A wide range of technologies can be used to provide remote access. Some of the traditional technologies include Frame Relay, leased lines, ISDN, and dialup links. Newer technologies include DSL, cable modems, and wireless technologies such as 802.11x.

VPNs have also become a significant technology in the past few years. They can provide an alternative to expensive leased lines in a central office/branch office scenario. Also, they can provide security to users who use them over DSL and cable modem networks to connect to the central office. IPSec VPNs are a good example of the latter scenario.

Frame Relay

Over the past few years Frame Relay has been one of the most popular remote-access technologies. It offers a high-speed connection between a central office and a branch office.

One of Frame Relay's benefits is built-in congestion control to combat bursty traffic. As bandwidth needs have increased over the years, this technology has proven very popular. Also, Frame Relay circuits can be ordered from providers in a variety of bandwidths. Starting at 56 kbps, these are usually fractions of a T1. This allows for flexibility when planning.

Some of the services that can be used over Frame Relay networks are data, voice over IP, voice over Frame Relay, and IP Multicast.

Frame Relay operates at Layer 2 by encapsulating Layer 3 traffic such as IP within a Frame Relay frame. To improve performance, Frame Relay relies on higher-layer protocols such as TCP to overcome corrupt or dropped frames that occur during transmission. This is different from protocols such as X.25 that have built-in error checking/correction. Often Frame Relay is described as a successor to X.25.

Frame Relay employs its own addressing scheme at Layer 2 to specify a frame's destination. This feature is called a Data Link Connection Identifier (DLCI). This field in the Frame Relay header tells the Frame Relay switch where to route the frame. The DLCI can be thought of as the Media Access Control (MAC) address in the Frame Relay network.

Another advantage of Frame Relay is its capability to establish one-to-many connections. This is often called point-to-multipoint. This capability can potentially allow the redirection of traffic around an outage, provided that a partially-meshed network exists.

One of Frame Relay's drawbacks is the high cost of provisioning links. The high cost can possibly be justified in a branch office scenario, but it might be unsuitable for single remote users.

Serial Links

This type of network has also been historically popular in connecting branch offices. These lines can be ordered from a fractional T1 such as 56 kbps up to DS3s. Possible fractional T1 line speeds include 56 kbps, 128 kbps, 256 kbps, and so on. A full DS3 has a speed of 45 Mbps.

These networks do not provide any of the congestion control and error-detection capabilities that Frame Relay provides. The onus is completely on the higher-layer protocols to provide such services.

As with Frame Relay, cost is also an issue with these links.

ISDN

Integrated Services Digital Network (ISDN) remains one of the most flexible and widely offered services today. Providers all over the world offer ISDN services to users.

ISDN basically comes in two varieties:

• Basic Rate Interface (BRI), which consists of two 64-kbps Bearer (B) channels and one 16-kbps Data (D) channel. This is often represented as 2B+D, for the two B channels and one D channel. This has traditionally been the choice of many remote users who connect to the office from their residences. It remains popular, especially with users who do not have DSL or cable services available at their residences.

• Primary Rate Interface (PRI), which in the U.S. consists of 23 64-kbps B channels and one 64-kbps D channel. This is often represented as 23B+D. In Europe, PRI consists of 30 64-kbps B channels and one 64-kbps D channel. PRI services are often used when greater bandwidth is needed, such as when a connection is needed between a central office and a branch office.

Even though ISDN is offered all around the world, there are differences in the switches that providers use to provide ISDN service. When configuring ISDN, make sure that the design and configuration match the switch type and service being offered.

ISDN makes use of the same wiring used by analog phone lines. However, because ISDN is digital, the signal transmitted across the line is digital instead of analog. This allows for much higher transmission speeds. In addition, call setup for ISDN is very quick compared to that of an analog line. This is because of the use of the separate D channel. The setup is done out-of-band on the D channel, it does not disturb existing user traffic, and it takes a short amount of time. The combination of these factors makes ISDN ubiquitous, fast, and convenient.

ISDN is useful when a variety of applications need to be supported. The higher bandwidth can support applications such as videoconferencing, web browsing, e-mail, and voice services. Also, ISDN can support multiple data sources, as opposed to analog, which typically can support only one data source at a time.

ISDN lends itself to a variety of applications in the remote-access arena. Users who want to connect from home or users in a small office/home office (SOHO) typically can use BRI connections to do so.

However, in a scenario where a branch office needs a connection to a central office, PRI services can be used either as a primary link or as a backup connection that can be activated when the primary line goes down or when additional bandwidth is needed. This is often called a remote office/branch office (ROBO) scenario.

ISDN does have some drawbacks. A variety of standards are supported in different parts of the world. This results in a variety of equipment needed to support these standards and interfaces. You have to be careful when ordering, configuring, and maintaining equipment that connects to ISDN providers in different parts of the world.

Another drawback of ISDN is its cost. Because ISDN is charged on a per-usage basis, it can be expensive to operate. This is one of the reasons why ISDN is used in many scenarios as a backup to a serial link that has a flat per-month cost.

Lately ISDN has been replaced in many homes and SOHO environments by technologies such as DSL and cable modems, which offer much higher transfer rates. These services are also cheaper because they offer flat-rate pricing. The combination of these factors has made these technologies more attractive than ISDN.

