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Joined: Mar 2004
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Joined: Mar 2004
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I can't get my phone to stop this 1 second ecco. It only occures on out of office calls, not on interoffice calls. Our installer can not fix it. Anyone out there with some ideas. It makes my phone useless.


John Reynolds
SleepQuest, Inc.
Atcom VoIP Demo
VoIP Demo
Joined: Mar 2004
Posts: 5
Joined: Mar 2004
Posts: 5
echo problems are the toughest to deal with IP phones are especially a problem if they also go out over a T1. Here is a long tech note maybe it can help you look in the right direction. The bottom line is echo has to travel quite a distance to be noticed that is why it does not happen on internal calls.

Topic:VoIP Tech Notes
Author:Mingwei Hsu/Steven Shearer


Internet Telephony Glossary

Echo Compensation
Echo in a telephone network is caused by signal reflections generated by the hybrid circuit that converts between a 4-wire circuit and a 2-wire circuit. These reflections of the speaker's voice are heard in the speaker's ear. Echo is present even in a conventional circuit-switched telephone network. However, it's acceptable because the round trip delays through the network are smaller than 50 ms, and the echo is marked by the normal side tone every telephone generates.

Echo becomes a problem in voice packet networks because the round trip delay through the network is almost always greater than 50 ms. Thus, echo cancellation techniques are always used. ITU standard G.165 defines performance requirements that are currently required for echo cancellers. The ITU is defining much more stringent performance requirements in the G.IEC specification.

Echo is generated toward the packet network from the telephone network. The echo canceller compares the voice data received from the packet network with voice data being transmitted to the packet network. The echo from the telephone network hybrid is removed by a digital filter on the transmit path into the packet network.
Lost Packet Compensation
Lost packets can be an even more severe problem, depending on the type of packet network that is being used. Because IP networks do not guarantee service, they will usually exhibit a much higher incidence of loss voice packets than ATM networks. In current IP networks, all voice frames are treated like data. Under peak loads and congestion, voice frames will be dropped equally with data frames. The data frames, however, are not time sensitive and dropped packets can be appropriately corrected through the process of re-transmission. Lost voice packets, however, cannot be dealt with in this manner.

Some schemes used by Voice over Packets software to address the problem of lost frames are:

Interpolate for lost speech packets by replaying the last packet received during the interval when the lost packet was supposed to be played out. This scheme is a simple method that fills the time between non-contiguous speech frames. It works well when the incidence of lost frames is infrequent. It does not work very well if there are a number of lost packets in a row or a burst of lost packets.
Send redundant information at the expense of bandwidth utilization. The basic approach replicates and sends the nth packet of voice information along with the (n+1)th packet. This method has the advantage of being able to exactly correct for the lost packet. However, this approach uses more bandwidth and also creates greater delay.
A hybrid approach uses a much lower bandwidth voice coder to provide redundant information carried along in the (n+1)th packet. This reduces the problem of the extra bandwidth required, but fails to solve the problem of delay.
Quality of Service
Delay causes two problems - echo and talker overlap.
Echo is caused by the signal reflections of the speaker's voice from the far end telephone equipment back into the speaker's ear. Echo becomes a significant problem when the round trip delay becomes greater than 50 ms. Since echo is perceived as a significant quality problem, voice over packer systems must address the need for echo control and implement some means of echo cancellation.
Talker overlap (or the problem of one talker stepping on the other talker's speech) becomes significant if the one-way delay becomes greater than 250 ms. The end-to-end delay budget is therefore the major constraint and driving requirement for reducing delay through a packer network.

Following are sources of delay in an end to end Voice over Packer call:

Accumulation Delay (sometimes called algorithmic delay): This delay is caused by the need to collect a frame of voice samples to be processed by the voice coder. It is related to the type of voice coder used and varies from a single sample time (125 s) to many milliseconds. A representative list of standard voice coders and their frame times follows:
G.726 ADPAM (16, 24, 32, 40 Kbps) - .125 ms
G.728 LD-CELP (16 Kbps) - 2.5 ms
G.729 CS-ACELP (8 Kbps) - 10 ms
G.723.1 Multi-Rate Coder (5.3, 6.4 Kbps) - 30 ms
Processing Delay: This delay is caused by the actual process of encoding and collecting the encoded samples into a packer for transmission over the packer network. The encoding delay is a function of both the processor execution time and the type of algorithm used. Often, multiple voice coder frames will be collected in a single packer to reduce the packet network overhead. For example, three frames of G.729 codewords, equaling 30 ms of speech, may be collected and packed into a single packet.
Network Delay: This delay is caused by the physical medium and protocols used to transmit the voice data, and by the buffers used to remove packet jitter on the receive side. Network delay is a function of the capacity of the links in the network and the processing that occurs as the packets transit the network. The jitter buffers add delay which is used to remove the packet delay variation that each packet is subjected to as it transits the packet network. This delay can be a significant part of the overall delay since packet delay variations can be as high as 70-100 ms in some Frame Relay network and IP network.
The delay problem is compounded by the need to remove jitter, a variable inter-packet timing caused by the network a packet traverses. Removing jitter requires collecting packets and holding them long enough to allow the slowest packets to arrive in time to be delayed in the correct sequence. This causes additional delay.

