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This chapter describes the PA-VXB and the PA-VXC port adapters and contains the following sections:
The PA-VXB and the PA-VXC (see Figure 1-1 and Figure 1-2) are multichannel packet voice port adapters that allow Cisco 7200 series, Cisco 7200 VXR, and Cisco 7500 series routers to become dedicated packet voice hubs or packet voice gateways that connect to both private branch exchanges (PBXs) and the Public Switched Telephone Network (PSTN). This allows packet voice and packet fax calls to be placed over the wide-area network (WAN) and sent through the gateway into the traditional circuit-switched voice infrastructure.
The PA-VXB and the PA-VXC are single-width port adapters with two universal ports that are configurable for either T1 or E1 connection. The PA-VXB contains 12 high-performance digital signal processors (DSPs) that support up to 48 medium-complexity or 24 high-complexity channels of compressed voice. The PA-VXC contains 30 high-performance DSPs that support up to 60 medium-complexity or 120 high-complexity channels of compressed voice.
In Voice over IP, the DSP segments the voice signal into frames, which are then coupled in groups of two and stored in voice packets. These voice packets are transported using IP in compliance with ITU-T specification H.323. Because Voice over IP is a delay-sensitive application, you must have a well-engineered network end-to-end to use it successfully. Fine-tuning your network to adequately support Voice over IP involves a series of protocols and features geared toward quality of service (QoS). Traffic shaping considerations must be taken into account to ensure the reliability of the voice connection.
The PA-VXB and the PA-VXC have the following features:
DSP features:
Signaling supported for H.323 environments:
Port configured as T1 features:
Port configured as E1 features:
Full bit-error-rate testing capabilities on each E1/T1
Supported loopbacks:
Channel-associated signaling (CAS)—A form of signaling used on a T1 line. With CAS, a signaling element is dedicated to each channel in the T1 frame. This type of signaling is sometimes called Robbed Bit Signaling (RBS) because a bit is taken out (or robbed) from the user's data stream to provide signaling information to and from the switch.
Codec—Coder-decoder compression scheme or technique. In Voice over IP, it specifies the voice coder rate of speech for a dial peer.
Dial peer—An addressable call endpoint. In Voice over IP, there are two kinds of dial peers: POTS and VoIP.
DS0—A 64-kbps channel on an E1 or T1 WAN interface.
DTMF—Dual tone multifrequency. Use of two simultaneous voice-band tones for dial (such as touch tone).
FRF.11—Frame Relay Forum implementation agreement for Voice over Frame Relay (Version 1.0, May 1997). This specification defines multiplexed data, voice, fax, DTMF digit-relay and CAS/Robbed Bit Signaling frame formats but does not include call setup, routing, or administration facilities.
FRF.12—Frame Relay Forum implementation agreement (also known as FRF.11 Annex C) developed to allow long data frames to be fragmented into smaller pieces and interleaved with real-time frames. In this way, real-time voice and non-real-time data frames can be carried together on lower-speed links without causing excessive delay to the real-time traffic.
Multilink PPP—Multilink Point-to-Point Protocol. This protocol is a method of splitting, recombining, and sequencing datagrams across multiple logical data links.
PBX—Private branch exchange. Privately owned central switching office.
POTS dial peer—Dial peer connected through a traditional telephony network. POTS peers point to a particular voice port on a voice network device.
PSTN—Public Switched Telephone Network. PSTN refers to the local telephone company.
PVC—Permanent virtual circuit.
QoS—Quality of service, which refers to the measure of service quality provided to the user.
RSVP—Resource Reservation Protocol. This protocol supports the reservation of resources across an IP network.
To understand Cisco's voice implementations, it helps to have some understanding of analog and digital transmission and signaling. This section provides some very basic, abbreviated voice telephony information as background to help you configure Voice over IP, Voice over Frame Relay, and includes the following topics:
The standard PSTN is basically a large, circuit-switched network. It uses a specific numbering scheme, which complies with the ITU-T E.164 recommendations. For example, in North America, the North American Numbering Plan (NANP) is used, which consists of an area code, an office code, and a station code. Area codes are assigned geographically, office codes are assigned to specific switches, and station codes identify a specific port on that switch. The format in North America is 1Nxx-Nxx-xxxx, where N is a digit between 2 and 9 and x is a digit between 0 and 9. Internationally, each country is assigned a one- to three-digit country code; the country's dialing plan follows the country code. In Cisco's voice implementations, numbering schemes are configured using the destination-pattern command.
Until recently, the telephone network was based on an analog infrastructure. Analog transmission is not particularly robust or efficient at recovering from line noise. Because analog signals degrade over distance, they need to be periodically amplified; this amplification boosts both the voice signal and ambient line noise, resulting in degradation of the quality of the transmitted sound.
