Bandwidth consumption of passthrough and relay calls is one of the most overlooked aspects of VoIP network design, and it can have a major impact on network capacity planning. Often, the VoIP network is designed with only traditional VoIP calls in mind. Modulated traffic such as faxes is often overlooked completely or the improper assumption is made that modulated communications and voice traffic can be accounted for in the same manner.

All passthrough and relay calls start out as voice calls using the user-defined codec. For this reason, it is important to know how much bandwidth a call is consuming before it switches over to passthrough or relay. Although bandwidth concerns might not be as critical in LAN environments, this is not usually the case in WANs.

Table 7-2 highlights the bandwidth consumed by some common voice codecs when being transported via the WAN protocol, Frame Relay. Even though Table 7-2 might not apply to your specific voice network, it is still essential to understand how much bandwidth your voice calls consume. If any of your voice calls transition to passthrough or relay, the bandwidth utilized per call can drastically change, which can impact bandwidth provisioning over a lower-speed link.

Table 7-2 Bandwidth Consumption for a VoIP Call over Frame Relay

Table 7-2 highlights the bandwidth consumed by some common voice codecs when being transported via the WAN protocol, Frame Relay. Even though Table 7-2 might not apply to your specific voice network, it is still essential to understand how much bandwidth your voice calls consume. If any of your voice calls transition to passthrough or relay, the bandwidth utilized per call can drastically change, which can impact bandwidth provisioning over a lower-speed link.

Table 7-2 Bandwidth Consumption for a VoIP Call over Frame Relay

Codec (bit rate)



Packets per Second

Bandwidth per Call (Kbps)

G.711 (64 Kbps)





G.711 (64 Kbps)





G.729 (8 Kbps)





G.729 (8 Kbps)





G.723 (6.3 Kbps)





The bandwidth calculations for each codec in Table 7-2 assume IP, UDP, and RTP header overhead to be 40 bytes and for the Frame Relay overhead to be 7 bytes (including a flag byte). However, assuming a constant header overhead, you can see how increasing the packetization interval includes more 10 ms DSP samples per packet and this in turn decreases the bandwidth used per call. For example, if G.711 uses the default 20 ms packetization interval, each call uses 82.8 Kbps of bandwidth. However, changing to a 30 ms packetization interval on the voice gateways lowers the bandwidth to 76.5 Kbps. Note that the initial sample value for each codec in Table 7-2 is the default on the Cisco voice gateways.

TIP has a useful tool known as the Voice Codec Bandwidth Calculator that is available to registered users. This tool allows you to select from a number of different codecs, Layer 2 protocols, and other parameters, and it then calculates the amount of bandwidth consumed for the selected number of VoIP calls. You can find the Voice Codec Bandwidth Calculator at

Because passthrough always forces the use of the high-bandwidth G.711 codec, you can see how this can be a problem if only G.729 voice calls are planned across the WAN. A single fax passthrough call at 82.8 Kbps consumes more bandwidth than three G.729 calls at 26.8 Kbps each. If you plan on transporting fax, modem, or text telephony traffic using a passthrough transport mechanism such as modem passthrough, fax pass-through, or text over G.711, ensure that the proper amount of bandwidth is taken into account.

The calculations in Table 7-2 do not take into consideration the use of bandwidth reduction mechanisms such as Voice Activity Detection (VAD) and the Compressed Real-Time Transport Protocol (CRTP). VAD can dramatically reduce voice bandwidth by not transmitting voice packets when silence is occurring. Unfortunately, VAD causes problems during passthrough (because of signal clipping) and therefore it cannot be used when G.711 is transporting modulated data. In the cases of modem passthrough and fax pass-through, VAD is automatically disabled as part of the switchover.

If CRTP is used, the amount of bandwidth consumed per call can be reduced at the expense of CPU cycles on the voice gateway. For example, CRTP enabled for a standard G.711 call drops the bandwidth over Frame Relay from 82.8 Kbps to 67.6 Kbps. Although CRTP is effective at reducing bandwidth for voice, passthrough, and even Cisco fax relay calls, caution should be exercised if CRTP is to be enabled for large numbers of calls on a single voice gateway.

