The foundations of WAN communication technologies begin with a discussion of data transmission itself. There are two distinct types of data transmission: analog and digital. Even digital transmissions are in the end in the form of electromagnetic waves through a wire and in a sense are analog in nature also. It is the interpretation of those waves that makes them digital in nature. An analog wave is one in which the information conveyed is in real time and varies according to the harmonic waveform components frequency inversely related to the wavelength and amplitude which means wave height. Sound and what you hear are examples of information that reaches you and the multiple waves change in wavelength and wave height forming the sentence that someone is saying or the music that you are enjoying. The problem with analog information is that it is extremely difficult to store and reproduce faithfully and the computer needs to store and manipulate numbers which must never be mistaken for a different number because they are so close. In an analog world we see these two boxes:

Which would be extremely difficult to tell apart if they were not side-by-side. In HTML the background colors are precisely defined because the system is digital as CCCC99h for the box on the left and CCCC98h for the box on the right. Whichever color is chosen it can always be faithfully reproduced because it is a specific digital value. The code will reproduce the color more accurately than even the monitor because the cathode ray tube is an analog device.

In Ethernet the waveforms that travel between systems are based on the voltages placed on the wire by the transmitter. Presence of the voltage is considered a "1" for example and absence of voltage is considered a "0" Because no information is conveyed in the changing wave height in real time as it is with analog transmissions like sound, then the waves change very quickly from voltage to no voltage and form a square wave like this:

Analog waveforms on the hand rise and fall with a shape that carries information such as this sine wave (a sine wave is mathematically perfect or symmetrical):

Analog waves of different frequencies, changing frequencies, different amplitude or height and changing height can all be combined into one medium and be distinguished by an analog receiver. This is why you can hear a symphony orchestra play and the music is created by the constant mixture of the individual waves. Digital systems on the other hand only react to the height change from zero to one and back again. Because of this multiple waves or streams of information are not possible on a purely digital line like an Ethernet cable. Because of this Ethernet cables are a baseband communication medium meaning they can only carry one "band" or channel of information to which all transceivers in the system must be "tuned."

Analog transmission media on the other hand can carry multiple simultaneous waves each carrying a stream of data. An example of this is cable television in which the cable can bring to the house hundreds of separate and independent channels all arriving on the cable simultaneously. In order to view a particular channel the television must set its receiver circuitry to the desired frequency which the user calls "changing the channel" in order to pick the program out of the stream. This type of transmission is called broadband meaning that the medium can carry many channels of information simultaneously. Note that the term broadband is often used in networking to refer to very high throughput WAN technologies that are in fact digital and technically baseband. They are using the term to signify high throughput. This usage of the term is incorrect, but so common in networking that it is accepted.

There three methods of switching used in large-scale mesh networks to get data from one particular source node to a particular destination node on the network.

• Circuit Switching - The source node transmits the destination node's address at the beginning of the transmission. The intermediate devices determine the route to the destination node and switch open a solid and continuous circuitry path between the source and destination nodes. With this physical circuit connected between the two nodes they can communicate directly until either node closes the transmission carrier waves. At that point the intermediate circuit switching devices disconnect the switched together circuit. The telephone system uses circuit switching.
• Packet Switching - In this type of network, information is exchanged in discrete blocks of data called packets. Each packet holds a conplete header containing source and destination node addressing information. The intermediate devices read the information within each individual packet in order to forward along to the next intermediate device until one of these can forward the packet to the destination node. TCP/IP based computer networks including Ethernet LANs up to the entire Internet use packet switching.
• Message Switching - Can be thought of as a hybrid of the other two switching techniques. An entire transmission from one node to the other is packaged into a message whose header contains the sources and destination node addressing information. The source will transmit this message to the first intermediate device which will receive the entire message. The connection between the source node and the first intermediate device is then dropped and the intermediate device stores the message until it can establish a connection with the next intermediate device. The message will then be forwarded to it. Each intermediate device is therefore responsible for storing and then forwarding the message when a connection becomes available. E-mail works on this principle though it is further converted into TCP/IP packets to travel across the packet switched network of the Internet. No modern computer networking system uses a pure message switching technology because of the processing and storage burden on the intermediate devices and transient traffic conditions which can cause long delays in the transfer of the message to the destinaton node.

