UNIX systems provide excellent support for high-performance T1/E1 and some T3/E3 interface cards, as well as for some High-Speed Serial Interface (HSSI) adapters (up to 52 Mbps) and PRIs. Several vendors provide dedicated drivers and management software for open-source Unices. The NICs integrate nicely into a Cisco WAN network and provide varying telecommunication characteristics:
Physical interfaces: V.35, V.36, X.21, RS-232
With or without integrated CSU/DSU
RAS option: with or without integrated digital modems (DSPs)
Almost all synchronous serial NICs support the following Layer 2 encapsulation formats:
LAPB (X.25 Layer2)
Frame Relay services are deployed by carriers up to T3 bandwidth in the United States and up to E1 bandwidth in Europe and most other countries in 56/64-kbps or sometimes even smaller increments (subrates/derived channels). Configuration of X.25 or Frame Relay is similar to Cisco configurations with regard to virtual/subinterface concepts and topology (point to point, point to multipoint, and so on).