Last Updated on August 10, 2026
Our previous What is ISDN? article defined ISDN as a set of communications standards that uses digital communication to transmit voice, data, and signaling over the PSTN. It described the two channels of ISDN. The first is the B channel to transmit, or “bear” data, at a rate of 64k and is used for voice, video, data, or multimedia. The second is the D channel to transmit data at a rate 16k or 64k and is used for signaling. There are two interfaces called Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI provides two B channels and one D channel whereas North America and Japan PRI provide 23 B channels, 32 in Europe, and one D channel.
This article will dive deeper into ISDN, identify, and describe the multiple layers of ISDN.
What is Point-to-Point Protocol (PPP)?
Point-to-Point Protocol (PPP) is a data link layer communications protocol. PPP provides data compression, authentication, encryption, and loop detection between two routers. PPP is utilized in ISDN to transmit packets utilizing an encapsulation process. ISDN utilizes circuit switching to establish a dedicated PPP connection between two nodes.
What is the Open Systems Interconnection (OSI) model?
The Open Systems Interconnection (OSI) model is a framework that standardizes the coordination of interconnection for devices within a network. The International Organization for Standardization (ISO) developed the OSI model for communication. The components of ISDN are established in seven layers of abstraction according to the OSI model.
What are the layers in ISDN?
There are two layers known as the Media layer and Host layer. The Media layer consists of three sub layers known as the Physical (Layer 1), Data Link (Layer 2), and Network (Layer 3). The Host layers are known as Transport (Layer 4), Session (Layer 5), Presentation (Layer 6), and Application (Layer 7). The Media layers facilitate the physical transmission of data while the Host layers define how data is formatted and presented to the user.
Layer 1: Physical layer
Layer 1 is the physical connection between devices such as network interface controllers, Ethernet hubs, switches, cables, and other hardware. These devices transmit digital bits from one device to another. This layer also defines how encoding occurs and the bit rate control such as simplex, half duplex, and full duplex.
Layer 2: Data link layer
Layer 2 manages data transfer between two devices, or nodes, within the same network. It provides error detection and correction that may have occurred in the physical layer. It also defines the protocol and flow control to establish and terminate connections between two nodes.
Layer 3: Network layer
Layer 3 establishes logical device addresses on the network and identifies the best route for data to travel within and between networks. It also determines the best method to transfer the data such as splitting large messages into fragments, sending fragments independently, and reassembling them at another node. It may optionally report delivery errors.
Layer 4: Transport layer
Layer 4 manages network traffic for host-to-host application communications. The rules for packet delivery are executed and data that was broken down are reassembled. Protocols such as TCP and UDP control data volume, transfer rate, and destination. This prevents congestion, provides error checking, and retransmission requests when necessary.
Layer 5: Session layer
Layer 5 manages the setup, connection control, and tears down the communication between two or more devices for a specified period of time. For example, in a web conference, the Session layer creates the connection among multiple devices, monitors audio and video flow to ensure they are synced, and tears down the connection at the end.
Layer 6: Presentation layer
Layer 6 is sometimes called the syntax layer. It translates data into a format specified by Layer 7 and the network. The Presentation layer provides data encryption / decryption, data compression / decompression, graphic commands, and translates the data to make the information usable for Layer 7 applications.
Layer 7: Application layer
Layer 7 is where the application is utilized by the end user. It provides the interface for communicating with user facing applications and the server.
How to record calls in an ISDN environment
Scenario # 3 is a method to record call on and ISDN environment utilizing Versadial Solutions. Analog lines are tapped between the trunk and the PBX in this environment by a VSLogger Recording Server. The VSLogger Recording Server contains a Network Interface Card (NIC) and is connected to a network switch via the SPAN/Mirror Port between the trunk and the PBX in a VoIP and SIP environment. The VSLogger records all calls via a passive method meaning the call recorder does not actively participate in the recording process. Learn more about Active vs Passive call recording.
Contact us to learn more about deploying this environment or setting up a call recording solution for your business.
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Analog Signaling vs Digital Signaling. How Analog and Digital Signaling work in Telephony systems
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Last Updated on August 10, 2026