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33 Categorize WAN technology types and properties

Different technologies are used to construct a WAN. Each technology varies in terms of speed, costing and difficulty in implementing. The various technologies that are available are:

- Frame Relay: It is a technology that is a fast means of packet switching, operating at the data link layer of the OSI model. Using this technology Frame Relay allows data transmission between LAN and the end points of a WAN. It uses Permanent Virtual Circuits (PVC's). It sends the transmission over a known route and as a result the need to find the best route to the destination is done away with. The addressing information is resorted to determine where the packet is supposed to go. It is flexible and can be used on many of the WAN technologies. A frame relay is composed of many parts. The figure given below illustrates the components of a frame relay.

Figure 53: Components of a Frame Relay

The components of a frame relay as can be seen in the above figure are:

- Frame Relay Access Device or Frame Relay Assembler/Disassembler (FRAD): A device on the local area network, performing the task of modifying data packets. The header and the trailer information is also placed on the packets that are going out. The process of adding all this information to the data packets is known as encapsulation and stripping of the information at the receiving end is known as decapsulation.

- Frame Relay Link: This component refers to the media that is used for connecting to the local network to the Frame Relay switch. This can be a T1/T3 link, ISDN or fiber.

- Frame Relay Switch: The frames are routed using the Frame Relay Switch. Before a frame gets to reach its destination, it may have to pass through many switches. It can be a part of a public or a private network.

- Backbone Media: The Frame Relay switches are connected using the backbone. T1/T3 links or even fiber is used for the backbone link. It follows a mesh topology. Mesh topology allows redundant links to interconnect to the Frame Relay Switches.

- Virtual Circuit: The starting point of a virtual circuit is the local network. It is connected to the FRAD on the receiving end. PVC is often a virtual link.

- Data Terminal Equipment (DTE): This is the terminating equipment located at the company's network. It includes hardware as end user systems, servers, routers, bridges, and switches.

- Data Circuit-Terminating Equipment (DCE): The term DCE refers to the equipment that is owned by a carrier. It is responsible for the switching services for the network. This is actually responsible for transmitting the data.

- Data Link Connection Identifier (DLCI): This is the number that is used for identifying the logical circuit between the router and the Frame Relay switch.

- T-Carrier Lines: These are high speed lines that are available on lease from various telephone companies. These are used for transmitting both data as well as voice. The technology is referred to as T lines in the U.S.A, J lines in Japan and E lines in Europe. Mostly these are used for forming point to point networks. There are four types of carrier lines that are available:

- T-1: This line offers a transmission speed of 1.544Mbps and is mostly used for connecting LANs.

- T-2: This line offers a transmission speed of 6.312Mbps.

- T-3: This line offers a transmission speed of up to 44.73 6Mbps.

- T-4: This line offers a transmission speed of up to 274.176Mbps.

The most commonly used lines are the T-1 and T-3 lines.

- T-1 Line: The technology is referred to as T-1It is a dedicated digital circuit, which is leased by a telephone company. Its unique characteristic is that it is always available and open line between two points of contact. It is based on the technology known as multiplexing or muxing. This done by the use of a special device called the multiplexer. The limitation of one call per wire is not applicable due to multiplexing. The multiplexer divides the transmission into multiple transmissions, which are forwarded simultaneously. The signals are reassembled when they reach their destination. The advantages of this connection are- point-to-point communication; a constant connection; the data rates is up to 1.544 Mbps and the costs can be fixed. The disadvantage of this is that it is a costly affair to set up a point-to-point communication between two ends.

- T3 Line: Till a certain period of time the speed of 1.544 Mbps offered by the T-1 carrier line was sufficient, but as the needs of businesses started to increase so did the demand for speed. The answer to the demand for speed was T-3. It provided a speed of 44.736 Mbps. These provide very high and are preferred by large corporate houses and Internet service providers. The speed is the advantage and the cost is its limitation.

- SONET/OCx Levels: In the year 1984, the monopoly of AT&T was broken and it was adjudged by the U.S. Department of Justice to divide the company into smaller and competitive companies. The local companies had a challenge to deal with. The challenge was to connect with the independent long distance carriers, which were increasing with every passing day. This proved to be a challenge as every company had an interface of its own. Bell Communications Research delivered to this challenge and delivered SONET. It is a fiber-optic WAN technology device. It transmits voice, data, and even video. The speed varies between 51.84Mbps to 40 Gbps. Largely, standardization of access methods was brought about by SONET. With SONET Bell created a system that was used for defining data rates in terms of Optical Carrier (OCx) levels.

