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In a multisite deployment, with a single or multiple CUCM clusters, dial plan design
requires the consideration of several issues that do not exist in single-site deployments:
• Overlapping numbers: Users located at different sites can have the same directory
numbers assigned. Because directory numbers usually are unique only within a site, a
multisite deployment requires a solution for overlapping numbers.
10 Chapter 1: Identifying Issues in a Multisite Deployment
• Nonconsecutive numbers: Contiguous ranges of numbers are important to summarize
call-routing information, analogous to contiguous IP address ranges for route summarization.
Such blocks can be represented by one or a few entries in a call-routing table,
such as route patterns, dial peer destination patterns, and voice translation rules, which
keep the routing table short and simple. If each endpoint requires its own entry in the
call-routing table, the table gets too big, lots of memory is required, and lookups take
more time. Therefore, nonconsecutive numbers at any site are not optimal for efficient
call routing.
• Variable-length numbering: Some countries, such as the U.S. and Canada, have
fixed-length numbering plans for PSTN numbers. Others, such as Mexico and England,
have variable-length numbering plans. A problem with variable-length numbers is that
the complete length of the number dialed can be determined only by the CUCM route
plan by waiting for the interdigit timeout. Waiting for the interdigit timeout, known as
the T.302 timer, adds to the post-dial delay, which may annoy users.
• Direct inward dialing (DID) ranges and £.164 addressing: When considering
integration with the PSTN, internally used directory numbers have to be related to
external PSTN numbers (E. 164 addressing). Depending on the numbering plan (fixed
or variable) and services provided by the PSTN, the following solutions are common:
—Each internal directory number relates to a fixed-length PSTN
number: In this case, each internal directory number has its own
dedicated PSTN number. The directory number can, but does not have to,
match the least-significant digits of the PSTN number. In countries with
a fixed numbering plan, such as the North American Numbering Plan
(NANP), this usually means that the four-digit office codes are used as
internal directory numbers. If these are not unique, digits of office codes
or administratively assigned site codes might be added, resulting in five
or more digits being used for internal directory numbers.
Another solution is to not reuse any digits of the PSTN number but to
simply map each internally used directory number to any PSTN number
assigned to the company. In this case, the internal and external numbers
do not have anything in common. If the internally used directory number
matches the least-significant digits of its corresponding PSTN number,
significant digits can be set at the gateway or trunk. Also, general external
phone number masks, transformation masks, or prefixes can be configured.
This is true because all internal directory numbers are changed to
fully qualified PSTN numbers in the same way. Another example is if the
internal directory number is composed of parts of the PSTN number and
administratively assigned digits such as site codes plus PSTN station
Dial Plan Challenges 11
codes, or different ranges, such as PSTN station codes 4100 to 4180 that
map to directory numbers 1100 to 1180, or totally independent mappings
of internal directory numbers to PSTN numbers. In that case, one or more
translation rules have to be used for incoming calls, and one or more calling
party transformation rules, transformation masks, external phone
number masks, or prefixes have to be configured.
—No DID support in fixed-length numbering plans: To avoid the
requirement of one PSTN number per internal directory number when
using a fixed-length numbering plan, it is common to disallow DID to an
extension. Instead, the PSTN trunk has a single number, and all PSTN
calls routed to that number are sent to an attendant, auto-attendant,
receptionist, or secretary. From there, the calls are transferred to the
appropriate internal extension.
—Internal directory numbers are part of a variable-length number: In
countries with variable-length numbering plans, a typically shorter
"subscriber" number is assigned to the PSTN trunk, but the PSTN routes
all calls starting with this number to the trunk. The caller can add digits
to identify the extension. There is no fixed number of additional digits or
total digits. However, there is a maximum, usually 32 digits, which
provides the freedom to select the length of directory numbers. This
maximum length can be less. For example, in E.164 the maximum
number is 15 digits, not including the country code. A caller simply adds
the appropriate extension to the company's (short) PSTN number when
placing a call to a specific user. If only the short PSTN number without
an extension is dialed, the call is routed to an attendant within the
company. Residential PSTN numbers are usually longer and do not allow
additional digits to be added; the feature just described is available only
on trunks.
