Home > Articles > Networking > Network Design & Architecture

  • Print
  • + Share This

FRF.11 Voice over Frame Relay

With the basics defined (somewhat), it's time to move on to the good stuff. As mentioned, FRF.11 defines Voice over Frame Relay services. FRF.11 extends Frame Relay application support to include the transport of digital voice payloads.

One of the most common questions that arises is, "Why use VoFR rather than VoIP?" The answer is, of course, "It depends." In a VoFR environment, all the voice-capable routers must be Frame Relay network edge devices, and must be directly connected to the Frame Relay service. With VoIP, however, this is not the case because we can route voice traffic to the individual voice-capable devices just as easily as routing data. The routing process doesn’t change. Is one better than the other? It depends. What are you hoping to accomplish the technology? How is your network laid out? These questions contain too many variables to address in a short document such as this, so those questions will remain unanswered for now.

Frame Relay transports voice traffic over the same PVCs used for data traffic. This is the case unless separate PVCs have been purchased for both voice and data. If separate PVCs are provisioned for both, they can be treated differently (i.e. traffic shaped and prioritized differently if desired). Cisco's recommendation, however, is to purchase adequate bandwidth and keep voice and data on the same PVCs for simplified administration, as well as to allow the VoFR specifications (FRF.11 and FRF.12) to work together properly. A discussion of these specifications follows shortly.

This transport is accomplished by utilizing an association between a DLCI and a destination phone number.

Telephones are attached to Foreign eXchange Station (FXS) ports of the VoFR-capable device. FXS ports are ports into which any type of station equipment (i.e., phone, fax, caller ID box, answering machine, etc.) can be plugged. When an attached phone goes off-hook, the device detects it and returns dial tone. A phone number is dialed, as in any other phone call. The VoFR device must then interpret the digits and forward them if necessary.

If the call is local (i.e., the destination is also attached to the same VoFR-capable device), ring voltage is sent out the appropriate port. Local calling, however, isn't VoFR. The VoFR-capable device is configured with dial-peers—static definitions of phone numbers and their respective DLCIs on the Frame Relay network. The device examines its dial plan configuration for a match (or the longest or closest match) to the dialed numbers. Based on the static mapping of destination phone numbers to active DLCIs, a forwarding decision is made and the dialed digits are forwarded out the appropriate port with the appropriate DLCI.

Once at the other side of the Frame Relay network, the process is repeated. The remote VoFR device receives the call and processes the digits. The determination must be made as to whether this phone number is local or needs to be forwarded along another DLCI. If the destination is local to this device, ring voltage is generated on the appropriate port.

If it is determined that the digits must be forwarded again, the device examines its dial plan configuration. Once a match is found, it is forwarded back into the Frame Relay network on the appropriate DLCI. This process of one device forwarding a call from a remote source to a remote destination is known as tandem switching.

Should a device fail to match the dialed digits to its dial plan configuration at any time, a reorder tone is issued. This is (usually) the three-tone sound you hear just before a telephone company message is played to inform you that a number is no longer in service.

The Frame Relay voice frame is relatively small. Figure 2 illustrates the frame structure.

Figure 2 Voice over Frame Relay frame structure

The header is identical in voice and data frames. An FRF.11 Voice/Fax header exists that specifies the type of payload contained in the frame. It is important to note that FRF.11 differentiates between primary and supporting (or signaling) payloads. There are three types of primary payloads: encoded voice, encoded fax, and data.

Encoded voice payload is typically G.729 or G.729(a) compressed payload. This is not always the case, however. A single Frame Relay frame can carry a primary payload of three voice payloads. Each voice payload is carried in a sub-frame. This sub-frame has a sub-header, which is where the supporting (or signaling) payload type is indicated. These signaled payloads include information such as dialed digits, channel-associated signaling, in-band encoded FAX relay, and fault indications. Figure 3 illustrates this concept.

Figure 3 VoFR Frame and Sub-Frame relationship

Each sub-frame consists of a variable length header and a payload. The minimal sub-frame header is a single octet containing the least significant bits of the voice/data channel identification along with extension and length indications. An extension octet (1 Byte) containing the most significant bits of the voice/data channel identification and a payload type is present when the Extension Indication is set. A payload length octet is present when the Length Indication is set.

Dialed Digits

As mentioned previously, dialed digits are transported across in the sub-frame. There are two types of dialing: pulse and Dual-Tone Multi-Frequency (DTMF). Pulse dialing is rare in today's society, so we'll concentrate on DTMF. A common question is, "How are the DTMF tones sent across the network?" The answer is simple—they're converted to a binary value and forwarded as payload in the Frame Relay frame.

There are payload identifiers in the frame that specify a dialed digits payload. A string of 20ms windows is used to encode the edges when digits are turned on and off.

The timing of the transmission of dialed digits is dependent upon the Bellcore standard requiring that tones should be 23ms or shorter, with not less than 40ms of silence between tones. The silence is subject to an inter-digit timeout value, which is a value in seconds that the device collecting the digits should wait after receiving a digit before attempting to make a forwarding decision. If too few digits have been received or the digit pattern is not recognized, a fast busy signal results.

The 20ms window is the delta time—0ms (00000) to 19ms (10011)—from the beginning of the current frame in ms. If there is no transition, the edge location is set to 0 and the digit type of the previous windows is repeated. The dialed digits sub-frame contains a 3-bit digit type field. A value of 000 represents DTMF=OFF, whereas a value of 001 represents DTMF=ON.

When DTMF=ON, another field (digit code) takes the DTMF tone and encodes its equivalent binary value in the digit code field. Table 1 shows the characters and their equivalents.

Table 1 Digit Code Translation

Digit Code

DTMF Digit



































When the transmitter detects a validated digit or has addressing information to send, it will start sending a dialed digit payload every 20ms. Each payload covers 60ms of digit on/off information. There is a sequence number in the sub-frame that is incremented by one in each transmitted payload.

As mentioned, there are other sub-frame types for channel-associated signaling, fax relay, and fault indication. Due to length constraints, however, they will not be addressed in this document.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020