In networking connections between nodes at the ends of a cable, there are four basic dialog types (a session layer concept, but not necessarily tied to the OSI model layer 5):

• Simplex - The cable can only carry information in one direction. Devices at one end can send information to the others, but the others cannot respond. Similar to a football coach yelling orders to the players on the field using a bullhorn. Some networking technologies use two simplex cables one for each direction to allow duplex communication between nodes.
• Half-Duplex - Nodes at opposite ends of the cable can communicate back and forth with each other, but the cable will only carry information in one direction at a time. Walkie-talkie radios are half-duplex communication devices that allow either person to talk, but not both at the same time.
• Full-Duplex (or simply: Duplex) - Nodes at either end of the cable can transmit in either direction at the same time. Most examples of physical duplex actually involve two simplex or half-duplex cables between the nodes.
• Multiplex - Allows multiple nodes to establish communication across the cable in both directions simultaneously. A multiplexed line can carry multiple half or full duplex communications between pairs of nodes at the same time.

Some WAN technologies take advantage of processes that allow multiple simultaneous connections between nodes across a single cable, which a purely packet switched network like Ethernet only emulates. In Ethernet only one node can transmit at one time to one destination node. The ability of a cable to carry multiple simultaneous connections is handled using a technique called multiplexing. There are three main methods used to multiplex across the cable:

• TDM - Time Division Multiplexing - A TDM device is attached to each end of the cable. At a specific period at specific intervals node "A" may transmit and/or receive a transmission across the cable. The same holds for nodes "B", "C", etc.
• WDM - Wave Division Multiplexing - Transmissions are separated from each other in frequency of the waveform rather than giving nodes spaced "windows" of time in which they can communicate on the nedium. Cable TV transmits multiple channels on the cable using WDM.
• Statistical Multiplexing - A modification of TDM in which nodes that do not use their time slot often will have their number of "windows" reduced while busy nodes will get more. Some Statistical Multiplexing technologies use sophisticated traffic monitoring techniques and can adjust to changing conditions. Statistical multiplexing takes full advantage of what the cable is capable of carrying and does not waste bandwidth capacity with leaving too many empty "windows" from nodes that are not using them.

Another consideration in network node communication is the relationships between the nodes. When one node knows the exact address of the destination node to which it will transmit information, and the information is intended only for that node, the transmission will be made addressed to that specific node and is called a point-to-point or unicast transmission. When the node does not know the exact address of the destination node, or the transmission is intended for all nodes on the network, then node will transmit the information without a specific destination node address and all nodes can receive the transmission which is called a broadcast. Finally, when a node needs to transmit information to multiple specific destination nodes, but not all nodes on the network, it will transmit the information using virtual temporary addressing which more than one node can use simultaneously and therefore receive the information. This technique is referred to as multicasting a technique popularized on the Internet because TCP/IP addressing schema support it.

This module will cover all of the basic WAN networking connectivity technologies that make it possible for LANs or single remote nodes to exchange information and therefore participate in an integrated Wide Area Network. The WAN technology discussion will begin here with the PSTN - Public Switched Telephone Network. The advantages of using the PSTN, also referred to as the POTS (Plain Old Telephone Service), are:

• Ubiquity - The PSTN reaches far more locations worldwide than any other technology other than long range radio transceivers.
• Availability - The hardware devices required to attach systems as a WAN over the PSTN are common and inexpensive.
• Ease of Installation and Configuration - The hardware devices required to attach systems as a WAN over the PSTN are relatively easy to install and configure.

The disadvantages generally sited with PSTN are:

• Reliability - The PSTN is not considered reliable but this issue refers to the quality of the connection as well as whether the service is working at all.
• Throughput - The PSTN is one of the slowest choices for wide area network connectivity mainly due to the fact that the PSTN backbone was originally designed to carry voice analog sound waves and has a maximum encoded throughput of 56Kbps. FCC regulations place the throughput limit for transmissions at 53Kbps.
• Security - The PSTN is relatively easy to tap the transmission at various points along the circuit making its security level quite poor.

Signal quality between any two circuit switched nodes along the path within a PSTN connection can degrade rapidly such that even the 53Kbps connection can be rare depending on the infrastructure of the network between the nodes especially near the endpoints of the WAN connection. The long distance carriers as usually very reliable high bandwidth fiber-optic lines now but from the telephone company's CO - Central Office, to the actual site may still be using antequated and low quality analog copper lines.

X.25 was developed and internationally standardized by the ITU in the 1970's specifically for the purpose of permitting WAN connectivity between digital computer systems on a global scale. X.25 is an analog packet switched technology capable of 64Kbps. X.25 is a reliable WAN technology based on verifying the packet at every intermediate device along the path. It is this verification of the packet by all devices that hinders the performance of the X.25 technology and makes it impractical for time sensitive applications like streaming audio or video in which the packets cannot afford to be delayed. X.25 was never popular in the U.S. although it grew widely popular throughout the rest of the world and is still used in other countries. In 1992 X.25 was reworked to handle a maximum data transmission speed of 2048Kbps (2.048 Mbps) making it somewhat competitive with the other emerging technologies, but it is still realtively slow compared to some of the newer technologies available. Another consideration is the cost per bandwidth or $/bps may not be competitive versus some of the newer technologies either.