In response to the limitations of analog transmission, the telephony network migrated to digital transmission using pulse code modulation (PCM) or adaptive differential pulse code modulation (ADPCM). In both cases, analog sound is converted into digital form by sampling the analog sound 8000 times per second and converting each sample into a numeric code.
PCM and ADPCM are examples of "waveform" coder-decoder (codec) techniques. Waveform codecs are compression techniques that exploit the redundant characteristics of the waveform itself. In addition to waveform codecs, there are source codecs that compress speech by sending only simplified parametric information about voice transmission; these codecs require less bandwidth. Source codecs include linear predictive coding (LPC), code-excited linear prediction (CELP), and multipulse, multilevel quantization (MP-MLQ).
Coding techniques are standardized by the ITU-T in its G-series recommendations. The most popular coding standards for telephony and voice packet are:
In Cisco's voice implementations, compression schemes are configured using the codec command.
Each codec provides a certain quality of speech. The quality of transmitted speech is a subjective response of the listener. A common benchmark used to determine the quality of sound produced by specific codecs is the mean opinion score (MOS). With MOS, a wide range of listeners judges the quality of a voice sample (corresponding to a particular codec) on a scale of 1 (bad) to 5 (excellent). The scores are averaged to provide the mean opinion score for that sample. Table 1-1 shows the relationship between codecs and MOS scores.
| Compression Method | Bit Rate (kbps) | Framing Size | MOS Score |
|---|---|---|---|
G.711 PCM | 64 | 0.125 | 4.1 |
G.726 ADPCM | 32 | 0.125 | 3.85 |
G.728 LD-CELP | 16 | 0.625 | 3.61 |
G.729 CS-ACELP | 8 | 10 | 3.92 |
G.729 x 2 Encodings | 8 | 10 | 3.27 |
G.729 x 3 Encodings | 8 | 10 | 2.68 |
G.729a CS-ACELP | 8 | 10 | 3.7 |
G.723.1 MP-MLQ | 6.3 | 30 | 3.9 |
G.723.1 ACELP | 5.3 | 30 | 3.65 |
Although it might seem logical from a financial standpoint to convert all calls to low-bit rate codecs to save on infrastructure costs, you should exercise additional care when designing voice networks with low-bit rate compression. There are drawbacks to compressing voice. One of the main drawbacks is signal distortion due to multiple encoding (called tandem encoding). For example, when a G.729 voice signal is tandem encoded three times, the MOS score drops from 3.92 (very good) to 2.68 (unacceptable). Another drawback is codec-induced delay with low-bit rate codecs.
One of the most important design considerations in implementing voice is minimizing one-way, end-to-end delay. Voice traffic is real-time traffic; if there is too long a delay in voice packet delivery, speech is unrecognizable. Delay is inherent in voice networking and is caused by a number of different factors. An acceptable delay is less than 200 milliseconds.
There are basically two kinds of delay inherent in today's telephony networks: propagation delay and handling delay. Propagation delay is caused by the characteristics of the speed of light traveling through a fiberoptic-based or copper-based media. Handling delay (sometimes called serialization delay) is caused by the devices that handle voice information. Handling delays have a significant impact on voice quality in a packetized network.
Codec-induced delays are considered a handling delay. Table 1-2 shows the delay introduced by different codecs.
| Codec | Bit Rate (kbps) | Compression Delay (ms) |
|---|---|---|
G.711 PCM | 64 | 0.75 |
G.726 ADPCM | 32 | 1 |
G.728 LD-CELP | 16 | 3 to 5 |
G.729 CS-ACELP | 8 | 10 |
G.729a CS-ACELP | 8 | 10 |
G.723.1 MP-MLQ | 6.3 | 30 |
G.723.1 ACELP | 5.3 | 30 |
Another handling delay is the time it takes to generate a voice packet. In Voice over IP, the DSP generates a frame every 10 milliseconds. Two of these frames are then placed within one voice packet; the packet delay is therefore 20 milliseconds.
Another source of handling delay is the time it takes to move the packet to the output queue. Cisco IOS software expedites the process of determining packet destination and getting the packet to the output queue. The actual delay at the output queue is another source of handling delay and should be kept to under 10 milliseconds whenever possible by using whatever queuing methods are optimal for your network. Output queue delays are a quality of service (QoS) issue in Voice over IP for Cisco 7200 series, Cisco 7200 VXR, and Cisco 7500 series routers, and are discussed in the "Configure IP Networks for Real-Time Voice Traffic" section.
