There are two kinds of ISDN ports. Routers can also combine bandwidth from several channels in two connecting units. The majority of products use Multilink PPP for this purpose, but a few use a proprietary linking scheme, which usually requires the same vendor's product at both ends of the connection. Most routers include an important money-saving feature called dial on demand. Dial on de mand establishes a connection only when a user attempts to transmit data.
Compression can also save money by reducing the amount of time users spend transmitting data.
Most router vendors offer compression, but some sell it as an optional add-on. If you plan to purchase routers from different vendors, it's important to verify that their compression schemes interoperate. Some companies use a proprietary compression scheme, but most use Stac Inc.'s LZS compression protocol. However, because implementations of Stac LZS sometimes vary from vendor to vendor, support of the standard does not ensure that products will function together. Many routers include a feature called bandwidth on demand, which automatically adds a second channel to a connection if traffic on the first channel reaches a specified critical point.