- Asynchronous Transfer Mode (ATM): The ATM technology was introduced in the early 1990's and was seen as a complete solution in itself for both LAN as well as WAN environments. Its use ranged from a desk top format to a remote system. The ATM technology did not deliver to the expected levels for the reasons of implementation costs as well as lack of standardization. The growth of ATM was further stunted by the popularity of Gigabit Ethernet.

It uses the packet switching technology and can make transmissions at a speed ranging from 1.544 Mbps at the lowest and 622 Mbps at the highest. It can perform its functions with both fixed length packets and variable length packets, but its performance is best derived by the use of fixed length. Fixed length packets also make into more flexible and adaptable to other technologies. Virtual connections are used by ATM networks for forming connections between end points. PVC's or SVC's can be used for setting up ATM's.

- Integrated Services Digital Network (ISDN): It is a dial up technology and can be used for transmitting voice and data at the same time and over the same physical connection. In this technology, access is available for users to digital communication. This communication is available via both packet switching and circuit switching connections.

It allows transmission of voice and data over telephone wires. The advantage of ISDN is that it much faster than a dial-up modems connection. Another advantage is that the connection is formed very quickly. The limitation is that a dedicated phone line is required even though it can be paid for through a monthly payment. These monthly payments exceed the costs involved in dial up connections.

For setting up an ISDN connection, the number is dialed just as in the case of a telephone call or a dial up modem connection. To bring the conversation to end, one end has to disconnect again just as in the case of a telephone connection.

In the case of ISDN, the same media is used as a PSTN or a regular wire. The method in which the media is used is much different. PSTN was designed for data transfer, a dial up modem is used for converting PSTN analog signals to digital signals. The problem that is faced is that no communication is converted to digital. The communication traveling from the telephone exchange to the modem is in an analog form and from the modem to the computer travels in a digital form. However, in the ISDN form, communication takes place in the digital form right from the ISDN exchange. Data is transferred at a much faster rate as the entire transfer is in the digital form. The figure illustrated below illustrates a comparison between PSTN Links and ISDN Links.

Figure 54: Comparison of PSTN and ISDN Communication

There are two defined interface standards- Basic Rate Interface (BRI) and Primary Rate Interface (PRI)

- Basic Rate Interface (BRI): A communication line that utilizes three separate channels is termed as BRI. The three channels are - 2 B channels of 64 Kbps, which carry digital information and 1 D channel of 16 Kbps for out of band signaling. To use this, the connection point must be within 5,486 meters of the providers BRI service center.

- Primary Rate Interface (PRI): This form of ISDN is carried over a T1 line. It consists of 23 B channels and is able to handle transmission rates of up to 1.536Mbps. Every channel provides 64Kbps for data/voice, and one 64Kbps D channel.

- Internet Access Technologies: The internet is an essential part of businesses of today. Applications like email, videoconferencing, and VPN connections operate over the internet. Many internet access technologies are available. The decision on which type of internet connection is required has to be made after due considerations. Some of the important internet access technologies are:

- POTS Internet Access: Even though telephone lines and modems are slow means of connecting to the internet, but these are still used even in today's times. POTS as a means of internet access are used in areas where high-speed technologies are not available. Another reason for preferring POTS is limited usage of internet and only for purposes like emailing etc. A modem and a dial up access account through the internet service provider are required for forming a connection using the phone system.

Modem: A modem is a device that is used for converting the digital signals generated by a computer to an analog form allowing for data to travel across phone lines. Modems can be in built in a computer system or be attached as an external device.

The dial up: An ISP account is to be procured from the internet service provider. A number of plans are available depending on the usage of the facilities provided. The maximum that is provided by ISP's is 56Kbps. The service provider should be chosen with due care. It is not only the costs charged by the ISP and the number of free hours that should be considered while choosing an ISP, the number of lines that an ISP has should also be taken into consideration. Shortage of lines means busy signals and an inability to access the net.

- xDSL: Another Internet Access method is DSL. A standard phone line is used in this method to provide access at a high speed. It is not widely prevalent in today's times and is expected to catch up as the use of broadband spreads. In this technology, data is transferred over a standard phone connection. It is preferred in homes and small to mid size business as at very reasonable costs it provides excellent services in terms of speed. It enables the use of a different frequency meaning that the phone line can be used while the internet is being used. It has many variations to it and the group taken cumulatively is known as xDSL. The reason behind having these variations is that the needs of the users differ and keeping these specific needs in mind, variations of the same technology have been designed. The variations are:

- Asymmetric DSL (ADSL): ADSL is the most popular the DSL varieties. The word asymmetric in the name represents the different bands on the line. One of the bands that is used for POTS deals with the analog traffic, a second band provides for upload access, while the third band takes care of downloads. With this form of technology, downloads take place much faster than uploads. The characteristics of ADSL are:

