Capítulo I: Estado del Arte
I.3 Ventilación
I.3.3 Sistemas de ventilación
The destination pattern dial-peer command assigns phone numbers (patterns) to dial peers. The router's voice software then routes voice calls based upon these patterns. This is loosely
analogous to the assigning of IP addresses to a network interface in the router. Wildcards and other operators help to simplify the creation of dial plans. The full syntax for the command follows:
destination-pattern [+]string[t]
where
A leading + indicates that the string is an E.164 standard number; its use is optional.
Page 173 string is the dial string or phone number comprised of digits (0—9), the letters A through D, or the * or #; acceptable operators are (1) a comma calls for a 1-s pause in dialing, (2) a period indicates a single digit wildcard (a period within a string matches any valid character for that location).
A trailing t indicates that variable-length dial strings are in use and that the router should wait for the interdigit timeout to expire before acting on the string.
To specify a specific phone number for a voice port, the following configuration would be used.
destination-pattern 2905000
Alternately, the following command could be used:
destination-pattern +2905000
In a scenario where the voice port was handing off to a PBX system which supported all of the phone extensions from 2905000 through 2905999, the following command would be
appropriate:
destination-pattern 2905...
Session Target
As mentioned previously, the session target command identifies the remote end of the VoIP call. The destination can be specified using an IP address, a DNS name, or a loopback. The use of DNS can help simplify identification of remote peers in some scenarios. The use of loopbacks helps test connectivity and voice quality through different parts of the network. The full syntax for the session target dial-peer command follows:
where target could be either a remote VoIP peer or a loopback. The keywords and their descriptions are explained below.
Destination Based
ipv4: address where address is the actual IP address of the remote VoIP peer.
Page 174 dns:hostname where hostname is the DNS name for the destination router. For example, the following configuration would be used for the router to route the voice call to the IP address identified by rts36v1.company.com:
ip name-server 192.168.1.250 !
... !
dial-peer voice 669 voip
session target dns:rts36v1.company.com !
Also, the following operators can be used along with the DNS keyword: :$s$ specifies that the source pattern be used as part of the DNS name. :$d$ specifies that the destination pattern be used as part of the DNS name.
This allows an administrator to assign the device's phone number as its DNS name, which can simplify the identification of devices. In a network with multiple remote offices, it may be convenient to enter the remote sites' phone numbers as hostnames in DNS and group their destination patterns such that they could be identifiable with a common pattern prefix that is unique from other elements. The following DNS $d$ example demonstrates the flexibility this introduces.
EXAMPLE
In the network diagram shown in Figure 6-12, the remote sites each have a phone number that begins with 21229051. Therefore, the following configuration would allow the AS5300 at the central site to reach each remote site without requiring a separate dial-peer paragraph.
ip name-server 192.168.1.250 ip domain-name company.com !
... !
dial-peer voice 99 voip
destination-pattern 121229051.. session target dns:$d$.company.com !
The notation $u$ specifies that the unmatched portion of the destination pattern be used as the hostname portion of the DNS name. This is
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Figure 6-12
Sample network utilizing $d$ option.
advantageous for addressing multiple endpoints with a single dial peer and for shortening the DNS names used. The following DNS $u$ example demonstrates this.
EXAMPLE
In the previous example, the only part of the phone number which changed from site to site was the last two digits. This means that each site can be uniquely identified by those digits. Figure 6-13 illustrates this.
In this scenario, only the last two digits are used as the DNS hostname. It provides more of an extension-based DNS naming technique. The configuration excerpt which would support this is as follows: ip name-server 192.168.1.250 ip domain-name company.com ! ... !
dial-peer voice 99 voip
destination-pattern 121229051.. session target dns:$u$.company.com !
Figure 6-13
Sample network utilizing $u$ option. Loopback Based
loopback:rtp indicates that the VoIP session be looped back to the source at the RTP layer. This attribute is applicable to VoIP peers where the remote peer's traffic is looped back when it reaches the RTP layer. This verifies that the router has received the voice traffic and is capable of processing it as high as the RTP layer. This test verifies that proper network and IP connectivity are established. The path setup by this loopback is depicted in Figure 6-14.
loopback:compressed instructs the router to send the voice data through the compression software before looping it back to the source. This attribute is applicable to POTS peers and verifies that the compression/decompression software is working. It also provides a sampling of the representative quality of the voice signal after the compression/decompression cycle. The loopback path for this command is depicted in the left portion of Figure 6-15.
loopback:uncompressed instructs the router to loop the traffic back to the source before it reaches the compression software. This attribute is applicable to POTS peers and simply verifies physical connectivity through the voice port. The loopback path for this command is depicted in the right portion of Figure 6-15.
