrect termination of fiber optic media will result in diminished signaling distances or complete transmission failure.

Three common types of fiber-optic termination and splicing errors are:

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Misalignment - the fiber-optic media are not precisely aligned to one another when joined.

End gap - the media do not completely touch at the splice or connection.

End finish - the media ends are not well polished or dirt is present at the termination.

It is recommended that an Optical Time Domain Reflectometer (OTDR) be used to test each fiber-optic cable segment. This device injects a test pulse of light into the cable and measures back scatter and reflection of light detected as a function of time. The OTDR will calculate the approximate distance at which these faults are detected along the length of the cable.

A field test can be performed by shining a bright flashlight into one end of the fiber while observing the other end of the fiber. If light is visible, then the fiber is capable of passing light. Although this does not ensure the performance of the fiber, it is a quick and inexpensive way to find a broken fiber.

Cabling

There are three types of cabling. These are:

1) Straight-through cable

2) Crossover cable

3) Rolled cable

Straight-through cable: It is used to connect

• Host to switch or hub

• Router to switch or hub

Here the four wires used that only pin 1, 2, 3 and 6. We just connect 1 to 1, 2 to 2, 3 to 3 and 6 to 6.

Crossover cable: It is used to connect

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• Switch to Switch

• Hub to Hub

• Hub to Switch

• Router direct to Host

The same four wires used in this cable as in the straight-through cable, but we just connect

Different pins together like 1 to 3, 2 to 6 on each side of the cable.

Rolled Cable:

Although rolled cable is not used to connect any Ethernet connections together, you can use a rolled

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Ethernet cable to connect a host to a router console serial communication port.

Intermediately network devices

Hubs

A hub is one type of networking device that is installed at the Access Layer of an Ethernet network. Hubs contain multiple ports that are used to connect hosts to the network. Hubs are simple devices that do not have the necessary electronics to decode the messages sent between hosts on the network. Hubs cannot determine which host should get any particular message. A hub simply accepts electronic signals from one port and regenerates (or repeats) the same message out all of the other ports.

All of the ports on the Ethernet hub connect to the same channel to send and receive messages. Because all hosts must share the bandwidth available on that channel, a hub is referred to as a shared-bandwidth device.

Only one message can be sent through an Ethernet hub at a time. It is possible for two or more hosts connected to a hub to attempt to send a message at the same time. If this happens, the electronic signals that make up the messages collide with each other at the hub.

Switches

An Ethernet switch is a device that is used at the Access Layer. Like a hub, a switch connects multiple hosts to the network. Unlike a hub, a switch can forward a message to a specific host. When a host sends a message to another host on the switch, the switch accepts and decodes the frames to read the physical (MAC) address portion of the message.

A table on the switch, called a MAC address table, contains a list of all of the active ports and the host MAC addresses that are attached to them. When a message is sent between hosts, the switch checks to see if the destination MAC address is in the table. If it is, the switch builds a temporary connection, called a circuit, between the source and destination ports. This new circuit provides a dedicated channel over which the two hosts can communicate.

Router

At the center of the network is the router. Stated simply, a router connects one network to another network. Therefore, the router is responsible for the delivery of packets across different networks. The destination of the IP packet might be a web server in another country or an e-mail server on the local area network. It is the responsibility of the routers to deliver those packets in a timely manner. The effectiveness of internetwork communications depends, to a large degree, on the ability of routers to forward packets in the most efficient way possible.

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Each network that a router connects to typically requires a separate interface. These interfaces are used to connect a combination of both Local Area Networks (LANs) and Wide Area Networks (WANs). LANs are commonly Ethernet networks that contain devices such as PCs, printers, and servers. WANs are used to connect networks over a large geographical area. For example, a WAN connection is commonly used to connect a LAN to the Internet Service Provider (ISP) network.

In addition to packet forwarding, a router provides other services as well. To meet the demands on today's networks, routers are also used to:

• Ensure 24x7 (24 hours a day, 7 days a week) availability. To help guarantee network reachability, routers use alternate paths in case the primary path fails.

• Provide integrated services of data, video, and voice over wired and wireless networks. Routers use Quality of service (QoS) prioritization of IP packets to ensure that real-time traffic, such as voice, video and critical data are not dropped or delayed.

• Mitigate the impact of worms, viruses, and other attacks on the network by permitting or denying the forwarding of packets.

Web server

The term web server can mean one of two things:

1. A computer program that is responsible for accepting HTTP requests from clients, which are known as web browsers, and serving them HTTP responses along with optional data contents, which usually are web pages such as HTML documents and linked objects (images, etc.).

2. A computer that runs a computer program as described above.

Although web server programs differ in detail, they all share some basic common features.

1. HTTP: every web server program operates by accepting HTTP requests from the client, and providing an HTTP response to the client. The HTTP response usually consists of an HTML document, but can also be a raw file, an image, or some other type of document (defined by MIME-types); if some error is found in client request or while trying to serve the request, a web server has to send an error response which may include some custom HTML or text messages to better explain the problem to end users.

