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This chapter is from the book

This chapter is from the book

Public WAN Infrastructure (2.2.3)

In this topic, public WAN infrastructures are discussed, including DSL, cable, wireless, 3G/4G cellular, as well as the need to secure data using site-to-site VPNs and remote-access VPNs.


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, and then an IP connection is made through the Internet to the enterprise. Security risks are incurred in this process, but can be mediated with security measures.

The topology in Figure 2-23 displays a sample DSL WAN connection.

Figure 2-23

Figure 2-23 Sample DSL Topology

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.

The topology in Figure 2-24 displays a sample cable WAN connection.

Figure 2-24

Figure 2-24 Sample Cable Topology

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 Kbps 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.

    Figure 2-25 displays an example of a WiMAX network.

    Figure 2-25

    Figure 2-25 Sample Wireless Topology

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 smartphones 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. As shown in Figure 2-26, these devices use radio waves to communicate through a nearby mobile phone tower.

Figure 2-26

Figure 2-26 Sample Cellular Topology

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 third-generation and fourth-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.

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 2-27. 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.

    Figure 2-27

    Figure 2-27 Sample Site-to-Site VPN Topology

  • 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. Each host (Teleworker 1 and Teleworker 2) typically has VPN client software loaded or uses a web-based client, as shown in Figure 2-28.

    Figure 2-28

    Figure 2-28 Sample Remote-Access VPN Topology

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