Terminology

 

• A subscriber, in this context, is a company that arranges for WAN services from a service provider.

• Data Communications Equipment (DCE) – Also called data circuit-terminating equipment, the DCE consists of devices that put data on the local loop. The DCE primarily provides an interface to connect subscribers to a communication link on the WAN cloud.

• Data Terminal Equipment (DTE) – The customer devices that pass the data from a customer network or host computer for transmission over the WAN. The DTE connects to the local loop through the DCE.

• Demarcation Point – A point established in a building or complex to separate customer equipment from service provider equipment. Physically, the demarcation point is the cabling junction box, located on the customer premises, that connects the CPE wiring to the local loop. It is usually placed for easy access by a technician. The demarcation point is the place where the responsibility for the connection changes from the user to the service provider. When problems arise, it is necessary to determine whether the user or the service provider is responsible for troubleshooting or repair.

• Local Loop – The actual copper or fiber cable that connects the CPE to the CO of the service provider. The local loop is also sometimes called the “last-mile”.

• Central Office (CO) – The CO is the local service provider facility or building that connects the CPE to the provider network.

• Toll network – This consists of the long-haul, all-digital, fiber-optic communications lines, switches, routers, and other equipment inside the WAN provider network.

There are many types of devices that are specific to WAN environments, including:

• Dialup modem – Considered to be a legacy WAN technology, a voiceband modem converts (i.e., modulates) the digital signals produced by a computer into voice frequencies that can be transmitted over the analog lines of the public telephone network. On the other side of the connection, another modem converts the sounds back into a digital signal (i.e., demodulates) for input to a computer or network connection.

• Access server – Concentrates dialup modem, dial-in and dial-out user communications. Considered to be a legacy technology, an access server may have a mixture of analog and digital interfaces and support hundreds of simultaneous users.

• Broadband modem – Modulator/Demodulator, a Modem is a hardware device that allows a computer to send and receive information over telephone lines by converting digital data into an analog signal used on phone lines, and vice versa. Modem terminates an analog local loop. A type of digital modem used with high-speed DSL or cable Internet service. Both operate in a similar manner to the voiceband modem, but use higher broadband frequencies and transmission speeds.

• CSU/DSU(Channel Service Unit/Data Service Unit) – Digital-leased lines require a CSU and a DSU. A CSU/DSU can be a separate device like a modem or it can be an interface on a router. The CSU provides termination for the digital signal and ensures connection integrity through error correction and line monitoring. The DSU converts the line frames into frames that the LAN can interpret and vice versa.CSU/DSU provides clocking signal to the customer equipment interface and terminates the channelized transport media to a leased line. As a result, DSU/CSU converts one form of digital format to another digital format. Therefore CSU/DSU terminates a digital local loop.
• WAN switch – A multiport internetworking device used in service provider networks. These devices typically switch traffic, such as Frame Relay or ATM and operate at Layer 2.

• Router – Provides internetworking and WAN access interface ports that are used to connect to the service provider network. These interfaces may be serial connections, Ethernet, or other WAN interfaces. With some types of WAN interfaces, an external device, such as a DSU/CSU or modem (analog, cable, or DSL), is required to connect the router to the local service provider.

• Core router/Multilayer switch – A router or multilayer switch that resides within the middle or backbone of the WAN, rather than at its periphery. To fulfill this role, a router or multilayer switch must be able to support multiple telecommunications interfaces of the highest speed used in the WAN core. It must also be able to forward IP packets at full speed on all of those interfaces. The router or multilayer switch must also support the routing protocols being used in the core.
  

Note: The preceding list is not exhaustive and other devices may be required, depending on the WAN access technology chosen.

Circuit switching

If the circuit carries computer data, the usage of this fixed capacity may not be efficient. For example, if the circuit is used to access the Internet, there is a burst of activity on the circuit while a web page is transferred. This could be followed by no activity while the user reads the page, and then another burst of activity while the next page is transferred. This variation in usage between none and maximum is typical of computer network traffic. Because the subscriber has sole use of the fixed capacity allocation, switched circuits are generally an expensive way of moving data.

The two most common types of circuit-switched WAN technologies are the public switched telephone network (PSTN) and the Integrated Services Digital Network (ISDN).

Packet switching

In contrast to circuit switching, packet switching splits traffic data into packets that are routed over a shared network. Packet-switching networks do not require a circuit to be established, and they allow many pairs of nodes to communicate over the same channel.

The switches in a packet-switched network (PSN) determine the links that packets must be sent over based on the addressing information in each packet. The following are two approaches to this link determination:

• Connectionless systems – Full addressing information must be carried in each packet. Each switch must evaluate the address to determine where to send the packet. An example of a connectionless system is the Internet.

