Chapter 10: Digit Manipulation

You can perform digit manipulation only on H.323 and SIP gateways; on MGCP gateways, the call agent must perform any manipulation because it controls the calls. Cisco CallManager Express and routers that are using Survivable Remote Site Telephony (SRST) offer some additional manipulation options to those discussed in this chapter, such as dial plan patterns. CallManager Express techniques are beyond the scope of this book. Chapter 13, "SRST and MGCP Gateway Fallback," discusses SRST digit manipulation in greater detail. You can also use Toolkit Command Language (Tcl) scripts to manipulate the called and calling numbers, as described in Chapter 15, "Using TCL Scripts and VoiceXML."

Table 10-1 Digit Manipulation Techniques

Digit Stripping

VoIP dial peers transmit all digits in the called number by default; however, POTS dial peers remove, or strip, any outbound digits that explicitly match their destination pattern. For instance, given a destination pattern of55512..the called number transmitted to the PSTN would contain just the last two digits. The first five digits,55512, would be stripped. Only explicitly matched digits are stripped. Given a destination pattern of555[2-9]..and a digit string of555422, only the555would be stripped. If all the digits in a called number are removed, the caller hears only a reorder tone.

Digit stripping is the default behavior of POTS dial peers. It can work to your advantage, as long as you understand its effect. For instance, if users dial9to reach an outside number, you would not want the number9sent to the PSTN as part of the called number. When you configure the destination pattern of a POTS dial peer as9T,9is matched, so it is stripped. The remaining digits are transmitted. However, suppose that you have a dial peer for an emergency number, such as911in the United States. In that case, you would not want any of those digits removed, so you must disable the default behavior for that dial peer.

You can disable digit stripping with the commandno digit-stripunder POTS dial-peer configuration mode. Re-enable it with the commanddigit-strip. The relevant command syntax is as follows:

dial-peer voice 1 pots[no]digit-strip

Forward Digits

You can achieve more precise control over the number of digits in the called number that are transmitted to the PSTN with the following command, which applies only to POTS dial peers:

forward-digits[number|all|extra]

where

• numbergives the number of digits to be forwarded.

• allmeans to forward all digits.

• extratells the gateway to forward any digits that are longer than the length of the destination pattern.

This command lets you specify the exact number of digits to be forwarded. If the number of digits presented exceeds the number allowed, the rightmost digits are sent. One place this can be useful is when you must dial a code (such as9) to reach an outside number, and there is an emergency situation. The previous section showed how to ensure that the entire emergency number (such as911) is sent. In an emergency, a person might be confused about whether to dial the outside code. That is why dial peers are typically set up to match both the emergency number, and that number plus the code number. If the destination pattern of a dial peer is9911, you should send only911. You can use theforward-digitscommand to transmit only the last three digits,911, to the PSTN.

The following examples show what digits are sent when you dial the number111-222-3333and use various options of theforward-digitscommand.

For this first case, the rightmost seven digits, 222-3333, are sent.

Miami(config)#dial-peer voice 111 potsMiami(config-dial-peer)#destination-pattern 111222....Miami(config-dial-peer)#forward-digits 7

With the configuration that follows, the entire number,111-222-3333, is sent. This is an alternative to using theno digit-stripcommand shown in the previous section.

Miami(config)#dial-peer voice 111 potsMiami(config-dial-peer)#destination-pattern 111222....Miami(config-dial-peer)#forward-digits all

最后,最后的配置, because the destination pattern is now six digits long and the dialed number is ten digits, the "extra" digits—3333—are sent.

Miami(config)#dial-peer voice 111 potsMiami(config-dial-peer)#destination-pattern 111222Miami(config-dial-peer)#forward-digits extra

Prefix Digits

In some cases, you might need to transmit more than the dialed digits of a called number. For example, perhaps a call that would normally go across your VoIP network needs to be rerouted through the PSTN, requiring the addition of the appropriate area code and prefix. Or, a destination pattern might specify the first six digits of the number, with wildcards for the other digits. By default, those six digits are stripped. You can use theprefixstringcommand to replace some of those digits. This command is given under dial-peer configuration mode, and it is only for POTS dial peers. The prefixed string can be any number from 0 to 9 and a comma that inserts a one-second pause. The gateway prefixes digits after the outgoing dial peer is matched and after any digits are stripped, but before it sends out the call.

In Example 10-1, long-distance calls need to go to a particular carrier and must have a separate dial peer from external local calls. The destination pattern is91to capture long-distance traffic; however, the PSTN needs the number1transmitted as part of the called number to route the call properly. Thus, theprefixcommand is added to replace that digit.

Example 10-1 Prefixing Digits to an Outgoing Call

Miami(config)#dial-peer voice 91 potsMiami(config-dial-peer)#destination-pattern 91TMiami(config-dial-peer)#prefix 1Miami(config-dial-peer)#port 1/0:23

When a user dials the outside long-distance number9-1-111-222-3333,digits are manipulated before being sent to the PSTN. The original digits9and1are stripped, and a1is prefixed to the remaining number. Thus, the PSTN receives the number1-111-222-3333.

