Home > Articles > Cisco Network Technology > IP Communications/VoIP > Voice, IP, and ATM MPLS Features

Voice, IP, and ATM MPLS Features

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: May 17, 2002.

Voice Connections

A voice connection is an end-to-end permanent virtual circuit (PVC) that originates and terminates on MGX VISM endpoints. The connection receives PCM voice samples and signaling and converts them into ATM cells using AAL2 (voice traffic and CAS signaling) or AAL5 (CCS signaling) and transports the cells to the remote endpoint. All voice connections are bidirectional, meaning that traffic flows in both directions.

A single AAL2 voice connection can carry multiple voice calls. This is called AAL2 multiplexing. The channel identifier (CID) differentiates voice traffic streams on the connection.

You can add two types of voice connections in an MGX network: feeder and local. Feeder connections go from the VISM to the PXM trunk and are transported through the ATM backbone network (BPX or other) to the destination MGX switch. Local connections go from one endpoint on an MGX switch to another endpoint on the same switch. Local connections can be between endpoints on the same or different VISM cards. Feeder and local connections perform the same functions.

Endpoints

All VISM connections terminate on endpoints. An endpoint is a channel (DS0 or timeslot) on a T1 or E1 line. An endpoint is defined by its endpoint number and the T1 or E1 line and channel number. You must create VISM endpoints before you can terminate connections on them.

Channel Identifiers

A CID identifies a specific voice traffic stream. You use it when multiplexing multiple voice calls across a single ATM connection. On the MGX switch, the CID also links the connection to the endpoint. When you create a CID, you identify the compression type and other voice processing characteristics, such as echo cancellation and dual-tone multifrequency (DTMF) transport. Figure 22-1 shows the VISM with endpoints, connection, and CIDs.

Figure 22-1 Channel Identifiers

Feeder Example

A voice feeder connection has three connection segments:

  • A master connection between the VISM endpoint and PXM1 trunk on one MGX switch.

  • A routing connection through the ATM backbone network. The routing connection should be rt-VBR for voice traffic and nrt-VBR for signaling traffic.

  • A master connection between the VISM and PXM1 trunk on the other MGX switch.

Figure 22-2 shows a voice feeder connection in an MGX network.

Local Example

An ATM local connection has two connection segments:

  • A slave connection on one VISM. The slave connection must be added first.

  • A master connection on the other VISM pointing to the slave connection.

Figure 22-2 Feeder Example

How PCM Samples Are Converted into ATM Cells

How are ATM cells created on Voice over AAL2 (VoAAL2) services on the VISM?

Figure 22-3 shows a high-level view of the traffic flow of voice traffic on the VISM.

Figure 22-3 Traffic Flow

Pulse Code Modulation (PCM) voice samples come in from the line and are processed by echo cancellation digital signal processors (DSPs) to eliminate any echo (if echo cancellation is enabled). Next, the PCM samples are processed by the compression DSPs. This is where the 8-bit PCM samples are converted into a variety of different samples (length and frequency), depending on the compression method in use. The compressed samples go to the segmentation and reassembly (SAR) processor to be loaded into ATM cells for transport to the cell bus and across the network.

AAL2 Segmentation

ITU-T Recommendation I.363.2 specifies the basic AAL2 structure for VoAAL2. Figure 22-4 shows three voice samples (from the same or different voice calls) and how they are made into an ATM cell.

Figure 22-4 Voice Sample Conversion to ATM Cell

Table 22-1 describes each stage of the AAL2 segmentation process.

Table 22-1 AAL2 Segmentation Process

Stage

Description

Voice samples

The fixed-length voice samples come from the codec. In this example, each voice sample is 10 bytes long, but the length depends on the compression method used.

CPS (Common Part Sublayer) packet

A 3-byte CPS header is added to each voice sample. Because the voice samples could come from different traffic streams (channels or calls), the CPS header differentiates the channels with the CID.

CPS-PDU

The CPS-PDU includes a 1-byte start field at the beginning of the CPS packets. Padding is added to the data to make the CPS-PDU exactly 48 bytes long.

ATM cell

The 5-byte ATM header is added to the CPS-PDU (ATM cell payload), and the cell is then ready to transport through the network.

LI (Length Indicator)

The number of cells with a payload length that does not match the LI in the CPS packet header.


AAL2 Coding

Here are the coding or compression methods supported on the VISM:

  • G.711u—PCM with µ-law coding

  • G.711a—PCM with A-law coding

  • G.726—ADPCM

  • G.729—CS-CELP with 10-, 20-, and 30-millisecond (ms) cell times

The coding type you use affects the voice sample size and the sample and cell frequency. Ultimately, this determines how much bandwidth the voice traffic utilizes. Figure 22-5 shows each coding type and the ATM cell created from the voice samples, assuming that multiplexing (multiple voice streams on the same ATM connection) is not in use.

Figure 22-5 AAL2 Coding Types

6. Voice Over AAL2 Network | Next Section Previous Section

Cisco Press Promotional Mailings & Special Offers

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

Overview

Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Cisco Press 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.

Surveys

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.

Newsletters

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

Security

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

Children

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

Marketing

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.

Choice/Opt-out

Users can always make an informed choice as to whether they should proceed with certain services offered by Cisco Press. 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.ciscopress.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.

Links

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