• Connection-oriented systems – The network predetermines the route for a packet, and each packet only has to carry an identifier. The switch determines the onward route by looking up the identifier in tables held in memory. The set of entries in the tables identifies a particular route or circuit through the system. When the circuit is established temporarily while a packet is traveling through it, and then breaks down again, it is called a virtual circuit (VC). An example of a connection-oriented system is Frame Relay. In the case of Frame Relay, the identifiers used are called data-link connection identifiers (DLCIs).
  The cost of packet switching is lower than that of circuit-switching. However, delays (latency) and variability of delay (jitter) are greater in packet-switched networks than in circuit-switched networks.

WAN connection options

• Private WAN infrastructure – Service providers may offer dedicated point-to-point leased lines, circuit-switched links, such as PSTN or ISDN, and packet-switched links, such as Ethernet WAN, ATM, or Frame Relay.
  1. Leased Lines: When permanent dedicated connections are required, a point-to-point link is used to provide a pre-established WAN communications path from the customer premises to the provider network. Point-to-point lines are usually leased from a service provider and are called leased lines.
     Different names such as leased circuits, serial link, serial line, point-to-point link, and T1/E1 or T3/E3 lines. Leased lines are available in different capacities and are generally priced based on the bandwidth required and the distance between the two connected points.
     In North America, service providers use the T-carrier system to define the digital transmission capability of a serial copper media link, while Europe uses the E-carrier system. For instance, a T1 link supports 1.544 Mb/s, an E1 supports 2.048 Mb/s, a T3 supports 43.7 Mb/s, and an E3 connection supports 34.368 Mb/s.
     Optical Carrier (OC) transmission rates are used to define the digital transmitting capacity of a fiber optic network.
     The advantages of leased lines include:
     • Simplicity – Point-to-point communication links require minimal expertise to install and maintain.
     • Quality – Point-to-point communication links usually offer high service quality, if they have adequate bandwidth. The dedicated capacity removes latency or jitter between the endpoints.
     • Availability – Constant availability is essential for some applications, such as e-commerce. Point-to-point communication links provide permanent, dedicated capacity which is required for VoIP or Video over IP.

     The disadvantages of leased lines include:

