General Passthrough and Relay Design Considerations

A number of design considerations must be looked at when designing VoIP networks that will successfully handle modulated communications such as fax, modem, and text. Not taking into account these design considerations from the beginning can cause problems later upon implementation.

This section focuses on passthrough and relay design considerations in a general sense, meaning that the information in this section applies equally to faxes, modems, and text devices. These considerations are summarized in Table 7-1. More specific design considerations directly applicable to fax, modem, and text communications are covered in subsequent sections of this chapter.

Table 7-1 General Passthrough and Relay Design Considerations Summary

Design Consideration

Explanation

Bandwidth

Utilizing much less bandwidth per call is one of the main

benefits of relay. Passthrough relies on the G.711 voice codec

for transport, and this uncompressed codec comparably

consumes much more bandwidth.

Call control protocol

Not all call control protocols support all the various passthrough and relay transport methods. Therefore, it is important to know the limitations of each call control protocol from a fax, modem, and text transport perspective.

Quality of service (QoS) The QoS requirements for modulated communications can be different from what is needed for a typical VoIP call. For example, fax traffic can handle a higher end-to-end delay than

a standard VoIP call, but it typically cannot tolerate the same degree of packet loss.

Redundancy

Relay protocols typically offer built-in redundancy options,

whereas the redundancy option with passthrough is less robust

and not always supported.

Resource utilization

Certain passthrough and relay calls can be resource intensive

to the voice gateway as certain thresholds are approached.

Secure Real-Time Transport

Fax and modem calls can use the secure RTP feature, but only

Protocol (SRTP)

for transport methods that make use of a full RTP header.

Timing and synchronization Certain clocking dependencies exist that can affect fax, modem, and text calls. This is especially true when a form of the pass-through transport method is implemented as digital signal processor (DSP) playout buffers may eventually slip on calls of a significantly long duration.

Timing and synchronization Certain clocking dependencies exist that can affect fax, modem, and text calls. This is especially true when a form of the pass-through transport method is implemented as digital signal processor (DSP) playout buffers may eventually slip on calls of a significantly long duration.

Each design consideration in Table 7-1 is discussed in more detail in the following subsections. In addition, as you read each subsection, you will notice differences between passthrough and relay. Design considerations for one transport method are not always applicable to the other. If you are trying to decide whether passthrough or relay should be used for a particular network design, these subsections provide some valuable information.

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