Frame Relay is a fully digital version of X.25 which is also a packet switching network. Another significant difference between Frame Relay and X.25 is that the intermediate nodes perform no error checking on the packets and so time dependent data streams can be sent across a frame relay network link. Frame relay was standardized in 1984 and became a very popular high bandwidth WAN technology alternative used mostly in the U.S. and Canada. The technology is available in either 1.544 Mbps or 45 Mbps. It is no longer as popular due to competition from other emerging WAN technologies.

The one advantage to modern frame relay WAN connections is that the customer is not forced to purchase the entire 1.544 Mbps or 45 Mbps link. Now the customer can purchase the service based on the bandwidth requirement marketed as a "fractional frame relay" or "fractional T-1" The customer can pay a flat rate for a guaranteed bandwidth through the link in multiples of 64Kbps so a customer could request a fractional T-1 with a bandwidth of 3 x 64Kbps = 192Kbps at a significant savings from having to invest in the full priced 1.544Mbps frame relay line. The guaranteed throughput is referred to as the CIR - committed information rate. The only problem with a fractional frame relay link is that the line is shared with other sites and in peak traffic times on the cable, bandwidth may fall well short of the CIR and there is no real way to control this or correct the transient throughput loss in performance.

ISDN or Integrated Digital Services Network is another international WAN technology standardized by the ITU. ISDN is provided by the telephone companies and was first offered in the mid-1980's and can support two independent telephone connections and digital connections simultaneously eliminating the need for separate services at a single location. It was thought that ISDN would ultimately replace the analog PSTN technologies but the high price of the service, around $100/mo. minimum, extensive hardware requirements, and unimpressive throughput stalled its potential spread. Now it is a fairly unattractive alternative versus newer much faster and less inexpensive emerging technologies. However, many sites worldwide still use ISDN as their existing and installed WAN connection.

ISDN service is divided into "B" or "bearer" channels and "D" or "data" channels. This can be confusing because all data whether digitized voice or computer connections travel over the B channels. The D channel, usually only one, is used to transmit session information concerning the connection to the network. B channels in general have a throughput of 64Kbps while the D channel can have a throughput of either 16Kbps or 64Kbps depending on the type of ISDN service installed. Some areas limit the B channels to 56Kbps.

In North America ISDN is bundled into two types of service called BRI - Basic Rate Interface ISDN and PRI - Primary Rate Interface ISDN. A third type was developed called Broadband ISDN or B-ISDN which offers higher throughput than either of the other types but most customers who need higher throughput opt for other technologies and B-ISDN is not very common. BRI consists of a bundle of 2 x B channels and 1 x 16Kbps D channel. The maximum data transfer throughput of a BRI line is then 128Kbps while the total throughput of the service is 128Kbps + 16Kbps = 144Kbps. The PRI line bundles 23 x B channels and 1 x 64Kbps D channel and offers a maximum data throughput of 1472Kbps and a total capacity of 1536Kbps although most references list the total capacity of PRI lines as 1.544Mbps. BRI lines are also called 2B+D ISDN and PRI lines are also called 23B+D ISDN.

The completely digital special circuit of the ISDN line must attach at the site to a special device referred to generically as a Network Termination 1 or NT1. This is the transceiver of the digital information on the ISDN network. ISDN systems do "dial" other ISDN locations to establish a connection and this is the device that is needed. The NT1 equipment will provide interfaces to either the Terminal Equipment or TE devices or to a TA - Terminal Adapter. A fax machine or the NIC on a PC that communicate in pure digital signals are forms of TE. The TA is required to attach a regular telephone or modem or any other analog device to the ISDN service.

The larger bundle of the PRI line must attach to an NT1 device which has an interface to an NT2 - Network Termination 2 device(s). NT2 devices are needed to separate the trunk of 23 separate channels. TE's and TA's attach to the NT2 equipment of the PRI line.

T carriers are the physical systems designed to carry a slightly different standard signal called a DSx or Digital Signal where x is an arbitrary numeric suffix that indicates which standard throughput it refers to. A DS0 (zero) is a single digital channel that can carry either voice or data at 64Kbps. DS1 is a signal that can carry 24 x 64Kbps channels. Although they sound roughly similar to ISDN, these signals are logically distinct and the physical layer technology required to carry them is the T-1 line.