     • Cost – Point-to-point links are generally the most expensive type of WAN access. The cost of leased line solutions can become significant when they are used to connect many sites over increasing distances. In addition, each endpoint requires an interface on the router, which increases equipment costs.
     • Limited flexibility – WAN traffic is often variable, and leased lines have a fixed capacity, so that the bandwidth of the line seldom matches the need exactly. Any change to the leased line generally requires a site visit by ISP personnel to adjust capacity.
  2. Dialup: Dialup WAN access may be required when no other WAN technology is available. Suitable when intermittent, low-volume data transfers are needed. The physical characteristics of the local loop and its connection to the PSTN limit the rate of the signal to less than 56 kb/s.
     Traditional local loops can transport binary computer data through the voice telephone network using a modem.
     The modem modulates the binary data into an analog signal at the source and demodulates the analog signal to binary data at the destination.
     The advantages of modem and analog lines are simplicity, availability, and low implementation cost.
     The disadvantages are the low data rates and a relatively long connection time.
  3. Integrated Services Digital Network (ISDN) is a circuit-switching technology that enables the local loop of a PSTN to carry digital signals, resulting in higher capacity switched connections. ISDN changes the internal connections of the PSTN from carrying analog signals to time-division multiplexed (TDM) digital signals. TDM allows two or more signals, or bit streams, to be transferred as subchannels in one communication channel. The signals appear to transfer simultaneously; but physically, the signals are taking turns on the channel.There are two types of ISDN interfaces:
     • Basic Rate Interface (BRI) – ISDN BRI is intended for the home and small enterprise and provides two 64 kb/s B channels and a 16 kb/s D channel. The BRI D channel is designed for control and often underused, because it has only two B channels to control.
       BRI has a call setup time that is less than a second, and the 64 kb/s B channel provides greater capacity than an analog modem link. If greater capacity is required, a second B channel can be activated to provide a total of 128 kb/s. Although inadequate for video, this permits several simultaneous voice conversations in addition to data traffic. More info.
     • Primary Rate Interface (PRI) – ISDN is also available for larger installations. In North America, PRI delivers 23 B channels with 64 kb/s and one D channel with 64 kb/s for a total bit rate of up to 1.544 Mb/s. This includes some additional overhead for synchronization.  In Europe, Australia, and other parts of the world, ISDN PRI provides 30 B channels and one D channel, for a total bit rate of up to 2.048 Mb/s, including synchronization overhead. More info.
       With PRI ISDN, multiple B channels can be connected between two endpoints. This allows for videoconferencing and high-bandwidth data connections with no latency or jitter. However, multiple connections can be very expensive over long distances.
       Another common application of ISDN is to provide additional capacity as needed on a leased line connection. The leased line is sized to carry average traffic loads while ISDN is added during peak demand periods. ISDN is also used as a backup if the leased line fails. ISDN tariffs are based on a per-B channel basis and are similar to those of analog voice connections.
  4. Frame Relay:Frame Relay is a simple Layer 2 non-broadcast multiaccess (NBMA) WAN technology used to interconnect enterprise LANs. A single router interface can be used to connect to multiple sites using PVCs(permanent virtual circuits). PVCs are used to carry both voice and data traffic between a source and destination, and support data rates up to 4 Mb/s, with some providers offering even higher rates.An edge router only requires a single interface, even when multiple virtual circuits (VCs) are used. The short-leased line to the Frame Relay network edge allows cost-effective connections between widely scattered LANs.Frame Relay creates PVCs which are uniquely identified by a data-link connection identifier (DLCI). The PVCs and DLCIs ensure bidirectional communication from one DTE device to another.
  5. ATM: Asynchronous Transfer Mode (ATM) technology is capable of transferring voice, video, and data through private and public networks. It is built on a cell-based architecture rather than on a frame-based architecture. ATM cells are always a fixed length of 53 bytes. The ATM cell contains a 5-byte ATM header followed by 48 bytes of ATM payload. Small, fixed-length cells are well-suited for carrying voice and video traffic because this traffic is intolerant of delay. Video and voice traffic do not have to wait for larger data packets to be transmitted.
     The overhead is higher because the ATM switch must be able to reassemble the packets at the destination. A typical ATM line needs almost 20 percent greater bandwidth than Frame Relay to carry the same volume of network layer data.ATM was designed to be extremely scalable and to support link speeds of T1/E1 to OC-12 (622 Mb/s) and faster.ATM offers both PVCs and SVCs(switched virtual circuit), although PVCs are more common with WANs. As with other shared technologies, ATM allows multiple VCs on a single leased-line connection to the network edge.
  6. Ethernet WAN: IEEE 1000BASE-LX standard supports fiber optic cable lengths of 5 km, while the IEEE 1000BASE-ZX standard supports up to 70 km cable lengths.Service providers now offer Ethernet WAN service using fiber optic cabling. The Ethernet WAN service can go by many names, including Metropolitan Ethernet (MetroE), Ethernet over MPLS (EoMPLS), and Virtual Private LAN Service (VPLS).
     Benefits of Ethernet WAN include:
     • Reduced expenses and administration – Ethernet WAN provides a switched, high-bandwidth Layer 2 network capable of managing data, voice, and video all on the same infrastructure. This characteristic increases bandwidth and eliminates expensive conversions to other WAN technologies. The technology enables businesses to inexpensively connect numerous sites, in a metropolitan area, to each other and to the Internet.
     • Easy integration with existing networks – Ethernet WAN connects easily to existing Ethernet LANs, reducing installation costs and time.
     • Enhanced business productivity – Ethernet WAN enables businesses to take advantage of productivity-enhancing IP applications that are difficult to implement on TDM or Frame Relay networks, such as hosted IP communications, VoIP, and streaming and broadcast video.

     Note: Ethernet WANs have gained in popularity and are now commonly being used to replace the traditional Frame Relay and ATM WAN links.

  7. MPLS:Multiprotocol Label Switching (MPLS) is a multiprotocol high-performance WAN technology that directs data from one router to the next, based on short path labels rather than IP network addresses.
     MPLS has several defining characteristics.
     1. It is multiprotocol, meaning it has the ability to carry any payload including IPv4, IPv6, Ethernet, ATM, DSL, and Frame Relay traffic.
     2. It uses labels which tell a router what to do with a packet. The labels identify paths between distant routers rather than endpoints, and while MPLS actually routes IPv4 and IPv6 packets, everything else is switched.
        • MPLS is a service provider technology. Leased lines deliver bits between sites, and Frame Relay and Ethernet WAN deliver frames between sites.
        • However, MPLS can deliver any type of packet between sites.
        • MPLS can encapsulate packets of various network protocols. It supports a wide range of WAN technologies including T-carrier / E-carrier links, Carrier Ethernet, ATM, Frame Relay, and DSL.The sample topology in the figure illustrates how MPLS is used.
          Notice that the different sites can connect to the MPLS cloud using different access technologies.
  8. VSAT( Very small aperture terminal ):What if an organization needed connectivity in a remote location where there are no service providers that offer WAN service?Very small aperture terminal (VSAT) is a solution that creates a private WAN using satellite communications.
     A VSAT is a small satellite dish similar to those used for home Internet and TV. VSATs create a private WAN while providing connectivity to remote locations.Specifically, a router connects to a satellite dish which is pointed to a service provider’s satellite in a geosynchronous orbit in space. The signals must travel approximately 35,786 kilometers (22,236 miles) to the satellite and back.