Although VAD and CRTP typically lower bandwidth requirements, redundancy is an option for some passthrough and relay transport methods and has the opposite effect. Redundancy increases the bandwidth consumed per call but provides the benefit of more reliable communications in networks where packet loss, jitter, and other impairments are present. The effect of redundancy on the amount of bandwidth consumed for passthrough and relay calls is covered in the section "Redundancy" later in this chapter.

Bandwidth calculations for passthrough-based calls are quite simple because passthrough always uses the G.711 codec. On the other hand, bandwidth calculations for relay calls can be a bit more complicated. Certain assumptions and worst-case scenarios have to be made to arrive at a bandwidth consumption value that will prevent oversubscription. However, despite this additional complication of calculating relay bandwidth consumption, a large reduction in the bandwidth consumed per call is gained when using a relay transport method compared to passthrough.

The consumption of less bandwidth is one of the major benefits of relay over passthrough, and it occurs because relay demodulates the incoming data. Therefore, only the necessary information is transported across IP, meaning that a 9600 bps fax call will only occupy 9600 bps plus the additional header overhead. Passthrough, on the other hand, does not make a discrimination of what is the actual modulated data, and it samples everything, consuming much larger amounts of bandwidth.

The reason that bandwidth calculations for relay are a bit more complicated has to do with the asymmetrical nature of most modulated communications to begin with. For example, during a fax call, a page is sent by the originating fax machine to the terminating fax machine. The bandwidth consumed during this page transmission will be at a maximum in one direction but zero in the other direction because a fax communication is half duplex. In addition, all the fax T.30 signaling messages occur at 300 bps, significantly slower than page transmission speeds. Therefore, bandwidth measurements for fax relay calls usually look at the maximum page transmission speed allowed for the call. However, you should realize that this peak bandwidth is not seen for the whole fax call, and when it does occur, it occurs in only one direction. Figure 7-1 highlights the varying and asymmetrical bandwidths for a T.38 fax relay call. Cisco fax relay is similar in nature.

The T.38 low-speed bandwidth of 8 Kbps and high-speed bandwidth of 25 Kbps as shown in Figure 7-1 are commonly used values in capacity planning for T.38 fax over Frame Relay or over Ethernet. In actuality, because the Frame Relay header is a few bytes smaller than Ethernet, using a Frame Relay encapsulation with T.38 saves a few additional kilobits of bandwidth. However, for the sake of making a network design estimate, the 25 Kbps value is widely used for both Ethernet and Frame Relay bandwidth calculations. This value assumes that the T.38 fax call has negotiated at its maximum speed of 14.4 Kbps.

As touched on previously, fax relay calls use less bandwidth if the fax endpoints negotiate a rate lower than 14400 bps. For example, a 7200 bps T.38 fax relay call consumes only about 18 Kbps of bandwidth compared to the 25 Kbps needed for a 14.4 Kbps fax call. However, unless you force all the fax calls to this lower rate using the fax rate command, you must budget for the maximum speed of 14.4 Kbps. More information on the fax rate command and how you can use it to restrict the page transfer speed and consequently the fax relay bandwidth can be found in Table 10-3 in Chapter 10, "Configuring Relay," as well as the section "Fax Relay Data Rate" in Chapter 12, "Troubleshooting Passthrough and Relay."

Figure 7-1 Low- and High-Speed Bandwidths for a T.38 Fax Relay Call

Originating Fax machine

Originating Fax machine

Cisco Voice Gateway eES

Low-speed fax messaging occupies approximately 8 Kbps of bandwidth.

T.38 Fax Relay Call

CED, DIS with Optional NSF and CSI

Cisco Voice Gateway l\

Terminating Fax machine

Terminating Fax machine

DCS with Optional TSI

Training (14.4 kbps)

High-speed training and page transmission occupies approximately 25 Kbps of bandwidth in a single direction for the maximum transmission speed of 14.4 Kbps.