Analog

Analog dialup service is the most ubiquitous remote access available. All you need is a phone line and a modem. Speeds, which started out around 300 bps, have steadily increased over the years to 56 kbps.

Users using analog dialup usually connect to an access server using a modem. The provider that operates the access server gives the user a phone number. The user connects to the access server using that phone number.

If in the same calling area, the user can connect to the provider using a local phone number. If the user is not in the same calling area, many providers have toll-free numbers. This allows users to connect without incurring long distance charges or using calling cards.

Some providers also offer software that has a list of phone numbers organized by country. Users can use this software to select the appropriate number for their location. They can then connect from all over the world.

Users can also connect to the Internet via dialup and then use VPNs to connect to their corporate networks. Many operating systems now offer native VPN solutions such as Microsoft's Point-to-Point Tunneling Protocol (PPTP).

The most obvious drawback of dialup services is the speedor lack thereof. With applications becoming more and more bandwidth-intensive and other broadband options becoming more cost-effective, users are turning away from dialup.

DSL

In the past few years, DSL has emerged as one of the technologies that can provide broadband services to homes. This technology can support both high-speed data and voice at the same time. It also can support data transfer rates of up to several megabits. Certain flavors of DSL can deliver speeds of up to 52 Mbps.

These transfer rates are made possible by using unused frequencies on copper telephone lines. The available bandwidth is divided into frequency ranges. One frequency range is used for voice, another is used for upstream data transmission, and another is used for downstream data transmission. For example, voice uses the frequency range of 0 to 3.4 kHz, and Asymmetric DSL uses the frequency ranges of 25 to 138 kHz in the upstream direction and 170 to 1104 kHz in the downstream direction. Splitters are sometimes used to separate these frequencies.

Another feature of DSL is that it is "always on." Unlike ISDN and analog, no dial-in is required. This is an attractive feature, especially for users who are accustomed to the cumbersome call setups and busy signals associated with analog dialup services.

DSL offerings can be broadly divided into two categories:

• Asymmetric DSL

• Symmetric DSL

Asymmetric DSL

In this category, the upload and download speeds differ. Here are some of the different Asymmetric DSL technologies:

• Asymmetric DSL (ADSL) As noted in the name, this technology offers differing upload and download speeds. This is the most common technology for residential and commercial use. It can be configured to reach rates of 6 Mbps.

• Rate-Adaptive DSL (RADSL) This technology uses ADSL modems that can adjust to differing line lengths and line qualities. The speed varies in this technology, depending on conditions, up to 7 Mbps.

• Very High Bit Rate DSL (VDSL) The fastest DSL technology, it has a maximum range of 4500 feet and can deliver rates of up to 52 Mbps.

• Consumer DSL (CDSL) This technology does not need a splitter like ADSL and RADSL. In those technologies, splitters are used to split the frequency ranges and protect the different ranges from interference. CDSL is slower than ADSL and offers downstream speeds of around 1 Mbps.

Symmetric DSL

In this category, the upload and download speeds are the same. Here are some of the different Symmetric DSL technologies:

• Symmetric DSL (SDSL) This technology is suited to environments that need higher upload speeds than those offered by ADSL. It is provided over a single telephone line and typically offers rates of around 768 kbps.

• Integrated Services Digital Network DSL (IDSL) As the name implies, this technology is similar to ISDN in that it can use the same terminal adapter. However, it is different in that it is always on. Also, IDSL is not metered like ISDN. It is a symmetric service offering rates of around 144 kbps.

• High Bit Rate DSL (HDSL) This technology delivers symmetric data rates of around 1.5 Mbps in both directions. It runs over two-wire pairs.

DSL has a wide range of offerings that users can choose from. Also, DSL's always-on characteristics and its support of a wide range of applications make it an attractive technology for many remote users.

DSL does have its drawbacks. Its distance limitation is a significant issue. DSL services cannot be offered beyond certain distances from the central office. Also, DSL is not as ubiquitous as other services, like dialup and ISDN.

Cable Modem Services

The demand for high-speed Internet access in the past few years has seen the rise of cable modem services as a broadband alternative. The technology takes advantage of the wide reach of cable infrastructure used to deliver television service.

Data is transmitted over the network as radio frequency (RF) signals. The cable modem converts these into digital signals. In addition to television and data signals, analog voice signals can be transmitted over the network. These systems can also perform full-duplex communications. The fiber coming from the homes of subscribers is usually aggregated in remote units, and fiber is used to connect these units to headend routers. This kind of hybrid network is also called a Hybrid Fiber-Coaxial (HFC) network.

Different frequency ranges are used to transmit in upstream and downstream directions. The cable modem uses channels in the 5-to-42 MHz range to transmit data in the upstream direction. Similarly, a TV channel in the 50-to-750 MHz range is used for downstream traffic.

Cable can support a significant amount of bandwidthenough bandwidth to allow subscribers to watch television and be on the Internet at the same time. The cable modem uses 10/100 Ethernet or USB to connect to the user's PC.

In addition to bandwidth, cable is also attractive because of the wide range of applications it can support. Data, voice, and video can all be supported by this medium.

Conversely, because cable is a shared medium, performance can be degraded if too many users are on the same segment. This is the most significant drawback of cable. The shared nature of the medium also raises security concerns, because traffic can potentially be captured using a packet sniffer.

Also, the frequency range used for upstream communications is vulnerable to interference caused by household appliances