• Public WAN infrastructure – Service provider may offer broadband Internet access using digital subscriber line (DSL), cable, and satellite access. Broadband connection options are typically used to connect small offices and telecommuting employees to a corporate site over the Internet. Data travelling between corporate sites over the public WAN infrastructure should be protected using VPNs.
  1. DSL: DSL technology is an always-on connection technology that uses existing twisted-pair telephone lines to transport high-bandwidth data, and provides IP services to subscribers. A DSL modem converts an Ethernet signal from the user device to a DSL signal, which is transmitted to the central office.Multiple DSL subscriber lines are multiplexed into a single, high-capacity link using a DSL access multiplexer (DSLAM) at the provider location. DSLAMs incorporate TDM technology to aggregate many subscriber lines into a single medium, generally a T3 (DS3) connection. Current DSL technologies use sophisticated coding and modulation techniques to achieve fast data rates.There is a wide variety of DSL types, standards, and emerging standards. DSL is now a popular choice for enterprise IT departments to support home workers. Generally, a subscriber cannot choose to connect to an enterprise network directly, but must first connect to an ISP.

     On the customer side, the DSL Transceiver, or ATU-R, or more commonly known as a DSL modem, is hooked up to a phone line. The telephone company connects the other end of the line to a DSLAM, which concentrates a large number of individual DSL connections into a single box.
     Like analog modems, DSL transceivers constantly monitor the quality of each channel and will add or remove them from service depending on whether they are usable. Once upstream and downstream circuits are established, a subscriber can connect to a service such as an Internet service provider or other network services.

     When the DSL modem powers up it goes through a series of steps to establish connections. The actual process varies from modem to modem but generally involves the following steps:

     1. The DSL transceiver performs a self-test.
     2. The DSL transceiver then attempts to synchronize with the DSLAM. Data can only come into the computer when the DSLAM and the modem are synchronized. The synchronization process is relatively quick (in the range of seconds) but is very complex, involving extensive tests that allow both sides of the connection to optimize the performance according to the characteristics of the line in use. External, or standalone modem units have an indicator labeled “CD”, “DSL”, or “LINK”, which can be used to tell if the modem is synchronized. During synchronization the light flashes; when synchronized, the light stays lit, usually with a green color.
     3. The DSL transceiver checks the connection between the DSL transceiver and the computer.
  2. Cable:Coaxial cable is widely used in urban areas to distribute television signals. Network access is available from many cable television providers. This allows for greater bandwidth than the conventional telephone local loop.Cable modems provide an always-on connection and a simple installation. A subscriber connects a computer or LAN router to the cable modem, which translates the digital signals into the broadband frequencies used for transmitting on a cable television network. The local cable TV office, which is called the cable headend, contains the computer system and databases needed to provide Internet access. The most important component located at the headend is the cable modem termination system (CMTS), which sends and receives digital cable modem signals on a cable network and is necessary for providing Internet services to cable subscribers.Cable modem subscribers must use the ISP associated with the service provider. All the local subscribers share the same cable bandwidth. As more users join the service, available bandwidth may be below the expected rate.
  3. Wireless:Wireless technology uses the unlicensed radio spectrum to send and receive data. The unlicensed spectrum is accessible to anyone who has a wireless router and wireless technology in the device they are using.Until recently, one limitation of wireless access has been the need to be within the local transmission range (typically less than 100 feet) of a wireless router or a wireless modem that has a wired connection to the Internet. The following new developments in broadband wireless technology are changing this situation:
     • Municipal Wi-Fi – Many cities have begun setting up municipal wireless networks. Some of these networks provide high-speed Internet access for free or for substantially less than the price of other broadband services. Others are for city use only, allowing police and fire departments and other city employees to do certain aspects of their jobs remotely. To connect to a municipal Wi-Fi, a subscriber typically needs a wireless modem, which provides a stronger radio and directional antenna than conventional wireless adapters. Most service providers provide the necessary equipment for free or for a fee, much like they do with DSL or cable modems.
     • WiMAX – Worldwide Interoperability for Microwave Access (WiMAX) is a new technology that is just beginning to come into use. It is described in the IEEE standard 802.16. WiMAX provides high-speed broadband service with wireless access and provides broad coverage like a cell phone network rather than through small Wi-Fi hotspots. WiMAX operates in a similar way to Wi-Fi, but at higher speeds, over greater distances, and for a greater number of users. It uses a network of WiMAX towers that are similar to cell phone towers. To access a WiMAX network, subscribers must subscribe to an ISP with a WiMAX tower within 30 miles of their location. They also need some type of WiMAX receiver and a special encryption code to get access to the base station.
     • Satellite Internet – Typically used by rural users where cable and DSL are not available. A VSAT provides two-way (upload and download) data communications. The upload speed is about one-tenth of the 500 kb/s download speed. Cable and DSL have higher download speeds, but satellite systems are about 10 times faster than an analog modem. To access satellite Internet services, subscribers need a satellite dish, two modems (uplink and downlink), and coaxial cables between the dish and the modem.
  4. 3G/4G cellular:Increasingly, cellular service is another wireless WAN technology being used to connect users and remote locations where no other WAN access technology is available. Many users with smart phones and tablets can use cellular data to email, surf the web, download apps, and watch videos.Phones, tablet computers, laptops, and even some routers can communicate through to the Internet using cellular technology. These devices use radio waves to communicate through a nearby mobile phone tower. The device has a small radio antenna, and the provider has a much larger antenna sitting at the top of a tower somewhere within miles of the phone.Common cellular industry terms include:
     • 3G/4G Wireless – Abbreviation for 3rd generation and 4th generation cellular access. These technologies support wireless Internet access.
     • Long-Term Evolution (LTE) – Refers to a newer and faster technology and is considered to be part of fourth generation (4G) technology.
  5. VPN technology:Security risks are incurred when a teleworker or a remote office worker uses broadband services to access the corporate WAN over the Internet. To address security concerns, broadband services provide capabilities for using VPN connections to a VPN server, which is typically located at the corporate site.A VPN is an encrypted connection between private networks over a public network, such as the Internet. Instead of using a dedicated Layer 2 connection, such as a leased line, a VPN uses virtual connections called VPN tunnels, which are routed through the Internet from the private network of the company to the remote site or employee host.Benefits of VPN include the following:
     • Cost savings – VPNs enable organizations to use the global Internet to connect remote offices and remote users to the main corporate site, thus eliminating expensive, dedicated WAN links and modem banks.
     • Security – VPNs provide the highest level of security by using advanced encryption and authentication protocols that protect data from unauthorized access.
     • Scalability – Because VPNs use the Internet infrastructure within ISPs and devices, it is easy to add new users. Corporations are able to add large amounts of capacity without adding significant infrastructure.
     • Compatibility with broadband technology – VPN technology is supported by broadband service providers such as DSL and cable, so mobile workers and telecommuters can take advantage of their home high-speed Internet service to access their corporate networks. Business-grade, high-speed broadband connections can also provide a cost-effective solution for connecting remote offices.

     There are two types of VPN access:

     • Site-to-site VPNs – Site-to-site VPNs connect entire networks to each other; for example, they can connect a branch office network to a company headquarters network, as shown in Figure 1. Each site is equipped with a VPN gateway, such as a router, firewall, VPN concentrator, or security appliance. In the figure, a remote branch office uses a site-to-site-VPN to connect with the corporate head office.
     • Remote-access VPNs – Remote-access VPNs enable individual hosts, such as telecommuters, mobile users, and extranet consumers, to access a company network securely over the Internet.

Service provider network infrastructure

Service provider networks are complex. They consist mostly of high-bandwidth fiber optic media, using either the Synchronous Optical Networking (SONET) or Synchronous Digital Hierarchy (SDH) standard. These standards define how to transfer multiple data, voice, and video traffic over optical fiber using lasers or light-emitting diodes (LEDs) over great distances.

Note: SONET is an American-based ANSI standard, while SDH is a European-based ETSI and ITU standard. Both are essentially the same and, therefore, often listed as SONET/SDH.

A newer fiber optic media development for long-range communications is called Dense Wavelength Division Multiplexing (DWDM).

Characteristic of DWDM:

• Enables bidirectional communications over one strand of fiber.

• Can multiplex more than 80 different channels of data (i.e., wavelengths) onto a single fiber.

• Each channel is capable of carrying a 10 Gb/s multiplexed signal.

• Assigns incoming optical signals to specific wavelengths of light (i.e., frequencies).

• Can amplify these wavelengths to boost the signal strength.

• Supports SONET and SDH standards.
• It’s used in long range communication, like connections between ISPs.

 

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