• Type of Number (TON) in ISDN: The calling number (the Automatic Number
Identification [ANI]) of calls being received from the PSTN can be represented in
different ways:
—As a seven-digit subscriber number
—As a ten-digit number, including the area code
—In international format with the country code in front of the area code
To standardize the ANI for all calls, the format that is used must be known, and the
number has to be transformed accordingly.
Chapter 1: Identifying Issues in a Multisite Deployment
• Optimized call routing: Having an IP WAN between sites with PSTN access at all
sites allows PSTN toll bypass by sending calls between sites over the IP WAN instead
of using the PSTN. In such scenarios, the PSTN should be used as a backup path only
in case of WAN failure. Another solution, which extends the idea of toll bypass and can
potentially reduce toll charges, is to also use the IP WAN for PSTN calls. With tail-end
hop-off (TEHO), the IP WAN is used as much as possible, and the gateway that is
closest to the dialed PSTN destination is used for the PSTN breakout.
NOTE Any two-way phone call has two phone numbers: the calling number, or
Automatic Number Identification (ANI), and the called number, or Dialed Number
Identification Service (DNIS). Any two-way call goes from the ANI to the DNIS. Digit
manipulation is the process of changing the ANI and/or the DNIS to any other number.
Overlapping and Nonconsecutive Numbers
In Figure 1-4, Cisco IP Phones at the main site use directory numbers 1001 to 1099,2000 to
2157, and 2365 to 2999. At the remote site, 1001 to 1099 and 2158 to 2364 are used. These
directory numbers have two issues. First, 1001 to 1099 overlap; these directory numbers
exist at both sites, so they are not unique throughout the complete deployment. This causes
a problem: If a user in the remote site dialed only the four digits 1001, which phone would
ring? This issue of overlapping dial plans needs to be addressed by digit manipulation. In
addition, the nonconsecutive use of the range 2000 to 2999 (with some duplicate numbers at
the two sites) would require a significant number of additional entries in call-routing tables
because the ranges can hardly be summarized by one (or a few) entries.
Figure 1-4 Dial Plan Challenges: Overlapping and Nonconsecutive Numbers
Main Site Remote Site
Dial Plan Challenges 13
NOTE The solutions to the problems listed in this chapter are discussed in more detail
in the next chapter.
Fixed Versus Variable-Length Numbering Plans
A fixed numbering plan features fixed-length area codes and local numbers. An open
numbering plan features variance in length of area code or local number, or both, within
the country.
Table 1-1 contrasts the NANP and a variable-length numbering plan—Germany's
numbering plan in this example.
Table 1-1 Fixed Versus Variable-Length Numbering Plans
Component Description
Fixed Numbering
Plan (NANP)
Variable-Length
Numbering Plan
(Germany)
Country code A code of one to three digits is
used to reach the particular
telephone system for each nation
or special service. Obtain the
E.164 standard from http://itu.org
to see all international country
codes.
1 49
Area code Used within many nations to route
calls to a particular city, region, or
special service. Depending on the
nation or region, it may also be
called a numbering plan area, subscriber
trunk dialing code,
national destination code, or routing
code.
Three digits Three to five digits
Subscriber
number
Represents the specific telephone
number to be dialed, but it does
not include the country code, area
code (if applicable), international
prefix, or trunk prefix.
Three-digit exchange
code plus a fourdigit
station code
Three or more digits
continues
Chapter 1: Identifying Issues in a Multisite Deployment
Table 1-1 Fixed Versus Variable-Length Numbering Plans (Continued)
Component Description
Fixed Numbering
Plan (NANP)
Variable-Length
Numbering Plan
(Germany)
Trunk prefix The initial digits to be dialed in
a domestic call, before the area
code and the subscriber number.
l 0
Access code A number that is traditionally
dialed first "to get out to the
PSTN," used in PBXs and VoIP
systems.
9 0
International
prefix
The code dialed before an
international number (country
code, area code if any, and then
subscriber number).
