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Taking advantage of advanced spoofing and filtering requires your router to actually "route" the protocols you use. Some products actually bridge--rather than route--the protocol. Look for routers that route the protocols you want to use. With bridging alone, it's often difficult to perform line optimiz ations.

Squeezing It All In Compression across ISDN, as in any WAN scenario, reduces the actual amount of data sen t across the connection. In effect, compression increases throughput, which improves user productivity and reduces line charges. Compression algorithms don't make your router or ISDN line faster; they simply reduce the amount of data you need to send.

The standard for compression in ISDN routers is Stac LZS, which offers up to 4:1 and 5:1 compression ratios. However, though many router vendors implement Stac LZS and most implementations interoperate, some vendors support Microsoft Corp.'s Stac (MS Stac) compression, which may not work with other Stac-based compression. Microsoft has adopted the standard and modified. Many vendors support both types, but there are a few that support only one.

The benefits of compression are sometimes overrated. Much of the data that is retrieved from the Internet is already compressed--.ZIP and .JPG files, for example--so additional compression is useless. Some proprietary compression algorithms might slow down the router's performance by trying to compress an already-compressed file. The good news is that Stac LZS will back off when it sees a compressed file.

Stac LZS compresses the data in the packet; other algorithms compress the protocol data. Van Jacob (VJ) header compression, for instance, is designed to compress the header data of IP traffic. The compressed data is then decoded at the other router.

Share and Share Alike A feature to look for in SOHO ISDN routers is IP-address sharing or address translation. A router that supports this feature will save you money and effort: You'll avoid IP-addressing headaches at the corporate site because several remote workstations will appear as one to the central end of the connection.

With IP-address sharing, each remote workstation has its own local IP addresses, but all outbound packets from the remote workstations share a single IP address, which is attached at the router. Inbound p ackets destined for those machines are sorted by the router and readdressed to those specific remote workstations. Because dial-in IS Ps typically charge by the IP address, this can save on operating costs.

The Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP) are security standards that establish password-based connections to other devices. Most corporate sites will employ one of these types of authentication protocols. CHAP is more popular because it uses encrypted passwords. PAP doesn't encrypt passwords and is considered a security hole by many organizations.

NT-1 units are the channel service units/data service units (CSU/DSUs) of ISDN premise equipment. Although an NT-1's operation is slightly different, it is required for operation in much the same way CSU/DSUs are required. Most ISDN router vendors offer built-in NT-1 units, also known as a "U" interface. Units without an internal NT-1 are known as "S/T" interface units. Unless you want to string a se ries of separate ISDN devices to the line, choose a router with an internal NT-1.

At the Corporate End It is possible to place a SOHO router at the corporate site to serve as the host router, but these routers haven't been designed for that operation. If your organization is small and has few remote users, a SOHO router may work fine. But a company with large dial-in networks hosting many users will require hardware that is capable of handling the larger-scale network. The router at the corporate site can be a normal router with an ISDN interface or it can be a dedicated ISDN router. What's most important is the router support the standards described for the SOHO routers and take full advantage of ISDN. Otherwise, dial-in users won't fully benefit from their ISDN equipment.