IP-Based Wireless Heterogeneous Networks

The primary mode of voice communication for a long time has been with 8 KHz band limited analog speech signals over circuit switched path. With advances in digitization of analog information, digital transmission and digital switching, and advances in computer technology, fully digital end-to-end voice communication over 64 kbps circuit switched path was possible. Such networks commonly known as public switched telephone networks (PSTN), and used ‘out-of-band’ signalling, commonly known as signalling system 7 (SS7).

Data communication in the beginning had a more over local area networks (LAN), connecting terminals to a central computer. While the wide area PSTN networks are circuit switched, the LAN-s that carried inherently digital bursty data, evolved with a packet switched character. Wide area data communication needs primarily for text and fax, was achieved by modulating the digital data over the analog waveform band-limited to 8 kHz, and transmitted and switched over the same PSTN network. The resulting digital channel speed was obviously very slow- from 300 bps onwards. As the possibility of multiplexing and high speed switching of digital data steams were realisably, convergence of networks were first envisaged in the conceptualization of circuit switched Integrated Services Digital Networks or ISDN (also known as narrowband-ISDN). N-ISDN was designed to carry real-time and conversational services, but the digital channels are naturally capable also to carry non-conversational and non-real-times data services. The need for a wide area packet data network (PDN) was specifically addressed by the International Standardization Organization (ISO) by coming up with a seven layer Open System Interconnection (OSI) model. This model can be applied to both circuit-switched packet data networks (CSPDN) or packet switched packet data networks (PSPDN). The seven layer ISO/OSI model as shown in Figure 2.1 provides the concept of peer-to-peer communication between protocol layers. This is facilitated by encapsulating a protocol entity or protocol data unit (PDU) of a higher layer by a header and a trailer of a lower and then passing on to the next lower layer. Actual communication between systems happens only over the physical layer (commonly known as the PHY). The present day Internet was initiated through an initiation by the United States Department of Defence Advanced Research Projects Agency (ARPA) of a wide area packet switched data network ARPANET. Internet is described next.