Friday, March 29, 2013

VPN (Virtual Private Network)


             Until recently, there has always been a clear distinction between public and private networks. A public network, like the plain old telephone service (POTS) or the Internet is a collection of unrelated systems that are allowed to exchange information freely with each other. A private network is composed of computers that are owned and administered by a single organization that share information with each other. Different sites in a private network are interconnected using dedicated leased lines to ensure that inter-site connectivity is always private. An enterprise that deploys a private network is assured that it is the only organization using the network.

             As it is most commonly defined, a virtual private network (VPN) allows two or more private networks to be connected over a publicly accessed network. In a sense, VPNs are similar to wide area networks (WAN) or a securely encrypted tunnel, but the key feature of VPNs is that they are able to use public networks like the Internet rather than rely on expensive, private leased lines. At they same time, VPNs have the same security and encryption features as a private network, while taking the advantage of the economies of scale and remote accessibility of large public networks.

  A VPN is an especially effective means of exchanging critical information for employees working remotely in branch offices, at home, or on the road. It can securely deliver information between vendors, suppliers, and business partners, who may have a huge physical distance between them. VPN connect both branch offices and telecommuters into an enterprise-wide corporate network via the Internet, and can eliminate all long distance charges, along with the management and security responsibilities of maintaining private networks. 

Saturday, March 23, 2013


Introduction : -

The name, General Packet Radio Service (GPRS) doesn't convey much information to the non-technical user. Describing it as providing a direct link into the Internet from a GSM phone, is much clearer. GPRS is to mobile networks what ADSL (Asymmetric Digital Subscriber Line) is to fixed telephone networks - the favoured solution for providing fast and inexpensive Internet links.
GPRS will undoubtedly speed up a handset's Internet connection - but it remains to be seen exactly how much speed can be wrung out of the system. GPRS works by amalgamating (aggregating) a number of separate data channels. This is feasible because data is being broken down into small 'packets' which are re-assembled by the receiving handset back into their original format. The catch is that the number of receiving channels does not necessarily have to match the number of sending channels. On the Internet, it is assumed that you want to view more information (such as a complicated Web page) than you want to send (such as a simple Yes or No response). So GPRS is an asymmetric technology because the number of ‘down’ channels used to receive data doesn’t match the number of ‘up’ channels used to send data.

The task of defining GPRS has been the responsibility of the Special Mobile Group (SMG) - part of the 3GPP initiative (3rd Generation Partnership Project).  Rather than wait for the final version of the SMG standard some manufacturers decided to go with GPRS handsets which conformed to an earlier version of the specifications known as SMG29. This basically offers two 'down' channels and a single 'up' channel. In practice each channel is offering around 12-13 Kbit/s so the top speeds works out to be around 26 Kbit/s. Most experts agree, however that full interoperability between products will come with SMG 31. This is capable of offering four 'down' channels which equates to a top speed of around 52 Kbit/s - the same as a high speed (V.90) landline modem.

GPRS is classified as a 2.5G (or 2G Plus) technology because it builds upon existing network infrastructure whereas with 3G networks it normally requires building an entirely new network. In order to compete against 3G networks, therefore, North- American operators have been looking to GPRS to provide high speed data links. Hence, manufacturers have been working on a related technology known as EDGE (Enhanced Data for Global Evolution). In order to compete with 3G, EDGE must offer links running at 384 Kbit/s and originally this equated to running GPRS three times faster. However, because GPRS has proved much slower than expected, it now needs to be seven times faster.

What does GPRS do?

GPRS provides a permanent connection where information can be sent or received immediately as the need arises, subject to radio coverage. No dial-up modem connection is necessary. This is why GPRS users are sometimes referred to be as being anytime-anywhere "always connected".The GPRS tariff structure is based on a fixed cost, dependent on the quantity of data required. In other words customers will be able to fix their operating costs without the concerns of variable billing. 
How GPRS works ?

The General Packet Radio Service is Mobile Data upgrade to a GSM mobile phone network. This provides users with packet data services (similar to the Internet) using the GSM digital radio network. Each voice circuit in GSM transmits the speech on a secure 14kbps digital radio link between the mobile phone and a nearby GSM transceiver station. The GPRS service joins together multiple speech channels to provide higher bandwidth data connections for GPRS data users. The radio bandwidth remains the same, it is just shared between the voice users and the data users. The network operator has the choice of prioritizing one or the other.
GPRS users will also benefit from being able to use GPRS while traveling as the GSM system should transparently hand over the GPRS connection from one base station to another.
Radio Interface :
Each GSM radio transceiver uses Time Division Multiplexing to deliver eight voice circuits on one radio channel. Each radio site may have one or more transceivers to provide sufficient channels to end users (maximum numbers are limited by many factors including - operators radio license, interference with other nearby GSM cells, cost of equipment, capacity of radio site infrastructure etc.)
A GPRS user may theoretically use all voice channels on one transceiver - (8 * 14 kbps) but radios to support this are not available and the operators will probably reserve at least some channels for voice circuits.
Each 14kbps channel may be shared by multiple 'connected' GPRS users (many users will be connected to the network but transmitting very little data). As a user's data requirements grow, they will use more of the available capacity within that timeslot, and then more available timeslots up to the maximum available or the maximum supported by their device.

In general the higher the data rate, the more power the mobile device will use and the shorter the battery life and the higher the transmitted RF power. If you are using GPRS with a mobile phone, do not keep it near your ear for long periods while data transfers are taking place.

Friday, March 22, 2013

Mobile IP


Mobile IP is a proposed standard protocol that builds on the Internet Protocol by making mobility transparent to applications and higher-level protocols like TCP. This paper aims at discussing the design principles of Mobile IP and how it can be incorporated with the already existing Internet architecture.
Mobile Internet Protocol is a new recommended Internet protocol designed to support the mobility of a user (host). Host mobility is becoming important because of the recent blossoming of laptop computers and the high desire to have continuous network connectivity anywhere the host happens to be. The development of Mobile IP makes this possible.
There are mainly three processes in Mobile IP:

1.Agent Discovery:
The process by which a Mobile node determines its current location and obtains the care of address.
The process by which a Mobile node request service from a foreign agent on foreign link and informs its home agent of its current care-off address.
The specific mechanism by which packets are routed to and from a Mobile node that is connected to a foreign link.

2.    Terminology

            Mobile IP introduces the following new functional entities:

            Mobile node – A mobile node is a host or a router that changes its point of attachment from one network or sub network to another. A mobile node may change its location without changing its IP address. It may continue to communicate with other Internet nodes at any location using its (constant) IP address, assuming link-layer connectivity to a point of attachment is available.

            Home agent – A home agent is a router on a mobile node’s home network that tunnels datagrams for delivery to the mobile node when it is away from home and maintains current location information for the mobile node.

            Foreign agent – A foreign agent is a router on a mobile node’s visited network that provides routing services to the mobile node while registered. The foreign agent detunnels and delivers datagrams to the mobile node that were tunneled by the mobile node’s home agent. The foreign agent may always be selected as a default router by registered mobile nodes.