2. Logging: usually web servers have also the capability of logging some detailed information, about client requests and server responses, to log files; this allows the webmaster to collect statistics by running log analyzers on log files.

In practice many web servers implement the following features also:

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1. Authentication, optional authorization request (request of user name and password) before allowing access to some or all kind of resources.

2. Handling of static content (file content recorded in server's filesystem(s)) and dynamic content by supporting one or more related interfaces (SSI, CGI, SCGI, FastCGI, JSP, PHP, ASP, ASP .NET, Server API such as NSAPI, ISAPI, etc.).

3. HTTPS support (by SSL or TLS) to allow secure (encrypted) connections to the server on the standard port 443 instead of usual port 80.

4. Content compression (i.e. by gzip encoding) to reduce the size of the responses (to lower bandwidth usage, etc.).

5. Virtual hosting to serve many web sites using one IP address. 6. Large file support to be able to serve files whose size is greater than 2 GB on 32 bit OS. 7. Bandwidth throttling to limit the speed of responses in order to not saturate the network

and to be able to serve more clients.

Proxy server

In computers, a proxy server is a server which services of requests of its clients by forwarding requests to other servers. A client connect to the proxy server, requesting some service, such as a file, connection, web page, or other resource, available from a different a server. The proxy server provides the resource by connecting to the specified server and requesting the service on behalf of the client. A proxy server may optionally after the clients requests or the server’s response, and sometime it may serve the request contacting the specified server. In this case, it would cache the first request to the remote server, so it could save the information for later, and make everything as fast as possible.

A proxy server that passes all request and replies unmodified is usually called a gateway or sometimes tunneling proxy.

An ADSL modem

An ADSL modem is very high speed data connection that uses the same wires as a regular telephone line. The ADSL divides up the available frequencies available on the phone line so that5 the high speed channel rates from 1.5 mbps to 10 mbps, while the slow speed channel rates range from 16 kbps to 1 mbps.

Core subsystem include:

• DSP/CPU-performs echo cancellation, error correction, digital coding, and rate adoption, and high speed device interface.

• Transmit channel front end-the function of this front end is to amplify and filter the analog signals created by the digital to analog converter and deliver them to the phone line at correct power levels.

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• Receive channel front end-the function of this front end is to preamp, filter, and digitize the analog signals from the phone line.

• Hybrid coupler-allows full duplex operation of the modem by separating Tx from Rx signals using the transformer and passive components.

• Network interface-allows the modem to high speed devices like Ethernet router, VoIp, and USB

• Power conversion-converts the input power from the ac adapter to run various functional blocks.

• ADSL modem : An asymmetric digital subscriber line transceiver, also known as an ADSL modem or DSL modem, is a device used to connect a single computer to a DSL phone line, in order to use an ADSL service. Some ADSL modems also manage the connection and sharing of the ADSL service with a group of machines: in this case, the unit is termed a DSL router or residential gateway. A DSL modem acts as the ADSL Terminal Unit or ATU-R, as the telephone companies call it. The acronym NTBBA (network termination broad band adapter, network termination broad band access) is also common in various countries. Because a DSL modem is a bridge, it has no interface and the IP address that is

configured to the computer it is attached to is assigned to the device.

Fig: ADSL Ethernet Modem

An Ethernet ADSL Modem is ideal for a single PC/MAC user looking for a cost-effective way to connect to ADSL Broadband. An Ethernet ADSL Modem is completely driver-free, so it works on any platform and under any operating system! Since there are no drivers to load, setup is simple – just plug it into the Ethernet port of your device/computer and configure the network settings through your web browser.

If you connect an Ethernet ADSL Modem to a Broadband router, you can share high-speed Internet access with every-one on your network either wired or wireless.

Key Specifications/Special Features:

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• Application: Gigabit Ethernet, 155 Mbps ATM.

• 4 pairs twisted pair cable.

• All cables meet or exceed the requirement proposed by TIA /EIA 568 B .2, ISO/IEC

11801 Categories 5e.

• 24AWG (0.51 mm) insulated copper conducts with PVC jacket...

• Twisted pairs are brightly colored and distinguished at a glance.

• Performance characterized to 350 MHz.

• Ultra smooth jacket makes this cable easy to pull in tight spaces.

• UL-Certified.

• 305 meters, packaged in full boxes

Concentrator:

• In telecommunication, the term concentrator has the following meanings:

• 1. in data transmission, a functional unit that permits a common path to handle more data

• Sources than there are channels currently available within the path. A concentrator usually provides communication capability between many low-speed, usually asynchronous channels and one or more high-speed, usually synchronous channels. Usually different speeds, codes, and protocols can be accommodated on the low-speed side. The low-speed channels usually operate in contention and require buffering.

• 2. A device that connects a number of circuits, which are not all used at once, to a smaller group of circuits for economy. ISP usually use concentrators to enable modem dialing, this kind of concentrator is sometimes called a modem concentrator or a remote access concentrator.

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