Home > Articles > Networking > Voice/IP Communications

  • Print
  • + Share This
This chapter is from the book

This chapter is from the book

Trunk Provisioning

The capacity of a SIP trunk is normally defined by the number of simultaneous calls supported and the bandwidth provided for the trunk. An enterprise uses the same Erlang calculations traditionally used in a TDM environment to determine the number of simultaneous calls required on a SIP trunk.

Generally service providers offer a tiered service based on capacity. One of the major benefits of a SIP trunk is that as an enterprise's needs expand, the number of simultaneous calls can be readily expanded without changing the physical interconnection, or even without an increase in provisioned bandwidth, provided excess bandwidth is already available.

Bandwidth Adjustments and Consumption

Bandwidth consumption for IP call traffic inbound from the PSTN on a TDM gateway is easily predicted and controlled because the codec assignment is done by the gateway (or by the enterprise call agent such as CUCM). The use of a CUBE can ensure that this capability is maintained when an enterprise adds a SIP trunk to its communications infrastructure.

CAC policies and features are deployed in the enterprise network based on predictable patterns of codec use by calls (that is, typically G.711 for calls within a site on the LAN and G.729A for calls that traverse the WAN between sites). The bandwidth consumption of inbound SIP trunk calls is partly based on the service provider's configuration, but an enterprise can use a CUBE to influence codec selection (also called codec filtering or stripping) or to transcode streams in the codec selections the enterprise prefers to use.

Call Admission Control (CAC)

Gateways connecting to the PSTN through a TDM interface provide an implicit form of CAC in both directions (inbound and outbound) by virtue of the limited number of channels (or timeslots) physically available on the analog, BRI, T1, or E1 interface. No more calls can simultaneously arrive from the PSTN into the enterprise than there are timeslots available on the gateway TDM trunks, providing implicit call admission control.

With a SIP trunk entering your network on a physical GE connection (possibly fiber or OC3 transport within the service provider's network before hand-off to your network), nothing physical limits the number of calls that could enter or exit your network at any one time.

Top-tier service providers exert CAC control in their networks, and how much protection this offers your enterprise network depends on who your service provider is and how well the controls are implemented. But there is virtually no physical limit, and it is strongly recommended that you protect your own network with your own CAC controls at your Border Element (especially if you are considering a SIP trunk offering without an explicit SLA). This protects against occasional unplanned bursts or surges in legitimate traffic and against potential malicious Dos attack traffic. Lack of CAC control could overrun bandwidth on your network and adversely impact network operations.

One general problem with CAC implementations is that many policies are often based on simple call-counting mechanisms (such as the CUCM Locations CAC feature) as opposed to bandwidth-based mechanisms (such as Resource Reservation Protocol [RSVP]). It is therefore important to control not only the number of calls arriving through the SIP trunk, but also the codec assigned to the calls.

In addition to transcoding and codec filtering, a CUBE can support the CAC policy of the enterprise in the following two ways:

  • Limiting calls per dial peer (per destination)
  • Limiting calls based on memory and CPU

Limiting Calls per Dial-Peer

You can configure the max-conn command on both the inbound and outbound dial-peers of the CUBE to ensure that no more than the configured number of calls connects at one time. Each call, regardless of codec or the direction of the call, counts as one call.

When a call arrives at a dial-peer and the current number of calls in the connected state exceeds the configured amount, the SIP INVITE request is rejected with a 503 result code to indicate that the gateway is out of resources.

Example 7-7 shows how to configure CAC per dial-peer.

Example 7-7. Using Dial-Peer CAC Mechanisms

dial-peer voice 1 voip
  max-conn 2

Global Call Admission Control

The CUBE can also be configured to monitor calls on a global basis; that is, without regard of which dial-peer the call might be active on. This global CAC control can be done based on:

  • A global system count of calls
  • A CPU threshold (as a percentage)
  • A memory threshold (as a percentage)
  • Any combination of the preceding three metrics

The CUBE checks these configurations and metrics before it completes the processing of a SIP INVITE request. If system resources used exceed the configured amount, the CUBE returns a result code in the SIP INVITE request, indicating that the gateway is out of resources.

Example 7-8 shows how to configure global CAC.

Example 7-8. Using Global CAC Mechanisms

call threshold global total-calls low 20 high 24
call threshold global cpu-avg low 68 high 75
call threshold global total-mem low 75 high 80
call threshold interface Ethernet 0/1 int-calls low 5 high 2500
call treatment cause-code no-resource
call treatment on

The call threshold global total-calls command controls the total number of calls to be supported on the CUBE. The command tracks the number of calls, rejecting the 25th call and not accepting calls again until the total number of calls falls below 20. The cpu-avg and total-mem options rejects the calls if the CPU or memory of the border element exceed the given thresholds regardless of the actual active call count. The call threshold interface command limits the number of calls over a specific IP interface.

The call treatment cause-code no-resource command correlates (by default) to a SIP 503 Service Unavailable message sent when calls are rejected.

