Last Updated on May 15, 2026

Our previous What are the ISDN Layers? article briefly touched upon the concept of Point-to-Point Protocol (PPP). This technology is used to transfer data. Data transfer using PPP may occur over a digital telephony system such as Integrated Services Digital Network (ISDN) or the standard Public Switched Telephone Network (PSTN). An important distinction is, as mentioned earlier, PPP is used to transfer data, however; it is not used in ordinary voice communication over the telephone.

This article will define what is PPP, the components involved in this technology, how it works, how PPP relates to ISDN, and recording calls.

What is Point-to-Point Protocol?

Point-to-Point Protocol (PPP) is a data link layer network communications protocol used to establish a direct connection between two devices. PPP is utilized to encapsulate IP packets to transmit over dedicated lines between two endpoints (point-to-point links). PPP also established a standard for data encapsulation, link configuration, network layer address negotiation, network authentication, error detection, and network protocol multiplexing. PPP facilitates these functions by providing Network Control Protocols (NCPs) and Link Control Protocol (LCP) to establish configuration parameters.

What are Point-to-Point Protocol Components?

Framing Method, LCP, and NCPs are the components of PPP. These components provide a method for transmitting data over point-to-point links and contain three components:

1. Framing Method – PPP encapsulates data by identifying the beginning and end of frame boundaries using a modified High-Level Data Link Control (HDLC) format.
2. LCP – Establishes and configures the connection of the telephone line.
3. NCP – Will amalgamate all the information from the different layers of protocol that have been used and will ensure the data be transmitted over the telephone line.

How does Point-to-Point Protocol work?

A connection originates within an ISDN network which is established using signaling protocols. PPP is used by sending LCP frames to configure and test the link connection. The originator’s device uses PPP to send NCP frames to configure network layer protocols when the link has been established. Data packets can be sent over the link when the network layer protocols have been configured. The link will remain connected until LCP frames close the connection, or some external event occurs.

What are the requirements for the PPP Physical Layer?

PPP can operate over many types of DTE (Data Terminal Equipment) and DCE (Data Circuit-terminating Equipment) interfaces. This includes older physical connection standards such as RS-232-C and modern telecom grade connections such as V.35. The primary requirement of PPP is the connection must flow in both directions for sending and receiving on a permanent or temporary line in a steady stream synchronous or start/stop asynchronous bit-serial mode. PPP will not impose any data speed restrictions on transmission rate. The speed is dependent upon the type of DTE/DCE interface in use.

PPP Link Layer

PPP utilizes a set of rules and formats created by the International Organization for Standardization (ISO). These rules describe how HDLC procedures (ISO 3309-1979) modified by ISO 3309:1984/PDAD1 should package data and be sent over a network. These rules were originally designed for systems where data was sent in a synchronous environment and have been updated to work in asynchronous environments. PPP uses these rules to organize data and control how devices communicate relying on standardized definitions ISO 4335-1979 and ISO 4335-1979/Addendum 1-1979 for how messages are structured.

The PPP Frame:

The PPP Frame is described as follows:

• Flag – Consists of the binary sequence 01111110 and indicates the beginning and end of a physical frame. Size is a single byte.
• Address – Consists of the binary sequence 11111111 as the Standard broadcast address. PPP is not broadcasting to multiple devices and doesn’t assign individual station address. Size is a single byte.
• Control – Consists of the binary sequence 00000011 and calls for the transmission of user data where the frame doesn’t use sequence numbers. Size is a single byte.
• Protocol – Identifies the type of data encapsulated in the information field of the frame and is the size of one or two bytes.
• Data – Contains the datagram stipulated in the protocol field and varies in size from zero or more bytes. The default maximum length of the information field is 1,500 bytes, however; it can be negotiated by consenting PPP implementations.
• Frame Check Sequence (FCS) – Checks the PPP packet transmission for errors and may be 16 bits or 32 bits in size.

What is PPP Link-Control Protocol?

LCP provides the method for establishing, configuring, maintaining, and terminating the point-to-point data link connection. There are four phases of LCP.

1. Link Establishment Phase – LCP opens the connection and negotiates the configuration parameters before any network data can be exchanged.
2. Link Authentication Phase – Authentication protocols such as Password Authentication Protocol (PAP) or Challenge-Handshake Authentication Protocol (CHAP) are used to determine whether the link can proceed to bring up network layer protocols. This is an optional phase.
3. Network Layer Protocol Configuration Phase – NCP negotiations occur and network layer protocols are configured. NCPs may be in an Open or Closed state and network layer data can be transmitted over PPP when the NCP enters the Opened state.
4. Link Termination Phase – The user or a physical event can terminate the connection at any time.

How does Point-to-Point Protocol (PPP) relate to ISDN?

ISDN is a set of communications standards that uses digital communication to transmit voice, data, and signaling. ISDN consists of two channels, the Bearer (B) channel and the Delta (D) channel. Computers send IP packets over the B channels in an ISDN environment utilizing PPP.

How Call Recording Systems capture the audio in an ISDN environment.

The most common method to record telephone calls in an ISDN environment is Scenario #3. This configuration places the VSLogger Recording Server between the Outside Telephone Trunk Lines and the PBX or phone system. Recording lines are tapped and connected to recording boards. This enables the server to record all phone lines connected to the PBX or phone system.

Contact us to learn more about deploying this environment or setting up a call recording solution for your business.

FAQs

1. How does telephone communication use point to point protocol?

PPP historically used telephone communications primarily in dial-up internet connections over telephone lines. This has been phased out by modern broadband, fiber, and mobile networks.

2. Is Point-to-Point Protocol (PPP) used in voice communications over telephone?

No, PPP is not typically used in voice telephone communications. Traditional telephone communications utilize circuit switched networks.

3. How does Point-to-Point Protocol (PPP) relate to ISDN?

ISDN and PPP are used together to provide digital communications. The role of ISDN is to provide digital network technology to deliver a point-to-point connection. PPP runs on top of ISDN to establish the internet and data session.

4. Does Point-to-Point Protocol (PPP) carry voice over ISDN telephone calls?

No, PPP is commonly a data-link protocol for computer networking, not for voice. ISDN networks typically carry digitized voice directly over their B channels.

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Last Updated on May 15, 2026