Number Expansion

Number expansion is another way to add digits to an outgoing called number; however, number expansion is applied to the gateway as a whole, and acts on all calls, not just those matching a designated dial peer. As with the previous three techniques, this manipulates only the called number. Number expansion manipulation occurs before any outbound dial peer is matched. Thus, you must configure outbound dial peers to match the expanded numbers, not the original ones.

The command for number expansion isnum-exporiginal-number expanded-number, configured at global configuration mode. Although it is callednumber expansion,manipulated number can contain fewer digits than the original, or it can contain more.

Example 10-2 shows two instances of number expansion configured. The first expands any four-number extension beginning with1to a seven-digit number beginning with5551. The second changes any seven-digit number beginning with5551to a four-digit extension.

Example 10-2 Number Expansion

Miami(config)#num-exp 1... 5551...Miami(config)#num-exp 5551... 1...! !Verifying the first number expansion Miami#show dialplan number 1111宏Exp。:5551111 !!验证第二个数字expansion Miami#show dialplan number 5551098Macro Exp.: 1098

In the results of theshow dialplan numbercommand, notice that although the number entered in the command is the original number, the router looked for a match to the expanded number. Theshow dialplan numbercommand is also useful for verifying that you have a dial peer to match the expanded number.

Voice Translation Rules and Profiles

You can accomplish more precise, granular manipulation using voice translation profiles, which contain voice translation rules. The techniques that were previously discussed can alter only the called number, or dialed number identification service (DNIS), digits. Translation profiles are much more powerful. They can change both the DNIS and the calling number, or automatic number identification (ANI), digits plus the redirecting number. With translation profiles you can adjust the numbering type and plan, reject unwanted calls based on a rule match, and remove specific digits (such as hyphens).

A legacy technique calledtranslation rules(without thevoice) can manipulate ANI and DNIS numbers. This technique is still supported, but the recommended method is to use voice translation rules and voice translation profiles.

Using voice translation profiles for digit manipulation requires three steps:

1. Create one or more voice translation rules and a prioritized list of translations associated with each rule. A maximum of 128 rules is supported, with 15 translations per rule.

2. Create one or more voice translation profiles and associate the translation rules to the profile. You can define up to 1000 profiles, each with its own unique name. Within the profile, you can apply one voice translation rule to calling numbers, one to called numbers, and one to redirected called numbers.

3. Apply the voice translation profile to all VoIP calls globally, a dial peer, a voice port, a trunk group, a source IP group, or an interface.

Creating Voice Translation Rules

To create a voice translation rule, use the commandvoice translation-ruletagin global configuration mode. Then create an ordered list of one or more rules with the following command:

荷重软化eprecedence/match pattern/ /replace pattern/[type {match-type replace-type}[plan {match-type replace-type}]]

You can enter rules in any order; theprecedence value determines the order in which the rules are executed. You can configure up to 15 rules.

A basic voice translation rule replaces one group of digits with another. Each group is delineated with//(frontslash) characters. In Example 10-3, voice translation Rule 1 replaces 111 with 222, and 333 with 444. A test of the translations shows that they are carried out as specified.

Example 10-3 Using a Basic Voice Translation Rule

Boise(config)#voice translation-rule 1Boise(cfg-translation-rule)#荷重软化e 1 /111/ /222/Boise(cfg-translation-rule)#荷重软化e 2 /333/ /444/Boise#test voice translation-rule 1 111Matched with rule 1 Original number: 111 Translated number: 222 Original number type: none Translated number type: none Original number plan: none Translated number plan: none Boise#test voice translation-rule 1 13335Matched with rule 2 Original number: 13335 Translated number: 14445 Original number type: none Translated number type: none Original number plan: none Translated number plan: none

When the number1335is tested, notice that the number patterns are translated no matter where they appear in the string. Notice also that any digits that are not expressly matched—the numbers 1 and 5 in this example—are simply carried through. For more precise control, you can use some special characters and wildcards to build regular expressions, as shown in the next section.

Building Regular Expressions

A regular expression is a text-parsing tool that combines a string of literal characters and special characters, calledmetacharacters. Voice translation rules can use regular expressions to find matches in the digit strings, and replace a match with a different string. Table 10-2 lists some commonly used special characters and their meaning.

Table 10-2 Regular Expression Special Characters

Previously in Example 10-3, the string333was matched even when it was in the middle of the list. If you wanted to match333only when it appeared at the beginning of a string, you could use the caret character:

荷重软化e 2 /^333/ /444/

To match a string containing only333, add the dollar sign at the end and the caret at the beginning:

荷重软化e 2 /^333$/ /444/