Quality of Service (QoS)

Cisco provides many methods of measuring and ensuring QoS in an enterprise IP network. You should always use these methods internally when designing a UC system, and you should also extend them to the interconnect point when using a SIP trunk to connect to a service provider. Consider several areas of QoS including:

  • Traffic marking
  • Delay and jitter
  • Echo
  • Congestion management

Traffic Marking

QoS on IP networks depends on the QoS marking on the IP packets. As with codec settings, QoS markings on voice signaling and media IP packets on IP call traffic inbound from the PSTN on a TDM gateway is easily predicted and controlled by the configuration on your gateway. On SIP trunks, the default packet markings are whatever the service provider sets them to and this might not be in line with your enterprise policies.

The CUBE can re-mark all media and signaling packets that enter you network or exit your network to comply with the SP UNI specification. Re-marking can be done on a per-dial-peer basis (that is, per voice call destination) or per interface (either ingress or egress or both).

Example 7-9 shows how to mark packets per dial-peer.

Example 7-9. Marking QoS on a Dial-Peer

dial-peer voice 40800011 voip
  destination-pattern 408.......
  session protocol sipv2
  session target ipv4 :10.10.1.1
  dtmf-relay rtp-nte
  ip qos dscp ef media     
  ip qos dscp cs4 signaling
  no vad

Delay and Jitter

The telephone industry standard specified in ITU-T G.114 recommends the maximum desired one-way delay be no more than 150 milliseconds. With a round-trip delay of 300 milliseconds or more, users might experience annoying talk-over effects.

When using SIP trunks, you should consider the IP delay of both the enterprise and service provider networks. In some cases, centralized SIP trunk services cannot be effectively deployed because of the resulting increase in latency. A border element device at the customer premises is required to ensure that latency in the service provider network and enterprise network can be independently measured and controlled.

Echo

An echo is the audible leak-through of your own voice into your own receive (return) path. The source of echo might be a TDM loop in the call path or acoustic echo that applies to all-IP calls. Acoustic echo can come from improper acoustic insulation on the phone, headset, or speakerphone (all Cisco IP Phones have an acoustic echo canceller) and is common on PC-based softphones.

A border element demarcation point at a customer site can help you determine if a problem with echo is occurring at the customer premises or in the service provider's network.

Congestion Management

When using a single connection for both voice and data, you should carefully consider congestion management (for example, queuing techniques such as Low-Latency Queuing [LLQ]) and bandwidth allocation to prevent data traffic from affecting the voice quality of SIP trunk calls.

The end-to-end voice quality experience of your SIP trunk calls depend on congestion management techniques in both your network and in the service provider's delivery network to your premises. A enterprise border element can help you determine in which network jurisdiction a problem lies.

Voice-Quality Monitoring

To ensure business class voice quality within the enterprise network and to determine if a service provider is meeting an agreed-upon SLA, your enterprise should monitor some metrics. Each enterprise might choose to monitor different metrics, but an effective method of collecting the metrics independent from the service provider is important.

Table 7-1 describes some of the important metrics you can monitor. These metrics can be gathered by using various features previously discussed in this chapter in the "Statistics" and "Billing" sections. You can use these basic metrics from the network to calculate the more typical voice quality measurements such as Mean Opinion Score (MOS) or Perceptual Evaluation of Speech Quality (PESQ) to quantify with a single number the voice quality attained by the network.

Table 7-1. Voice-Quality Monitoring Attributes

Metric

Goal

Definition

Method to Monitor

Round-trip delay (RTD)

100–300 ms

The RTD is the delay for a packet sent from the originating endpoint at the customer location to the terminating endpoint at the service provider and back again.

This metric can be monitored through the RTD metric in Cisco IOS Software; it is provided per call and is also available through IP-SLA probes.

Jitter

50–100 ms

Jitter is a measurement of the change in the delay of one packet to another during a call.

Jitter is measured in the per-call statistics; the maximum jitter detected during the call is recorded.

Packet loss

1 percent or lower

Packet loss is the number of packets lost during any given call, including UDP and TCP packets.

This metric can be monitored by SNMP in Cisco IOS Software; it is provided per call and can also be tracked with IP-SLA probes.

Uptime

99.999 percent

Uptime is the percentage of time that a path is available for the customer to complete a call to the PSTN.

When uptime is measured, planned outages should be accounted for, and it should be measured as the number of unplanned minutes of outages and monitored with trouble tickets.

Answer seizure rate (ASR) or call success rate (CSR)

Varies

The ASR can be recorded as the number of calls made divided by the number of calls that complete a voice path. This number varies greatly because of calling numbers that are unassigned or busy. CSR is the percentage of calls successfully completed through a service provider. The CSR rate should be more than 99 percent. The ASR rate is typically approximately 60 percent.

ASR can be measured by a summary of call activity at the end of the month. The specific value of ASR is not as important as whether there are large swings in the ASR from one month to another that might indicate a problem with end-to-end network connectivity.

  • + 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.

Overview


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.

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@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.

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 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.

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