VoIP systems and networking systems are often alike as these technologies originate from legacy circuit-based telephone infrastructure. These communications systems have protocols and layers that govern their implementation. Among these specifications is the Open Systems Interconnection standard developed in 1984, which contains the OSI Reference Model defining the various stages that data must pass through when travelling from one device to another device over a network.

The OSI Reference Model layers

The seven layers – arranged for easier understanding on a two-set, top-down arrangement with each layer relying on the layer beneath for support while giving support to the layer on top of it – with their corresponding functions are the following:

Application Set

Layer 7: Application – allows application to interact with the network for transferring files, opening web pages and other network-related tasks

Layer 6: Presentation – takes the input data from the Application layer and formats this into a type that can be decoded by other layers

Layer 5: Session – establishes connection with the receiving device, maintains the connection, and terminates the connection after the task is completed

Transport Set

Layer 4: Transport – maintains data flow control from various applicationa and ensures error checking and recovery of data between the devices

Layer 3: Network – determines the path the data will take and handles logical addressing and error synchronization to ensure that the data is correctly sent

Layer 2: Data – assigns the physical addressing protocol to the data, and defines the network type and sequencing of packets

Layer 1: Physical – defines the physical characteristics of the network such as connections, voltage levels and timing at the level of the actual hardware

The layers in a VoIP environment

Voice calls inevitably pass through the OSI layers even before the introduction of VoIP. The only difference is that users now have to contend with the various problems that may arise in VoIP calls, whereas these issues were the exclusive domain of telephone companies before. In short, users must now be concerned with call quality problems, packet routing, bandwidth allocation, and physical problems of the hardware and cables.

The roles of the layers in a VoIP setting and the possible problems that may be encountered in VoIP calls are the following:

First Layer – electrical shorts in the cables and failure in any of the equipment along the way may generate static, latency and line noise in the Physical layer

Second Layer – since a mere cursory check of the packets is done here, only minimal latency, jitter and packet loss is introduced in the Data layer

Third Layer – the Network layer has routers that may introduce latency, jitter and packet loss as well as distortion in signals, resulting in volume discrepancies

Fourth Layer – in the Transport layer, packet loss and jitter are likely to occur in sending of datagrams via User Datagram Protocol (UDP) and sending/receiving of media (voice) in Real-time Transport Protocol (RTP), resulting in robotic voices

Fifth Layer – VoIP calls are started with the Session Initiation Protocol (SIP) in the Session layer, but it’s Secession Description Protocol (SDP) together with RTP in the previous layer that are the workhorses that do most of the processing of the voice component in the transmission, and hence, they are susceptible to most latency and jitter

Sixth Layer – the Presentation layer is where data is encryped/decrypted and audio encoded/decoded using VoIP codecs and has inherent latency due to the delays in the conversions that must take place

Seventh Layer – the Application layer is where the conversion of packets into sound occurs, which may introduce additive noise such as those from background noises or subtractive noise resulting from lost packets

In VoIP systems, it appears that latency or delay is the most persistent issue that affects call quality and it can be introduced at any of the layers. Thus, designers and architects of VoIP systems for businesses must use this information to address and minimize future problems.

Andrew Wiggin is an IP Communications and business software expert. He enjoys promoting VoIP and recommends finding great hosted PBX and SIP providers for a fantastic experience with this emerging technology.