Understanding the Point-to-Point Protocol (PPP) for WAN Connectivity

Are you wondering what WAN protocol your company uses to connect to the internet? If you have a T1 leased line or a private network between locations, chances are you're utilizing one of these three common WAN protocols: HDLC, PPP, or Frame Relay. In this article, we'll focus on the Point-to-Point Protocol (PPP).

What is PPP? 

The Point-to-Point Protocol (PPP) is a data link layer (layer 2) protocol that encapsulates network layer frames for transmission over various physical links, such as serial cables, cellular networks, and fiber optic cables. PPP offers more features than HDLC and is an open standard protocol.

Key Features of PPP:

• Link Quality Management: Monitors link quality and shuts down the link if too many errors occur.
• Authentication: Supports PAP (Password Authentication Protocol) and CHAP (Challenge-Handshake Authentication Protocol) for user authentication.
• Multilink: Allows bundling of multiple physical links into one logical link for increased bandwidth.

PPP Operation: PPP operation relies on three main components:

1. Frame Encapsulation: Uses HDLC framing for encapsulating packets.
2. Link Control Protocol (LCP): Establishes, configures, and tests the data link.
3. Network Control Protocols (NCPs): Negotiate network layer protocols (e.g., IP, IPX, AppleTalk) to be used over the PPP link.

Link Control Protocol (LCP): LCP is the core protocol used by PPP for link establishment and configuration. It operates at the data link layer and performs the following functions:

• Determines maximum packet size for transmission
• Detects link configuration errors
• Terminates the link
• Determines link failure
• Negotiates PPP options like authentication, error detection, and compression

Network Control Protocol (NCP) Layer: NCPs enable PPP to transport various network layer protocols such as IP, IPX, and AppleTalk over the PPP link.

Establishing a PPP Session:

1. LCP negotiates PPP configuration options and acknowledges the link setup.
2. LCP optionally tests the link quality to ensure it meets requirements for layer 3 protocols.
3. NCP configures the network layer protocols to be used over the PPP link.

Configuring PPP: To configure PPP, you'll need to set up the appropriate PPP options on the serial links between routers and PCs. This typically involves configuring authentication methods, link quality monitoring, and network layer protocols.

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