Figure 6-14 VOIP loopback path.
Figure 6-15 Voice-port loopback paths.
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Prefix
To add a prefix for outgoing calls through a POTS dial peer, the prefix command is used. This is helpful when the voice-enabled router is connected to a PBX system which requires a 9 for accessing an outside line, or another number for accessing a specific trunk line. It can also be
used to prepend an access code for dialed calls such as a 10-10-xxx code. The syntax for the dial-peer command is as follows:
prefix string
where string can be any number of digits or a comma. The comma provides a momentary pause in dialing.
The simple example of prepending 9 for outgoing calls through a PBX follows:
!
dial-peer voice 2 pots prefix 9,
dest-pat 1212317.... port 0:D
!
QoS Commands
The dial-peer construct allows for the specification of quality of service techniques including IP precedence and RSVP. This allows QoS mechanisms to be set more granularly.
IP Precedence In Chapter 4, it was discussed how the setting of IP precedence bits in the IP header interworks with various IP QoS mechanisms such as weighted fair queuing, weighted random early detection, access lists, and others. One way of setting these bits for outgoing voice-over-IP calls is to set it within the dial-peer paragraph. The syntax for this dial-peer command follows:
ip precedence n
where n is a digit from 0 through 5. The significance of each IP-precedence value is presented in Table 6-9, but, in general, voice traffic should be given a precedence of 5.
A sample configuration for setting the IP precedence bit follows:
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TABLE 6-9
IP Precedence Values
Value Definition
0 Routine (normal traffic)
1 Priority
2 Immediate
3 Flash
4 Flash override
5 Critical
6 Internet (for network control traffic) 7 Network (for network control traffic)
!
dial-peer voice 101 voip
destination-pattern 140852645.. session target ipv4:192.168.1.1 ip precedence 5
!
RSVP The use of RSVP for voice traffic can be enabled on a per-call basis by configuring it under the dial peer construct. The dial/peer, when activated, initiates an RSVP request to support the call. Since the request is made at the dial-peer level, it does not account for the bandwidth savings introduced by compressing RTP headers or other external techniques. This results in RSVP requests for 24 kbps when using G.729 compression instead of the actual 11 kbps which is realized when using CRTP.
As discussed in Chapter 4, an RSVP client can request two different types of QoS: controlled load or guaranteed delay. Cisco recommends the controlled-load model for voice traffic. The dial-peer command for enabling RSVP for a given dial peer is the following:
req-qos { best-effort | controlled-load | guaranteed delay }
where
The best-effort option is the default and does not make a bandwidth reservation.
The controlled-load option provides preferential service to the associated RSVP flow such that the performance for the flow mimics that of an unloaded network
The guaranteed delay option attempts to guarantee queuing delay throughout the network
Page 180 An example configuration using RSVP at the dial-peer level follows:
!
dial-peer voice 25 voip destination-pattern 29052.. req-qos controlled-load
session target ipv4:10.1.1.254 !
Voice Encoding/Compression
To select the type of voice encoding and compression to be used for voice calls to a given destination, the codec command is used. It is configurable on a dial-peer basis so that different compression techniques may be used for different destinations. For instance, in a LAN
environment where bandwidth is plentiful, G.711 PCM encoding may be used to achieve slightly higher voice quality. However, for remote calls, G.729 may be selected to conserve bandwidth over WAN links. The dial-peer command to select the voice encoding algorithm is
codec { g711ulaw | g711alaw | g729r8 }
where
g711ulaw specifies 64-kbps PCM using the mu-law compander g711alaw specifies 64-kbps PCM using the a-law compander g729r8 specifies 8-kbps CS-ACELP compression and is the default
Voice-Activity Detection
As discussed in Chapter 3, voice-activity detection can be used to reduce the amount of
bandwidth consumed by the session. Voice-activity detection identifies silent periods when the participant is listening and not speaking, and stops the transmission of voice packets during the silent period. The effectiveness of VAD is limited by its ability to identify the start and stop of conversations. As a result, VAD can sometimes clip voice signals and add a choppy nature to conversations. The dial-peer command used to enable VAD is simply
vad
One side effect of enabling VAD is that the remote user hears pure silence when silence is detected. This can be disconcerting since users are accustomed to hearing background noise. To combat this, generic
Page 181 noise can be transmitted over the local line to simulate background noise. The dial-peer
command to generate the simulated background noise is
comfort-noise
Summary
This chapter introduced the concepts of voice ports and dial peers. Voice ports handle the physical-layer communications with telephony devices while dial peers handle the logical connections between voice ports and VoIP destinations. It is important to understand the distinction between the two constructs and how the two work together to form an end-to-end voice network.
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