In Voice over Frame Relay, you need to make sure that voice traffic is not crowded out by data traffic. Strategies on how to manage Voice over Frame Relay voice traffic are discussed in the "Voice over Frame Relay Configuration Example" section
Jitter is another factor that affects delay. Jitter occurs when there is a variation between when a voice packet is expected to be received and when it actually is received, causing a discontinuity in the real-time voice stream. Voice devices such as Cisco 7200 series, Cisco 7200 VXR, and Cisco 7500 series routers with PA-VXB and PA-VXC port adapters compensate for jitter by setting up a playout buffer to play back voice smoothly. Playout control is handled through RTP encapsulation, either by selecting adaptive or nonadaptive playout-delay mode. In either mode, the default value for nominal delay is sufficient.
Figuring out the end-to-end delay is not difficult if you know the end-to-end signal paths or data paths, the codec, and the payload size of the packets. Adding the delays from the endpoints to the codecs at both ends, the encoder delay (which is 5 milliseconds for G.711 and G.726 codecs and 10 milliseconds for the G.729 codec), the packetization delay, and the fixed portion of the network delay yield the end-to-end delay for the connection.
Echo is hearing your own voice in the telephone receiver while you are talking. When timed properly, echo is reassuring to the speaker; if the echo exceeds approximately 25 milliseconds, it can be distracting and cause breaks in the conversation. In a traditional telephony network, echo is normally caused by a mismatch in impedance from the four-wire network switch conversion to the two-wire local loop and is controlled by echo cancelers. In voice packet-based networks, echo cancelers are built into the low-bit rate codecs and are operated on each digital signal processor (DSP). Echo cancelers are limited by design by the total amount of time they wait for the reflected speech to be received, which is known as an echo trail. The echo trail is normally 32 milliseconds.
In Cisco's voice implementations, echo cancellers are enabled using the echo-cancel enable command. The echo trails are configured using the echo-cancel-coverage command. For example, Voice over IP has configurable echo trails of 16, 24, and 32 milliseconds.
Although there are various types of signaling used in telecommunications today, this document describes only those with direct applicability to Cisco's voice implementations. The first one involves access signaling, which determines when a line has gone off-hook or on-hook (in other words, dial tone). Foreign Exchange Office (FXO) and Foreign Exchange Station (FXS) are types of access signaling. There are two common methods of providing this basic signal:
In Cisco's voice implementations, access signaling is configured using the signal command.
Another signaling technique used mainly between PBXs or other network-to-network telephony switches is known as E&M. There are five types of E&M signaling, as well as two different wiring methods. Cisco's voice implementation supports E&M types I, II, III, and V, using both two-wire and four-wire implementations. In Cisco's voice implementations, E&M signal types are configured using the type command.
For specific information on hardware and software requirements, parts and tools you need to perform the port adapter installation, and safety and ESD-prevention guidelines, see "Preparing for Installation."
As shown in Figure 1-3 and Figure 1-4, the PA-VXB and PA-VXC port adapters have four LEDs on the faceplate: a green enabled LED, a bicolor alarm LED, and two bicolor port status LEDs, one for each port. Table 1-3 lists the colors and functions of the LEDs.
| LED Label | Color | State | Function |
EN | Green | On | Indicates the PA-VXB and PA-VXC port adapters are powered up. |
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| Off | Indicates the PA-VXB and PA-VXC port adapters are not ready or disabled. |
AL | Amber | On | Indicates an alarm condition exists on the remote end of one of the T1/E1 ports. |
| Red | On | Indicates an alarm condition exists locally on one of the T1/E1 ports. |
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| Off | Indicates no alarms detected on either port. |
0 or 1 | Green | On | Indicates the port is enabled and in frame. |
| Yellow | On | Indicates the port is in loopback. |
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| Off | Indicates that the port is not enabled, the received signal is bad, or an alarm condition exists. |
The T1/E1 interface receptacles on the PA-VXB and PA-VXC port adapters are RJ-45 for both T1 (100-ohm) and E1 (120-ohm).
After you properly connect a port to a line, it takes approximately 30 seconds for the Cisco IOS to report that the line is up.
Each connection supports T1(100-ohm) or E1(120-ohm) interfaces that meet T1.403 and ACCUNET TR62411 standards. The RJ-45 connection does not require an external transceiver. The DS1 ports are T1 interfaces that use foil twisted-pair cables.
Shielded cables (FTP [foil twisted-pair]) with 120-ohm impedance are required to comply with CE marking requirements.
Figure 1-5 shows the PA-VXB and PA-VXC port adapter interface cable connector. See the "Cisco 7500 SeriesRemoving and Installing an Interface Processor" section for directions on connecting the cables to a PA-VXB or PA-VXC port adapter.
Table 1-4 lists the signal pinouts and descriptions for the RJ-45 connector.
| Pin | Signal |
|---|---|
1 | RX tip |
2 | RX ring |
3 | No connection |
4 | TX tip |
5 | TX ring |
6 | No connection |
7 | No connection |
8 | No connection |
This section discusses port adapter slot locations on the supported platforms. The illustrations that follow summarize slot location conventions on each platform.