The device that attaches directly to the T-1 cabling itself is called a CSU/DSU: Channel Service Unit/Data Service Unit and generates and receives the actual transmissions on the cable; nothing else. It can be thought of as the T-1 transceiver. The input and output of the CSU/DSU must then go to the Multiplexer which can separate the channels. As described earlier then this will be either a TDM or a statistical multiplexer. So T-1 lines are in theory a single cable carrying a high frequency mutliplexed digital stream of information whereas a PRI ISDN line would bring a trunk or 24 lines to the site. The multiplexer will be equipped with the interfaces for either network devices such as routers, a company's PBX - Private Branch eXchange telephone system, or both. As mentioned above, the T-1 physical circuit can be used to carry "fractional T-1" which is really frame relay, a form of completely digital X.25, a packet switched network. True T-1 however, is a constantly active dedicated circuit. T-1's are quite expensive and only those companies that need the bandwidth and the reliability opt for the service as a T-1 costs over $1000/mo.

Here are the T carriers and their associated digital signal standards:

One of the newer emerging technologies that is competing with many of the older technologies mentioned above in both price and bandwidth is DSL - Digital Subscriber Line. DSL uses a new physical layer modulation technology to achieve extraordinary throughput for a relatively short distance on the existing PSTN installed cabling. The longest distance that DSL can function from repeater to repeater is about 25,000 feet or roughly 4 3/4 miles and this is the slowest version available. DSL is a constantly active line for the digital signal and coexists on the same line as the PSTN voice line which can still dial out and receive calls simultaneously. In order to facilitate this, all regular telephone attachments require filters to screen the audible white noise harmonics generated by the digital channel's carrier frequency.

Once activated at the local provider's side of the circuit all that is required is the DSL "modem" on the customer's end. This device can be either an external peripheral or an internal expansion card, although the external devices are much more common and easier for the average user to connect and use. These devices usually provide a standard Ethernet interface for connecting systems to the DSL line using Ethernet NIC's.

At the provider end of the cable the signal is sent to a DSLAM - DSL Access Multiplexer which channels multiple DSL lines into another single highspeed high bandwidth service onto the Internet.

There are basically two main categories of DSL: Symmetric and Asymmetric. Asymmetric has a much lower upload throughput than download throughput and is suitable to end users that only need Internet access to view, browse and download. Symmetric DSL services offer upload throughput equal to the download throughput (hence the term: symmetric) and are more suitable for businesses and other services that need to transmit or provide large amounts of information to the Internet or other sites. The main asymmetric DSL types offered are: ADSL (Asymmetric DSL), "G.lite", and VDSL (Very high bitrate DSL, also called VHDSL). The main symmetric products are: SDSL (Symmetric DSL) and HDSL (High bitrate DSL). All forms of DSL are often referred to collectively as xDSL.

Another emerging technology that is competing with the older WAN technologies is CATV Access or simply "Cable". The existing Cable Television network supports very high analog bandwidth for downstream transmission with no provision for upstream transmissions at all. However, the cable to the end users location can support an analog upstream transmission but this would be limited in effective distance to still be compliant with FCC regulations governing radio frequency transmitting equipment.

Another issue is the extremely high bandwidth of the cable network which typically offers up to 36Mbps downstream and 10Mbps upstream throughput. At the central local node where the end users transmissions are collected then there would be a very high quantity of traffic; too high for the existing cables.

To solve this issue the cable company must install a local node within an area to which all of the end users can be attached using the existing cabling. Each of these connections are called a cable drop. These nodes however, must be attached to the cable company's local main office, called the head-end by an entirely new cabling called hybrid fiber-coax. HFC cable supports the throughput from the nodes back to the head-end. From the head-ends central point, existing very high throughput technologies can forward the traffic to the Internet.

At the end users location the cable is attached to a cable "modem" which can provide a standard network interface usually for 100BaseTX Ethernet to which local computers can be interfaced to the network. The main problem with cable network access is that the cable modem is nothing more than a transceiver of the digital data meaning that all local computers attached to it are attaching to an extended Ethernet LAN. Without taking proper security precautions it is possible for other systems in the local area to detect and possibly access another person's computer. Another problem with cable is that during peak traffic conditions if a large percentage of the local nodes systems are active then bandwidth could suffer dramatically. In comparing cable versus DSL, since both services are comparable in price, most businesses are opting for DSL mainly because it is more widely available than cable, it is more secure, and bandwidth does not get choked off as often because each end node has a separate circuit to the DSLAM. The DSLAM is however, a single point of failure that can under peak conditions also cause reductions in available bandwidth. However, as DSL continues to grow it is most likely that the provider will continue to improve the infrastructure improving performance.