Oil 00 or + (+ is used by
cell phones)
Examples:
• Within the U.S.: 9-1-408-555-1234 or 1-555-1234 (within the same area code)
• U.S. to Germany: 9-011-49-404-132670
• Within Germany: 0-0-404-132670 or 0-132670 (within the same area code)
• Germany to the U.S.: 0-00-1-408-555-1234 (Note: the 1 in 00-1-408 is the U.S.
country code, not the trunk prefix.)
The NANP PSTN number is 408-555-1234, DID is not used, and all calls placed to the
main site are handled by an attendant. There is a remote site in Germany with the E.164
PSTN number +49 404 13267. Four-digit extensions are used at the German location, and
DID is allowed because digits can be added to the PSTN number. When calling the German
office attendant (not knowing a specific extension), U.S. users would dial 9-011-49-404-
13267. Note how the + is replaced by the international prefix 011 and the access code 9. If
the phone with extension 1001 should be called directly, 9-011-49-404-13267-1001 has to
be dialed.
NOTE In the examples shown following Table 1-1, dialing out from the U.S. illustrates
the common practice of dialing 9 first as an access code to dial out. This use is common
but optional in a dial plan. However, if the access code is used, the 9 must be stripped
before reaching the PSTN, whereas the other dialed prefixes must be sent to the PSTN
for proper call routing.
Dial Plan Challenges 15
Variable-Length Numbering, E.164 Addressing, and DID
Figure 1-5 illustrates an example in which the main site with CUCM resides in the U.S. and
a remote site without CUCM resides in Germany. The NANP PSTN number in the U.S. is
408-555-1234. Note that DID is not used, because all calls placed to the main site are
handled by an attendant. A remote site in Germany has PSTN number +49 404 13267. Fourdigit
extensions are used at the German location, and DID is allowed because digits can be
added to the PSTN number. When calling the German office attendant (not knowing a
specific extension), U.S. users would dial 9-011-49-404-13267. If the phone with extension
1001 should be called directly, 9-011-49-404-13267-1001 has to be dialed.
Figure 1-5 Variable-Length Numbering, E.164 Addressing, and DID
The logic of routing calls by CUCM over the WAN or through the PSTN is appropriately
transparent to the phone user.
Optimized Call Routing and PSTN Backup
There are two ways to save costs for PSTN calls in a multisite deployment:
• Toll bypass: Calls between sites within an organization that use the IP WAN instead
of the PSTN. The PSTN is used for intersite calls only if calls over the IP WAN are not
possible—either because of a WAN failure or because the call is not admitted by Call
Admission Control (CAC).
Chapter 1: Identifying Issues in a Multisite Deployment
• Tail-end hop-off (TEHO): Extends the concept of toll bypass by also using the IP
WAN for calls to the remote destinations in the PSTN. With TEHO, the IP WAN is used
as much as possible, and PSTN breakout occurs at the gateway that is located closest
to the dialed PSTN destination. Local PSTN breakout is used as a backup in case of IP
WAN or CAC.
CAUTION Some countries do not allow the use of TEHO or toll bypass because it is
illegal to bypass their international tariff collections, which would deprive their operators
of international inbound revenues. When implementing either, ensure that the deployment
complies with legal requirements of that country.
In the example shown in Figure 1-6, a call from Chicago to San Jose would be routed
as follows:
1. The Chicago CUCM Express user dials 9-1-408-555-6666, a PSTN phone located
in San Jose.
2. The call is routed from Chicago CUCM Express Router to the San Jose CUCM cluster
over the IP WAN with either SIP or H.323.
3. The San Jose CUCM routes the call to the San Jose gateway, which breaks out to the
PSTN with what now becomes a local inexpensive call to the San Jose PSTN.
4. The San Jose PSTN Central Office routes the call, and the phone rings.
Figure 1-6 Tail-End Hop-Off (TEHO) Example
NAT and Security Issues 17
If the WAN were unavailable for any reason before the call, the Chicago Gateway would
have to be properly configured to route the call with the appropriate digit manipulation
through the PSTN at a potentially higher toll cost to the San Jose PSTN phone.