Wide Area Networks
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A wide area network is a network spread over a larger area than a LAN. A wide area network will typically consist of two or more LAN’s linked together. Another main difference between the local area networks and the wide area networks is the fact that the wide area network uses switching equipment. WAN’s can and will usually be spread over different countries. For communication purposes a WAN will be ideal for a company that may be based in the United States but has an office in the U.K. If this is the case the organizations in question will need to communicate. A WAN is an ideal solution for a company that needs a constant flow of digital information between the 2 offices. If the WAN is set up for a company the company will use its own servers etc. This is usually the case as it keeps the network independent and also means that the companies own engineers can work on any problems which may arise.
WAN technologies function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer. A point-to-point link provides a single, pre-established WAN communications path from the customer premises through a carrier network, such as a telephone company, to a remote network. A point-to-point link is also known as a leased line because its established path is permanent and fixed for each remote network reached through the carrier facilities. The carrier company reserves point-to-point links for the private use of the customer. These links accommodate two types of transmissions: datagram transmissions, which are composed of individually addressed frames, and data-stream transmissions, which are composed of a stream of data for which address checking occurs only once.
Circuit switching is a WAN switching method in which a dedicated physical circuit is established, maintained, and terminated through a carrier network for each communication session. This is another difference from the LAN network which does not incorporate this switching technique. This is used extensively in telephone company networks; circuit switching operates much like a normal telephone call.
This is a simplified diagram of a WAN:
Circuit switching is a WAN switching method in which a dedicated physical circuit is established, maintained, and terminated through a carrier network for each communication session. Circuit switching accommodates two types of transmissions: datagram transmissions and data-stream transmissions. Used extensively in telephone company networks, circuit switching operates much like a normal telephone call. Integrated Services Digital Network (ISDN) is an example of a circuit-switched WAN technology.
The Packet Switched WAN appeared in the 1960’s, and defined the basis for all communication networks today. The principle in Packet Switched Data Network (PSDN) is that the data between the nodes is transferred in small packets. This principle enables the PSDN to allow one node to be connected to more than one other node through one physical connection. That way, a fully connected network, between several nodes, can be obtained by connecting each node to one physical link, as shown in the diagram below.
Another advantage for Packet Switching was the efficient use of resources by sharing the Network bandwidth among the users (instead of dividing).
The first communication Packet Switched Networks were based on the X.25 packet switching protocol. X.25 networks became the de facto standard for non permanent data communication and was adopted by most PTT’s.
X.25 networks enabled cheaper communication, since their tariff was based on the communication time and the amount of data transferred. X.25 networks used the PTT’s transmission networks more efficiently since the bandwidth was released at the end of the connection, or when no data was transmitted. Another advantage of X.25 was that it allowed easy implementation of international connections enabling organizations to be connected to data centers and services throughout the world. By the 1980’s, X.25 networks were the main international channel for commercial data communication.
Today, X.25 transfer rates are considered to be very low, and this service is expected to be replaced with new services by the end of the century.
The communication target today is the ATM (and B-ISDN services). However, until applications and technologies for ATM become more developed, two main mid-time services are popularly used in the world today.
The first service is Frame Relay, which is considered to be the next generation for X.25, and enables faster communication rate (up to T3/E3) and better communication protocol. Until all its standards will be completed, Frame Relay is mainly a point-to-point service and replaces the leased lines.
The second network service is ISDN, which is a fully digitized service, enabling communication for most types of data (voice, computer data and images) at all the network nodes (meaning in every house). This service is at its peak today and is been implemented mostly in Europe.
Those two communication networks are not fully developed yet, and will be spread in the world in the next years.
Current public and packet switched telephone services
STN (public switched telephone network) is the world’s collection of interconnected voice-oriented public telephone networks, both commercial and government-owned. It’s also referred to as the Plain Old Telephone Service (POTS). It’s the aggregation of circuit-switching telephone networks that has evolved from the days of Alexander Graham Bell. Today, it is almost entirely digital in technology except for the final link from the central (local) telephone office to the user.
In relation to the Internet, the PSTN actually furnishes much of the Internet’s long-distance infrastructure. Because Internet service providers ISPs pay the long-distance providers for access to their infrastructure and share the circuits among many users through packet-switching, Internet users avoid having to pay usage tolls to anyone other than their ISPs.
Diagram of a public switched telephone network
ISDN (Integrated Services Digital Network) is a set of CCITT/ITU standards for digital transmission over ordinary telephone copper wire as well as over other media. Home and business users who install an ISDN adapter (in place of a modem) can see highly-graphic Web pages arriving very quickly (up to 128 Kbps). ISDN requires adapters at both ends of the transmission so your access provider also needs an ISDN adapter. ISDN is generally available from your phone company in most urban areas in the United States and Europe.
There are two levels of service: the Basic Rate Interface (BRI), intended for the home and small enterprise, and the Primary Rate Interface (PRI), for larger users. Both rates include a number of B-channels and D-channels. Each B-channel carries data, voice, and other services. Each D-channel carries control and signalling information.