SONET stands for Synchronous Optical NETwork. SONET is also an international WAN standard although in Europe it is called SDH - Synchronous Digital Hierarchy. SONET uses the same data form as the T carriers and in fact it integrates well with T carriers. SONET however, is physically different and special interface devices are necessary to accomplish this.

SONET uses fiber optic cabling and nodes are attached in a dual ring topology. If a break is detected in the primary ring, information will automatically be rerouted through the back up ring. This feature of SONET is referred to as "self-healing" and it makes SONET highly reliable.

SONET requires an interface terminal equipment at the end users site and also a multiplexer since it uses very similar multiplexing techniques to what the T carriers also use. SONET is by far the most expensive broadband solution costing many thousands of dollars to install, many thousands of dollars for the terminal equipment and multiplexers and many thousands of dollars a month for the service. It does however provide far higher dedicated, secure and reliable information throughput than any other widely available standard. SONET is leased on circuits referred to as OCx - Optical Carrier where x is the numerical reference for the particular specification as follows:

Note that an OC24 SONET carrier has a throughput of over 1.2 gigabits per second and could carry 6 full 100BaseTX networks traffic at full duplex capacity. SONET provides the physical layer infrastructure for ATM - Asynchronous Transfer Mode, another packet switching WAN technology that depends on the use of virtual circuits that the end user can also lease. The ATM backbone is usually reported to have a throughput of 622Mbps indicating that the providers use an OC12 infrastructure for the ATM network.

Since WAN technologies in general are not compatible and highly diverse in features and can be exhorbitently expensive, choosing a WAN technology is by far the most critical step in its implementation. Investing large sums of money to install the service and purchase or even lease the terminal equipment only to end up not needing the bandwidth can be a costly mistake. Investing in a technology that is not efficiently scalable to meet the company's needs can be equally disastrous if the company suddenly needs far more bandwidth than he existing infrastructure can provide and must be completely replaced by another expensive backbone technology.

In this module the students will establish remote connectivity using the PSTN technologies by installing a modem on a Windows 2000 Server and setting it up as a simple RAS server. Then students will also install a modem on a Windows 98 machine and set it up as a RAS client and dial up the server through the telephone line emulator available in the lab. The links to all of the appropriate hands on exercises to prepare the two systems can be found in the preparations section of this module.

There are three distinct remote access networking methods that can be employed over the PSTN:

• Dial-Up to LAN - This is the system implemented in the related modules in which the remote node will directly dial the modem line of the RAS server and log onto it in order to gain access to the resources on the LAN.
• Dial-Up to Node - In this scenario the remote user dials directly to the modem of a particular workstation and does not enter through a RAS server. This system would require much more hardware than a centralized RAS server if multiple users need remote access to resources on the network and it is very difficult to manage, maintain or secure.
• Extranet Connectivity - Remote users can access the network resources and authenticate using any Internet access that is available to them through webserver on the LAN. This solution requires sophisticated configuration of the web portal server but once configured requires almost no additional configuration at the remote node as long as it can access the Internet. Online banking is an example of using this form of access.

A technology that is rising in popularity and making the extranet solution a much more secure and reliable WAN alternative is the VPN - Virtual Private Network. A VPN can be established over the Internet or even any of the other shared access technologies described above like a fractional T-1 and interconnect nodes securely even though the traffic is traveling over a public network. This is achieved using public/private key encryption technologies integrated into the network layer stacks of the participating nodes. Packets are encrypted and then encapsulated into normal IP packets that are then sent over the public medium. If they are intercepted their data payload is strongly encrypted using some form of asymmetric encryption and cannot be read or cracked because of the large size of the keys that are used.

The two main protocols used over the PSTN by dial-up networking are SLIP - Serial Line Internet Protocol and PPP - Point-to-Point Protocol. SLIP was developed around 1980 to allow remote control console access to UNIX servers using telnet. SLIP can only encapsulate IP packets into its frames and does not support any form of encryption. SLIP only functions on the serial port (physical or emulated) of the PC also and is not flexible so that it could be implemented on any other physical network layer. Because of these flaws it has fallen into disfavor and most systems have been converted to some other protocol usually PPP. PPP is a purely data link layer protocol that is not limited to use over any particular physical network layer. PPP is a fully modular data link layer technology that supports being encapsulated by any other protocol, which SLIP does not, as well as the ability to encapsulate any other protocol, which again SLIP cannot do. PPP supports encryption protocols and can establish secure connections whereas in SLIP the username and password are exchanged in plain text which is obviously a security risk.

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