Figure 1-6 shows a Cisco 7206 with port adapters installed. In the Cisco 7206, port adapter slot 1 is in the lower left position, and port adapter slot 6 is in the upper right position. (The Cisco 7202 and Cisco 7204 are not shown; however, the PA-VXB and PA-VXC port adapters can be installed in any available port adapter slot.)
Figure 1-7 shows a partial view of a VIP motherboard with installed port adapters. With the motherboard oriented as shown in Figure 1-7, the left port adapter is in port adapter slot 0, and the right port adapter is in port adapter slot 1. The slots are always numbered 0 and 1.
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Note In the Cisco 7507, and Cisco 7513 chassis, the VIP motherboard is installed vertically. In the Cisco 7505 chassis, the VIP motherboard is installed horizontally as shown in Figure 1-8. |
Interface processor slots are numbered as shown in Figure 1-8.
This section describes how to identify interface addresses for the PA-VXB and PA-VXC port adapters in supported platforms. Interface addresses specify the actual physical location of each interface on a router or switch.
Interfaces on the PA-VXB and PA-VXC port adapters installed in a router maintain the same address regardless of whether other port adapters are installed or removed. However, when you move a port adapter to a different slot, the first number in the interface address changes to reflect the new port adapter slot number.
Table 1-5 explains how to identify interface addresses.
| Platform | Interface Address Format | Numbers | Syntax |
|---|---|---|---|
Cisco 7200 series and Cisco 7200 VXR routers | Port-adapter-slot-number/interface-port-number | Port adapter slot—0 through 6 (depends on the number of slots in the router)1 Interface port—0 or 1 | 1/0 |
VIP2 and VIP4 in | Interface-processor-slot-number/port-adapter-slot-number/interface-port-number | Interface processor slot—0 through 12 (depends on the number of slots in the router) Port adapter slot—always 0 or 1 Interface port—0 or 1 | 3/0/0 |
| 1Port adapter slot 0 is reserved for the Fast Ethernet port on the I/O controller (if present). |
This section describes how to identify the interface addresses used for the PA-VXB and PA-VXC port adapters in Cisco 7200 series and Cisco 7200 VXR routers. The interface address is composed of a two-part number in the format port-adapter-slot-number/interface-port-number. See Table 1-5 for the interface address format.
In Cisco 7200 series and Cisco 7200 VXR routers, port adapter slots are numbered from the lower left to the upper right, beginning with port adapter slot 1 and continuing through port adapter slot 2 for the Cisco 7202, slot 4 for the Cisco 7204 and Cisco 7204VXR, and slot 6 for the Cisco 7206 and Cisco 7206VXR. (Port adapter slot 0 is reserved for the optional Fast Ethernet port on the I/O controller—if present.)
The interface addresses of the interfaces on the PA-VXB and PA-VXC port adapters in port adapter slot 1 are 1/0 or 1/1 (port adapter slot 1 and interfaces 0 or 1). If the PA-VXB or PA-VXC was in port adapter slot 4, these same interfaces would be numbered 4/0 or 4/1 (port adapter slot 4 and interfaces 0 or 1).
The interface addresses of the interfaces on the PA-VXB and PA-VXC port adapters in port adapter slot 2 are 2/0 and 2/1 (port adapter slot 2 and interfaces 0 and 1). If the PA-VXB or PA-VXC was in port adapter slot 1, these same interfaces would be numbered 1/0 and 1/1 (port adapter slot 1 and interfaces 0 and 1).
This section describes how to identify the interface addresses used for the PA-VXB and PA-VXC on a VIP in Cisco 7500 series routers.
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Note Although the processor slots in the 7-slot Cisco 7507 and the 13-slot Cisco 7513 and Cisco 7576 are vertically oriented and those in the 5-slot Cisco 7505 are horizontally oriented, all Cisco 7500 series routers use the same method for slot and port numbering. |
See Table 1-5 for the interface address format. The interface address is composed of a three-part number in the format interface-processor-slot-number/port-adapter-slot-number/interface-port-number.
If the VIP is inserted in interface processor slot 3, and port adapter slot 0, then the interface addresses of the PA-VXB and PA-VXC are 3/0/0 or 3/0/1 (interface processor slot 3, port adapter slot 0, and interfaces 0 and 1). If the port adapter was in port adapter slot 1 on the VIP, these same interface addresses would be numbered 3/1/0 and 3/1/1.
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Note If you remove the VIP with the PA-VXB or PA-VXC (shown in Figure 1-8) from interface processor slot 3 and install it in interface processor slot 2, the interface addresses become 2/1/0 or 2/1/1. |
Posted: Thu Sep 7 09:37:18 PDT 2000
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