PPP stands for point-to-point protocol. Its “packets” are actually network data, delivered over a switched network. PPP connections allow two routers to communicate over a long distance without using a router-based network. To understand how this works, you must understand the difference between logical and data link layers in the open systems interconnect model.
Bridging Two Worlds
All networking technologies are described in regards to the open systems interconnect, or OSI, model. Data traveling moves up the OSI process when sending and back down the OSI model when received. This seven-layer model includes the data link and logical layers, with network devices designed to operate at a certain layer. Routing happens at layer three, the logical layer, with data forwarded based on an arbitrary, changeable IP address. By contrast, hardware addresses are what layer two, or data link-level, devices use to communicate. These hardware addresses are unique to every connecting device in the world, burnt into the network adapter at the factory. A layer three address is like your apartment, while your layer two address like a social security number. Logical links are well-suited for wide-ranging, scalable network configurations, while data links are best suited for local, directly-connected networks. PPP bridges these two layers.
How PPP Works
PPP bridges the worlds of data and logical layers through encapsulation. When a router receives a packet, it determines the next best step in sending that packet to its intended destination. When that next-best step is a PPP link, the router places information around the packet, making it appear like a data-link frame. The company operating the data-link network can then read the packet and forward on its internal network to the pre-arranged destination. Once it is at the destination, the data-link encapsulation information is removed, and the packet continues on its way as a logical-layer packet.
The Key Piece
Conversion of a packet does not normally work this way. To deal with the encapsulation of a packet, routers must have a Channel Service Unit/Digital Service Unit. The CSU/DSU is a separate device that handles the conversion of the packet. Both ends of the link need this device in order to function, and the device is typically configured in conjunction with ISP configuration.
Common Uses of PPP
Dial-up connections use PPP to connect. The modem acts as the CSU/DSU, converting a digital packet into a format that a telephone’s switched network can recognize, until it reaches the telephone company’s digital, logical-layer equipment. More commonly, however, PPP is used by companies that need to connect over wide distances. To accomplish this, they lease a PPP circuit from a telephone or cable company. Telephone and other communication companies are the only ones with the physical wires necessary to span large geographic distances. When a circuit is leased, a special path is made between the two destinations. PPP is less necessary today, as most service providers operate vast digital networks. Virtual private networks also provide an option for companies wishing to use the Internet to make a direct, secure connection between two points. VPN are cheaper than PPP circuits, but the bandwidth is not guaranteed.
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