RAS Buyer's Guide
Marcio Saito, V_1.3.0 Apr/00

About this document
   A Remote Access Server (RAS) is a network box that connects remote dialup clients to a Local Area Network (LAN. RAS are used by Internet Service Providers (ISP) and Corporate Network Managers to allow home users, small offices, and telecommuters to connect to the Internet or the corporate LAN from remote sites using analog phone lines or ISDN lines.

    This document discusses aspects to consider when selecting a digital RAS that best fits your needs. It also introduces you to the Cyclades-PR4000, which is a RAS specially designed for the medium and small-size ISP and corporate user. We have made an attempt to keep the analysis as objective and technical as possible, with the purpose of assisting you in making a responsible choice when selecting the right product for your budget and application.

Some history and background information
    Since the mid-90's, with the popularization of the Internet, the demand for dialup remote access has been growing exponentially. Until faster options are broadly available, analog phone lines and ISDN lines are the more common media to remotely connect personal computers or home offices to the LAN or to the Internet.
    A RAS is a box that attaches to the LAN at the central site and provides connection to dialup lines. It allows remote clients to access the LAN (and possibly the Internet) by using modems or ISDN terminal adapters.
    In the past, RS-232 Terminal Servers were used to provide remote access. A server-based or stand-alone box would attach to the LAN and provide multiple RS-232 serial interfaces. Those serial ports were connected to a modem bank, which on its turn, connected to the analog phone lines coming into the building.
    With the crescent demand, the number of required phone lines started to grow to a point where it was impractical to increase their number at the central site. The phone company, instead of delivering individual phone lines to you, bundle 23 or 30 channels in one T1 or E1 ISDN-PRI line.
    Typically, a RAS will have one or more Ethernet ports for connection to the LAN and one or more T1/E1/ISDN-PRI ports for the incoming calls (a T1/ISDN-PRI line is equivalent to 23 phone lines, while an E1/ISDN-PRI line is equivalent to 30 phone lines).
    A RAS will typically have internal digital modems and will be able to terminate both analog and digital calls and connect to remote clients using Point-to-Point (PPP) protocol.

T1/E1 and ISDN-PRI
    Let's look at how the Public Switched Telephone Network (PSTN) is organized.

    Regular phone lines are copper wire pairs carrying information in analog form from the customer's premise to the phone carrier's Central Office (CO). At the Central office, the analog lines are connected to a phone switch, which converts analog information into digital form and consolidates multiple connections in higher-density digital trunks (such as T1 and T3).
    In the US and Japan, a digital trunk uses T1 speed of 1.5Mbps (equivalent to 24 analog channels). In Europe and most of the other countries, a digital trunk runs at E1 speeds of 2Mbps (equivalent to 32 analog channels). A digital trunk is Time Division Multiplexed (TDM) line that carries digitized information from several channels in digital form.
    At the Central Office closest to the destination, the call is converted back to analog form and carried to the customer premises by a copper wire pair.
    Notice that phone connections are analog only in "the last mile", while it travels between the CO and the customer premises. Nowadays, most phone switches are digital and the information travels between COs in digital form as time slots within T1/E1 or T3/E3 TDM trunks.
    So, the solution for the problem of explosive demand for phone lines was not to convert the information from digital form back to analog at the CO. The phone carrier brings the digital T1 or E1 trunk directly to the customer premises.

    That way, only 2 pairs of copper wire can carry up the equivalent to 24 or 32 analog channels (because one or 2 channels are reserved for control purposes, the actual number of phone lines is usually 23 for T1 and 30 for E1).
    Since the access lines are not arriving as individual phone lines, we cannot use the traditional analog modem banks to provide connectivity. We need equipment that can interface directly to the T1/E1 trunk, separate the multiple channels in the TDM bus and process the modem connection in digital form.
    Those boxes are the Integrated Digital RAS and the Cyclades-PR4000 is one example. Being digital, it can terminate both analog (modem) and digital (terminal adapter, ISDN-BRI) calls.
    Another advantage of processing calls digitally is that it allows (the explanation is out of the scope of this document) support of new modem technologies (k56 flex, V.90) that can provide throughput higher than the V.34 limit of 33.6kbps.

Stand-alone or Server-Based?
    Traditionally, RAS are stand-alone, integrated and self-contained boxes running an Internetworking Operating System on a proprietary hardware specifically designed for Remote Access. They connect to the LAN and to the income dialup lines and all you need to do is to configure them properly to get them working.
    But there is also the possibility of building a RAS using standard server hardware (usually a PC) with a Networking Operating System (typically, Linux or Windows). The user installs PCI RAS adapter (with digital modems and ISDN-PRI interfaces) in a PC server, install the OS and the proper device drivers and configure the system to work as a Remote Access Server.
    So, maybe the first question you face when selecting a RAS solution is this: Stand-alone or server-based?
    In general, stand-alone solutions are better for the average user because of the following factors:
    Integration.
    The RAS solution is complete and there is no need for software or hardware integration. That eliminates the     possibility of hardware and software incompatibilities and installation problems and gives you a single point of contact for technical support and problem solving.
   Reliability.
    Stand-alone solutions run on hardware and software that was specifically designed for remote access. They are more integrated, more compact, dissipate less power, and, because of that, are more reliable than a solution based on general-purpose components.
    Cost.
   Usually, commodity PC hardware and open source software tend to drive costs down, making server-based solutions attractive for technical users who already have the PC and are willing to face the potential problems of integrating the solution from standard components. But, the PCI adapters for RAS available in the market today still carry a price tag that makes it difficult to justify a server-based solution.
    With today's technology, it is very difficult to design good RAS boards to be installed in standard PC hardware. Most of the board products in the market today are unreliable and expensive. The complexity of the board and the large number of components create large boards (sometimes with multiple circuit boards in a sandwich) that generate a lot of heat and have special power and mechanical requirements.
    As of today (early 2000), new technologies are becoming available and will allow hardware vendors to design more compact, cost-effective and integrated board products in the future. Until them, the use of server-based solutions will be restricted to very special cases.
    The Cyclades-PR4000 is a stand-alone RAS that doesn't depend on any additional hardware to connect T1/E1/ISDN-PRI access lines to the LAN.

