This chapter discusses the external interfaces of the gateway, i.e., what other systems it talks to, and how.

The gateway talks to, in principle, six different kinds of external entities: SMS centers, CSD routers, a configure/monitor/control workstation (which could also be considered part of the gateway, but isn't), content servers, clients sending SMS messages via HTTP, and an HTTP proxy. The SMS centers use a variety of mostly proprietary protocols (CIMD, EMI, SMPP) over TCP/IP, modem lines, or various other carriers. The gateway needs to support as many SMS center protocols as possible, and make it easy to add new ones. Communication with phones via CSD routers is plain UDP (of the TCP/IP stack), i.e., there is no special CSD protocol. The c/m/c workstation uses HTTP, where the gateway works as an HTTP server, and similarly for those sending SMS via HTTP. The content servers also use HTTP, but with them the gateway is a client. HTTP proxies also use HTTP (but a slightly different kind), and the gateway is then also a client.