Upload Speed	1Mbps
Download Speed	8Mbps
Type	Asymmetric

- Symmetric DSL (SDSL): Using this variation the user can enjoy the same speed for uploads and as well as downloads. It is most suited for applications such as web hosting, intranets, and ecommerce. A phone line cannot be shared while using this technology. The characteristics of SDSL are:

Upload Speed	1.5 Mbps
Download Speed	1.5 Mbps

- ISDN DSL (IDSL): This is a symmetric type of DSL. Environments where SDSL and ADSL are not available unavailable this variation is used. It does not support analog phones. The characteristics of IDSL are:

Upload Speed	144 Kbps
Download Speed	144 Kbps
Type	Symmetric

- Rate Adaptive DSL (RADSL): This is a variation on ADSL. The advantage of opting for this technology is that it can alter its transmission speeds based on the quality of the signal. It is asymmetric in nature and can support line sharing. The characteristics of RADSL are:

Upload Speed	1Mbps
Download Speed	7Mbps
Type	Asymmetric '

- Very High Bit Rate DSL (VHDSL): This variation is an asymmetric version of DSL. A phone line can be shared using this technology. High bandwidth applications such as VoIP and HDTV are easily accommodated on this. Data rates up to 10Mbps and above are easily achieved and as a result, it is a fast form of DSL. The characteristics of VHDSL are:

Upload Speed	1.6 Mbps
Download Speed	10 Mbps+
Type	Asymmetric

- High Bit Rate DSL (HDSL): This is a symmetric version of DSL. It allows for identical transmission rates in both directions, that is, while receiving and sending data. Line sharing is not accommodated in this technology. The characteristics of HDSL are:

Upload Speed	768 Kbps
Download Speed	768Kbps
Type	Symmetric

DSL can be incorporated in two ways:

- A Shared Link: A shared link is one which supports both voice and internet services. This is made possible by the use of two frequencies: high (for voice) and low (for internet)

- A Dedicated Link: Though this technology has its advantages it is not a cost effective option. It is not used while making regular voice transmissions.

Adaptive Rate is a term, which is often used while discussing DSL technology. This connotes that the speed of the connection is fluctuating. Several factors like the distance existing between the provider of DSL and the Internet, physical condition of the line etc cause fluctuations.

- Cable Internet Access: Areas that have access to digital cable television can enjoy access to the internet also using the cable connection. The advantages of using this technology are- it is reliable; it is pocket friendly. When this technology started the companies providing the service allowed unlimited data transfer but as the usage is increasing, ceilings are being put on the amount of data that can be transferred. Connectivity is established using a cable modem. The modem is turn uses a coaxial connection for connecting to the provider's outlet and a UTP for attaching directly to the system. The modems are provided by the cable companies on a nominal fee. The modems are able to supply a 10Mbps Ethernet connection for the home LAN though actually it may be much less. The speed factor makes it less popular for use in businesses and offices. The biggest disadvantage of using this technology is that the bandwidth is shared with others in the area. As a result, the link may weaken during rush hours.

- Satellite Internet Access: Internet facility is available through the satellite also. It is normally available where DSL, Internet through cable or broadband is not available. It is a connection which is always available and the claimed speed is 512Kbps for uploads and 2048Kbps for downloads. The drawback that is not allowing this technology to be popular is the cost factor. The advantage of the same is portability, which means that where ever one goes one can access the internet even if there is no phone line or cable. The same is offered by many companies under different packages designed on the basis of price, speed and service. Internet Satellite Services can be:

- One Way: A satellite card and a satellite dish are required to form a one way connection. The satellite dish is installed at the user end. Outgoing requests are sent on one link using the phone line and inbound traffic returns on the satellite link.

- Two Way: A two way connection provides for incoming as well as outgoing traffic. This also requires a satellite card and a dish to form the connection. Atmospheric conditions play a determining role in transmissions taking place using this technology

- Wireless Wide Area Networking (WWAN): A wireless LAN (WLAN) is a network allowing the users to form connection with the local network and move around that network without wires. Typically, these are effective over a certain area. For accessing local area networks from a remote location and to use the internet the WWAN is used. In comparison to a WLAN, it uses a much larger area. These permit remote access to the

which is mobile. From here, the data is transmitted to the appropriate network.

- WISP: The full form of WISP is Wireless Internet Service Provider. Public wireless internet access is provided, this is known as hotspot. Internet services are provided for mobile phones, handheld devices, cell phones in airports, conference rooms etc. By using one or many of the wireless AP's available near the hotspot a hotspot is created. Special application software may need to be installed for the purposes of billing and security.