The Basic Rate Interface consists of two 64 Kbps B-channels and one 16 Kbps D- channel. Thus, a Basic Rate user can have up to 128 Kbps service. The Primary Rate consists of 23 B-channels and one 64 Kbps D-channel in the United States or 30 B-channels and 1 D-channel in Europe.
Integrated Services Digital Network in concept is the integration of both analogue or voice data together with digital data over the same network. Although the ISDN you can install is integrating these on a medium designed for analogue transmission, broadband ISDN (BISDN) will extend the integration of both services throughout the rest of the end-to-end path using fibre optic and radio media. Broadband ISDN will encompass frame relay service for high-speed data that can be sent in large bursts, the Fibre Distributed-Data Interface (FDDI), and the Synchronous Optical Network (SONET). BISDN will support transmission from 2 Mbps up to much higher, but as yet unspecified, rates.
Example of an Integrated Services Digital Network
X.25 Packet Switched networks allows remote devices to communicate with each other across high speed digital links without the expense of individual leased lines. Packet Switching is a technique whereby the network routes individual packets of HDLC data between different destinations based on addressing within each packet.
A popular standard for packet switching networks. The X.25 standard was approved by the CCITT (now the ITU) in 1976. It defines layers 1, 2, and 3 in the OSI Reference Model.
Frame relay is a telecommunication service designed for cost-efficient data transmission for intermittent traffic between local area networks (LANs) and between end-points in a wide area network (WAN). Frame relay puts data in a variable-size unit called a frame and leaves any necessary error correction (retransmission of data) up to the end-points, which speeds up overall data transmission. For most services, the network provides a permanent virtual circuit (PVC), which means that the customer sees a continuous, dedicated connection without having to pay for a full-time leased line, while the service provider figures out the route each frame travels to its destination and can charge based on usage. An enterprise can select a level of service quality – prioritizing some frames and making others less important. Frame relay is offered by a number of service providers, including AT&T. Frame relay is provided on fractional T-1 or full T-carrier system carriers. Frame relay complements and provides a mid-range service between ISDN, which offers bandwidth at 128 Kbps, and Asynchronous Transfer Mode (ATM), which operates in somewhat similar fashion to frame relay but at speeds from 155.520 Mbps or 622.080 Mbps.
Short for Switched Multimegabit Data Services, a high-speed switched data communications service offered by telephone companies that enable organizations to connect geographically separate local-area networks (LANs) into a single wide-area network (WAN). Prior to SMDS’s arrival in 1995, the only way to connect LANs was through a dedicated private line. This is still the way most WANs are connected, but SMDS is becoming an increasingly attractive alternative because it is more flexible and in many cases more economical.
High Speed Broadband & Mobile Data Communication Technologies
In general, broadband refers to telecommunication in which a wide band of frequencies is available to transmit information. Because a wide band of frequencies is available, information can be multiplexed and sent on many different frequencies or channels within the band concurrently, allowing more information to be transmitted in a given amount of time (much as more lanes on a highway allow more cars to travel on it at the same time). Related terms are wideband (a synonym), baseband (a one-channel band), and narrowband (sometimes meaning just wide enough to carry voice, or simply “not broadband,” and sometimes meaning specifically between 50 cps and 64 Kpbs).
Various definers of broadband have assigned a minimum data rate to the term. Here are a few:
*Newton’s Telecom Dictionary: “…greater than a voice grade line of 3 KHz…some say [it should be at least] 20 KHz.”
*Jupiter Communications: at least 256 Kbps.
*IBM Dictionary of Computing: A broadband channel is “6 MHz wide.”
It is generally agreed that Digital Subscriber Line (DSL) and cable TV are broadband services in the downstream direction.
ATM (asynchronous transfer mode) is a dedicated-connection switching technology that organizes digital data into 53-byte cell units and transmits them over a physical medium using digital signal technology. Individually, a cell is processed asynchronously relative to other related cells and is queued before being multiplexed over the transmission path.
Because ATM is designed to be easily implemented by hardware (rather than software), faster processing and switch speeds are possible. The prespecified bit rates are either 155.520 Mbps or 622.080 Mbps. Speeds on ATM networks can reach 10 Gbps. Along with Synchronous Optical Network (SONET) and several other technologies, ATM is a key component of broadband ISDN (BISDN).
DSL (Digital Subscriber Line) is a technology for bringing high-bandwidth information to homes and small businesses over ordinary copper telephone lines. xDSL refers to different variations of DSL, such as ADSL, HDSL, and RADSL. Assuming your home or small business is close enough to a telephone company central office that offers DSL service, you may be able to receive data at rates up to 6.1 megabits (millions of bits) per second (of a theoretical 8.448 megabits per second), enabling continuous transmission of motion video, audio, and even 3-D effects. More typically, individual connections will provide from 1.544 Mbps to 512 Kbps downstream and about 128 Kbps upstream. A DSL line can carry both data and voice signals and the data part of the line is continuously connected. DSL installations began in 1998 and will continue at a greatly increased pace through the next decade in a number of communities in the U.S. and elsewhere. Compaq, Intel, and Microsoft working with telephone companies have developed a standard and easier-to-install form of ADSL called G.lite that is accelerating deployment. DSL is expected to replace ISDN in many areas and to compete with the cable modem in bringing multimedia and 3-D to homes and small businesses.
