In enterprises, the gateway node often acts as a proxy server and a firewall. The gateway is also associated with both a router, which use headers and forwarding tables to determine where packets are sent, and a switch, which provides the actual path for the packet in and out of the gateway.

A computer system located on earth that switches data signals and voice signals between satellites and terrestrial networks.

· An earlier term for router, though now obsolete in this sense as router is commonly used.

A node on a network that serves as an entrance to another network. In enterprises, the gateway is the computer that routes the traffic from a workstation to the outside network that is serving the Web pages. In homes, the gateway is the ISP that connects the user to the internet.

Working of Gateway

Gateways provide a radio connection between a fixed network and the mobile elements of an MSS system. They are the correct size of receive and process a large number of simultaneous calls, which necessitates a high degree of linearity in amplifiers and mixers to minimize adverse effect of intermodulation. Gateway receives operate over a wide dynamic range, as signals transmitted from mobile suffer from fading and are affected by variations in satellite antenna gain over the coverage area. Adaptive mobile power control and gain shaping of the antenna footprint alleviate demanding dynamic range requirements to a certain extent. Gateways support network related functions such as call set-up, radio resources management, user database management, mobility management, switching, and support to network and business management centers.

The system offers communication services to mobile users operating within a predefined service area. Users communication services with other mobile or with fixed users through one of the visible satellites. Users in the fixed network are accessed through large fixed stations called Gateways which carry large amount of traffic, whereas mobile are small portable units capable of supporting channels. Mobile terminals may be mounted on vehicle such as ship, aircraft, truck or carried by individuals.

Depending on the service area and application, the space segment may consist of one or more satellites. Telemetry and control ground station, used for monitoring and controlling satellite, constitute a part of space segment. To simplify the mobile terminals, complexity is shifted to the space segment and hence satellite tend to be large and complex a 3-4 kW geostationary satellite with 5-10 footprints (or spot beams) is quite typical of second generation system and 5 kW with 100-200 spot beams is typical of third generation systems. A geostationary satellite remains almost fixed with respect to the Earth and comprises of one or more static footprints. Fixed station can operate with a single antenna with minimal tracking while network topology is simple.

The main hardware support entities of a large earth station. The antenna system is usually a large parabolic dish several metres across with low noise temperature. The RF part of earth stations comprises a coder, modulator, up-converter high-power Amplifier and a parabolic antenna typically 10 m in diameter for the C band feeder link. Signals are received by the same antenna/feed system, amplified in a low noise amplifier, down-converted, and demodulated/decoded to reproduce the message. In systems where a reference signal is transmitted for frequency synchronization, receives incorporate an automatic frequency correction loop.

The tracking system is an essential feature of a Gateway. Common tracking techniques include step-track, intelligent track and monopulse. Step-track systems are simple and low cost and therefore widely used in MSS gateways. They are susceptible to amplitude fluctuations which may be caused for a variety of reasons, such as scintillation, rain, etc. Step-track systems incorporating intelligence offer a good compromise between cost and accuracy. Monopulse systems are highly accurate and agile and therefore best suited for Earth stations supporting applications where rapid response with high accuracy is essential, such as spacecraft operations.

Gateways have to comply with various RF and network-related functional specifications to ensure that the performance of participating Earth stations is mandatory include antenna side-lope patterns, antenna gain/system noise temperature (G/T), EIRP, frequency and EIRP stability, phase noise, frequency response and group delay.

The demand assigned SCPC frequency division multiplex accessing scheme is a common MSS access method. When using such an accessing scheme, gateways install a bank of single channel units commensurate with traffic requirements. A channel unit typically comprises a voice codec/transcoder and data multiplexer, scrambler, FEC encoder, frame synchronizer and modulator. For a TDMA system, a timing generator is essential in accordance with the network’s TDMA synchronization technique.

Gateways also support forward and return link signalling. Forward signalling to mobiles is usually done over a time division multiplexed (TDM) broadcast channel, which is suitable for demodulation at mobiles. Signalling supported in the return direction includes random access burst transmission for call set-up and in-band signalling for call management. Gateways exchange signalling with the NCS for radio resource and mobility management. The method of communication; some operator communicate on radio channels, others one terrestrial links, and yet others combine the two.

Depending on the architecture, an MSS network may incorporate either a central or a distributed management. In a centrally manage system, an NCS manages a number of functions, such as transmission of bulletin board, spot beam identifiers and frequency reference pilots, radio resource and mobility management, performance monitoring and communication with systems’ business management entities. Modern gateways rely heavily on software for control and system management.

A pilot signal is used by participating gateways in the forward direction for correcting there transmission frequency to avoid adjacent channel interference.

The most significant sources of frequency error in the forward direction are long/short-term drift in Earth stations and satellite oscillator, and Doppler frequency variance due to satellite motion. In the return direction, a pilot is used for canceling frequency uncertainties caused by satellite oscillator drift and Doppler, but not frequency errors introduced by MTs due to large

frequency deviations between Gateways.

Gateways implement internet protocol to forward datagrams between networks. Gateways also implement the Gateway to Gateway Protocol (GGP) [7] to coordinate routing and other internet control information.

In a gateway the higher level protocols need not be implemented and the GGP functions are added to the IP module.

+-------------------------------+

| Internet Protocol & ICMP & GGP|

+-------------------------------+

| |

+---------------+ +---------------+

| Local Net | | Local Net |

+---------------+ +---------------+

Gateway Protocols

A gateway is a secured connection between an internal network and an external network (such as the Internet), but may include connections to other (non-public) systems. It will usually comprise a number of items of computer equipment including a firewall host, proxy servers, routers, email hosts, etc.

Functions of gateway

· Connect the two computers that use different connection oriented transport protocols.

· Can copy the packets from one connection to the other ,reformatting them if required.

· Understand the format and contents of the data..

· Translate messages from one format to another.

Gateways and Remote Access

The term "gateway" is also used to describe a router. These gateways are also referred to as remote gateways. For example, Internet routers are called gateways. The term is also used for protocol converters. A modem gateway is another example of a remote access gateway. Modem gateways allow multiple remote asynchronous terminals to connect to a network through modems.

Gateways, or protocol converters, lie at the boundary of systems that do not use the same communications architecture. They operate at the OSI Network Layer and above. A gateway accepts a data stream which has data encapsulated by several layers of one communications architecture and must construct an encapsulated message with all of the headers required for the other architecture.