What about Cost?
    RAS equipment provides a lot of functionality. They integrate the TCP/IP routing and PPP server functions, the DSU/CSU units to connect to the communication lines, the digital modems that can support V.90 connections, the HDLC controllers to support ISDN digital connections.
    Because of that, RAS boxes usually are a significant expense, especially for the small ISP. Current (Mar/2000) market prices for a RAS box range from around US$5k to US$20k for a configuration with 1 T1/24 modems.
    When considering the cost of the solution, you have to analyze what are the features needed in your application. For example, you should not pay for extra scalability if you don't expect to grow above the limit of a lower-cost product.
    Besides the cost of the product itself, you should investigate the total cost to own and operate the solution. Some older products have special installation requirements. Most vendors will provide technical support and software updates only for a very limited time and will impose expensive maintenance contracts to support and update the product after that.
    The Cyclades-PR4000 uses newer hardware technology and has higher level of integration when compared to most of the products in the market. Also, because we have designed with the medium and small-size user, the product is simple, cost-effective, and easy to install and configure.
    Because of that, the PR4000 ranks among the most cost-effective in its class. As all Cyclades products, the PR4000 comes with free and unlimited technical support and software updates.

Digital and Analog Calls
    A Remote Access Server should be able to terminate both analog (V.90, V.34 and older modem standards) and ISDN-BRI digital calls (including V.110 rate adaptation, which is the standard for mobile phone digital access in Europe).
    Because there is a very wide range of client modems being used in the field, the digital modems should preferably be 100% software based. That way, it is relatively easy to change the modem software to accommodate changes in the modem standards and fix possible modem interoperability problems.
    Some of the RAS products in the field still use Rockwell and Lucent hybrid modem chips (which have only partial flexibility as far as modem software updates).
    The RAS needs to exchange signaling information with the switch at the Central Office. So, it is important to verify that the product is compatible with the signaling standards, which vary from country to country.
    The PR4000 supports up to 64 internal digital modems, which are implemented 100% in software running on general purpose DSPs. The modems are required only to terminate analog calls and don't need to be installed to terminate ISDN calls.
    It is compatible and certified for use with any phone switch that uses ISDN-PRI signaling, be it North American or European standard. It also supports the ITU/T standard for countries that still use R2D signaling, as well as support for several other countries that use specific R2D signaling.

Flexibility, Modularity and Scalability
    Even when looking for the most cost-effective solution, you should demand some level of flexibility, modularity and scalability so that the product can accommodate your changing requirements.
    The LAN interface should support both 10 and 100BT Ethernet connections. The number T1/E1/ISDN-PRI dialup trunks should match your needs. Each T1 line carries 23 connections while an E1 line carries 30 connections.
    The digital modems usually represent most of the cost of a RAS equipment and you should not need to get more modems than what you need (some products have always the 24/32 modems soldered on the main board).
    When remote uses use multiple lines to access the LAN (multiple phone lines or multiple ISDN B-channels), the RAS should be able to support Multilink PPP (MLPPP). MLPPP allows 2 or more channels to be bundled together to look like single faster link.
    Also, if the number of lines grow to the point where you need to stack more than one box (in the case of the PR4000, 2 T1/E1 lines), you should verify if the product supports Multi-chassis MLPPP to allow bonding of channels across different boxes.
    The PR4000 supports 2 Ethernet LAN ports (one is 10/100BT, including optional optical fiber support, and the other is 10BT), 2 T1/E1/ISDN-PRI and up to 64 digital modems (available in modules of 8 modems).
    Besides being a RAS, the PR4000 is a full featured router and supports one optional WAN routing interface, so that you can have a true "POP-in-a-box". If one of the built-in T1/E1 ports is not being used as an access trunk, it can also be configured as a WAN port.

Software features
    A RAS should support all the important protocols needed to connect your remote users.
    TCP/IP became the standard network protocol for the Internet and corporate networks. RIP and OSPF are the two most common routing protocols. PPP, MLPPP and Multi-chassis MLPPP are the encapsulation protocols used on the dialup connections.
    Authentication is a major concern when setting a remote access infrastructure. An authentication server can centralize the authentication process and provide logging and accounting information. RADIUS and TACACS are the most used authentication server protocols. PAP and CHAP authentication should be supported by the PPP protocol.
    The PR4000 supports all the protocols mentioned above, including RADIUS, TACACS and XTACACS for authentication server and PAP, CHAP and plain text for PPP authentication.

The Cyclades-PR4000