When planning for large numbers of fax relay calls, less bandwidth than the peak numbers discussed here will be seen for the aggregate number of calls. This occurs because all the faxes probably do not negotiate to the maximum 14.4 Kbps speed, and at any give moment not all the calls are consuming the maximum bandwidth with a page transmission. Recall that when pages are not being sent, a T.38 fax relay call needs only approximately 8 Kbps of bandwidth.

Although G3 fax calls are half duplex, modem calls are usually full duplex. However, modems rarely send and receive the maximum amount of data concurrently. For this reason, peak modem relay bandwidth is not used for planning bandwidth utilization by a modem. In fact, it is typically considered heavy modem usage when data is being sent and received more than 25 percent of the time. Therefore, an allotment of about 45 Kbps is generally allocated to each modem relay call. Table 7-3 highlights the peak bandwidth consumed by T.38 and Cisco fax relay and the average bandwidth for modem relay.

Table 7-3 Fax and Modem Relay Bandwidth Consumption

Bandwidth per

Relay Type

Call (Approximate)

T.38 fax relay (fax speed of 14.4 Kbps over Frame Relay, T.38

25 Kbps

redundancy disabled)

Cisco fax relay (Fax Speed of 14.4 Kbps Over Frame Relay with

48 Kbps

default 20 byte payload)

Cisco modem relay (V.34 modulation at a speed of 33.6 Kbps)

45 Kbps

In Table 7-3, the bandwidth for Cisco fax relay appears high because it uses a small 20 byte payload by default. However, the fax rate command has a bytes option that allows you to increase the payload size. Using the fax rate command to change the payload size from 20 to 40 bytes changes the Cisco fax relay bandwidth to a more manageable 32 Kbps per call.

TIP The bandwidth consumed by Cisco text relay is negligible, so it has not been discussed in this section. Like fax and modem relay, the bandwidth consumed is asymmetric because only one person types at a time. Fast typists may add an additional 3 Kbps of bandwidth to an existing voice call in one direction when full redundancy is enabled. In reality, the bandwidth typically used is much less than that. If only text traffic will be passed over a connection, enabling VAD for the voice call should stop all voice packets. Then, only a couple kilobits of periodic text traffic in each direction will be all the bandwidth that is consumed.

To proactively manage call bandwidths within a VoIP network, various call admission control (CAC) methods can be used. By tracking the number of calls across a link or destined to a particular location or zone, CAC ensures that network paths do not become oversubscribed. Common CAC methods include Resource Reservation Protocol (RSVP), an H.323 gatekeeper, or Cisco Unified Communications Manager (Unified CM) location-based CAC. Consult a comprehensive VoIP resource for additional information about these CAC methods or search for them at

In addition to CAC specifying bandwidth allocations for voice calls, fax and modem calls will usually have pre-assigned bandwidth allocation or adjustment values. With RSVP, a transition to T.38 fax relay causes an RSVP bandwidth adjustment to 80 Kbps, whereas transitions to modem passthrough, fax pass-through, or modem relay cause a bandwidth adjustment to 96 Kbps. If this bandwidth is unavailable, the call proceeds as best effort without RSVP.

An H.323 gatekeeper uses the same bandwidth adjustment values as RSVP. However, if bandwidth is unavailable, the transition does not occur, and the call proceeds using the original voice codec. Be aware that attempting to transport fax or modem calls using most voice codecs results in a call failure.

Unified CM can use a gatekeeper or locations-based CAC managing calls. However, Unified CM does not make any bandwidth adjustments after a voice call transitions to a fax or modem call. Whatever bandwidth has been allocated for the original voice codec from a CAC perspective will continue to be associated with the fax or modem call, too.

Implementing relay for fax, modem, or text will always save you bandwidth compared to a comparable passthrough call. In many network designs, especially those involving fax or modem traffic over a WAN, bandwidth is the overriding concern. If this is the case, a relay option will always be considered the best practice.

Was this article helpful?

0 0

Post a comment