Point-to-Point Protocol and Data Link Layer Design
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Point-to-Point Protocol (PPP)
PPP stands for Point-to-Point Protocol. It is the most commonly used protocol for point-to-point access. For instance, if a user wants to access the internet from home, the PPP protocol is typically employed.
As a Data Link Layer protocol residing in Layer 2 of the OSI model, it encapsulates Layer 3 protocols and all payload information for transmission across serial links. The PPP protocol functions on synchronous links like ISDN as well as asynchronous links like dial-up. It is primarily used for communication between two devices.
Physical Network Compatibility
PPP operates over various physical networks, including:
- Serial cables
- Phone lines
- Trunk lines
- Cellular telephones
- Fiber optic links (such as SONET)
Since the Data Link Layer identifies where a transmission starts and ends, Internet Service Providers (ISPs) use the PPP protocol to provide dial-up internet access.
Data Link Layer Design Issues
1. Services Provided to the Network Layer
The Data Link Layer (DLL) acts as a service interface to the Network Layer. Its primary function is transferring data from the sending machine's Network Layer to the destination machine's Network Layer via the DLL.
It provides three types of services:
- Unacknowledged and connectionless services
- Acknowledged and connectionless services
- Acknowledged and connection-oriented services
2. Frame Synchronization
The source machine sends data in blocks called frames. The destination machine must identify the start and end of each frame so that the data can be recognized correctly.
3. Flow Control
Flow control is implemented to manage the delivery of data frames at the receiver end. The source machine must not send data frames at a rate faster than the destination machine's capacity to accept them.
4. Error Control
Error control is used to prevent the duplication of frames. Any errors introduced during transmission from the source to the destination must be detected and corrected at the destination machine.