General Packet Radio Services (GPRS) is a packet-based wireless communication service that promises data rates from 56 up to 114 Kbps and continuous connection to the Internet for mobile phone and computer users. The higher data rates will allow users to take part in video conferences and interact with multimedia Web sites and similar applications using mobile handheld devices as well as notebook computers. GPRS is based on Global System for Mobile (GSM) communication and will complement existing services such circuit-switched cellular phone connections and the Short Message Service (SMS).
In theory, GPRS packet-based service should cost users less than circuit-switched services since communication channels are being used on a shared-use, as-packets-are-needed basis rather than dedicated only to one user at a time. It should also be easier to make applications available to mobile users because the faster data rate means that middleware currently needed to adapt applications to the slower speed of wireless systems will no longer be needed. As GPRS becomes available, mobile users of a virtual private network (VPN) will be able to access the private network continuously rather than through a dial-up connection.
3G is a short term for third-generation wireless, and refers to near-future developments in personal and business wireless technology, especially mobile communications. This phase is expected to reach maturity between the years 2003 and 2005.
The third generation, as its name suggests, follows the first generation (1G) and second generation (2G) in wireless communications. The 1G period began in the late 1970s and lasted through the 1980s. These systems featured the first true mobile phone systems, known at first as “cellular mobile radio telephone.” These networks used analogue voice signalling, and were little more sophisticated than repeater networks used by amateur radio operators. The 2G phase began in the 1990s, and much of this technology is still in use. The 2G cell phone features digital voice encoding. Examples include CDMA, TDMA, and GSM. Since its inception, 2G technology has steadily improved, with increased bandwidth, packet routing, and the introduction of multimedia. The present state of mobile wireless communications is often called 2.5G.
Ultimately, 3G is expected to include capabilities and features such as:
*Enhanced multimedia (voice, data, video, and remote control)
*Usability on all popular modes (cellular telephone, e-mail, paging, fax, videoconferencing, and Web browsing)
*Broad bandwidth and high speed (upwards of 2 Mbps)
*Routing flexibility (repeater, satellite, LAN)
*Operation at approximately 2 GHz transmit and receive frequencies
*Roaming capability throughout Europe, Japan, and North America
H.32x is a family of “umbrella-standards” for video telephony over different
network types. In the standards are specified how bridges between the different H.32x-standards should work as well as minimal compliance requirements.
H.323 is an umbrella standard for video telephony over unreliable packet networks (IP, Ethernet, Token Ring, etc). Its minimal scope consists of an audio standard plus multiplex (H.225.0?) and H.245 signalling. H.323 use RTP for transport over IP networks.
VoIP and IMTC recommends it’s users to use G.723.1 as default audio codec and ITU-T will incorporate this recommendation into H.323.
G.723.1 is owned by AudioCodes, DSP Group, France Telecom, University of Sherbrooke, ATT, Lucent, VoiceCraft, and NTT and have to be licensed.
VoIP (voice over IP – that is, voice delivered using the Internet Protocol) is a term used in IP telephony for a set of facilities for managing the delivery of voice information using the Internet Protocol (IP). In general, this means sending voice information in digital form in discrete packets rather than in the traditional circuit-committed protocols of the public switched telephone network (PSTN). A major advantage of VoIP and Internet telephony is that it avoids the tolls charged by ordinary telephone service.
VoIP, now used somewhat generally, derives from the VoIP Forum, an effort by major equipment providers, including Cisco, VocalTec, 3Com, and Netspeak to promote the use of ITU-T H.323, the standard for sending voice (audio) and video using IP on the public Internet and within an intranet. The Forum also promotes the user of directory service standards so that users can locate other users and the use of touch-tone signals for automatic call distribution and voice mail.
In addition to IP, VoIP uses the real-time protocol (RTP) to help ensure that packets get delivered in a timely way. Using public networks, it is currently difficult to guarantee Quality of Service (QoS). Better service is possible with private networks managed by an enterprise or by an Internet telephony service provider (ITSP).
A technique used by at least one equipment manufacturer, Adir Technologies (formerly Netspeak), to help ensure faster packet delivery is to use ping to contact all possible network gateway computers that have access to the public network and choose the fastest path before establishing a Transmission Control Protocol (TCP) sockets connection with the other end.
Using VoIP, an enterprise positions a “VoIP device” at a gateway. The gateway receives packetized voice transmissions from users within the company and then routes them to other parts of its intranet (local area or wide area network) or, using a T-carrier system or E-carrier interface, sends them over the public switched telephone network.