The two conflicting goals of minimizing delay and removing jitter have engendered various schemes to adapt the jitter buffer size to match the time varying requirements of network jitter removal. This adaptation has the explicit goal of minimizing the size and delay of the jitter buffer, while at the same time preventing buffer underflow caused by jitter.

Two approaches to adapting the jitter buffer size are detailed below. The approach selected will depend on the type of network the packets are traversing.
The first approach is to measure the variation of packet level in the jitter buffer over a period of time, and incrementally adapt the buffer size to match the calculated jitter. This approach works best with networks that provide a consistent jitter performance over time, such as ATM network.
The second approach is to count the number of packets that arrive late and create a ratio of these packets to the number of packets that are successfully processed. This ratio is them used to adjust the jitter buffer to target a predetermined allowable late packet ratio. This approach works best with the network with highly variable packet inter-arrival intervals, such as IP networks.

In addition to the techniques described above, the network must be configured and managed to provide minimal delay and jitter, enable a consistent quality of service.
Access Line
A communications line (e.g. circuit) interconnecting a frame-relay-compatible device(DTE) to a frame-relay switch (DCE). See also Trunk Line.

Access Rate (AR)
The data rate of the user access channel. The speed of the access channel determines how rapidly (maximum rate) the end user can inject data into a frame relay network.
ANI: Automatic Number Identification.
A phone call arrives at your home or office. At the front of the phone call is a series of digits which tell you the phone number calling you. These digits may arrive in a analog or digital form. They may arrive as Touch Tone digits inside the phone call. They may arrive in a digital form on a separate circuit. Whichever way they arrive, you will need some equipment to decipher the digits AND do "something" with them. That "something" might be throwing them in a database and throwing up your customer's record on a screen in front of your telephone agent as he answers the phone. "Good morning Mr. Smith." Some large users say they could save as much as 20 seconds on the average IN-WATS call if they knew early on the phone number of the person calling them. They wouldn't need to ask their regular customers for their address, phone number, credit card number, etc. It could all be there in the database. ANI is also called CLI, which stands for Calling Line Identification.

American National Standards Institute (ANSI)
Devises and proposes recommendations for international communications standards. See also Comite Consultatif International Telegraphique et Telephonique (CCITT).

A generic term for voice response equipment and services. Audiotex may be passive or interactive. In "passive," the classic applications fall into "Jokes, Scopes and Soaps." Dial a number, hear the joke of the day. Dial a number, hear your horoscope. Dial a number, hear what's happening on your latest TV soap. In its interactive form, audiotex is basically another term for Interactive Voice Response (IVR). You call a phone number. A machine answers. It presents you several options, "Push 1 for information on Plays, Push 2 for information on movies, Push 3 for information on Museums." If you push 2, the machine may come
back, "Push 1 for movies on the south side of town, Push 2 for movies on the north side of town, etc." Or it says, "To hear your present bank balance, enter your bank account number in now."

Asynchronous Transfer Mode (ATM)
See Tech Tip Number -

Backward Explicit Congestion Notification (BECN)
A bit set by a frame relay network to notify an interface device(DTE) that congestion avoidance procedures should be initiated by the sending device.

The range of frequencies, expressed in Kilobits per second, that can pass over a given data transmission channel within a frame relay network. The bandwidth determines the rate at which information can be sent through a channel - the greater the bandwidth, the more information that can be sent in a given amount of time.

A device that supports LAN-to-LAN communications. Bridges may be equipped to provide frame relay support to the LAN devices they serve. A frame-relay-capable bridge encapsulates LAN frames in frame relay frames and feeds those frame relay frames to a frame relay switch for transmission across the network. A frame-relay-capable bridge also receives frame relay frames from the network, strips the frame relay frame off each LAN frame, and passes the LAN frame on to the end device. Bridges are generally used to connect local area network (LAN) segments to other LAN segments or to a wide area network (WAN). They route traffic on the Level 2 LAN protocol (e.g., the Media Access Control address), which occupies the lower sub layer of the LAN OSI data link layer. See also Router.

In the context of a frame relay network, data that uses bandwidth only sporadically; that is, information that does not use the total bandwidth of a circuit 100 percent of the time. During pauses, channels are idle; and no traffic flows across them in either direction. Interactive and LAN-to-LAN data is bursty in nature, because it is sent intermittently, and in between data transmissions the channel experiences idle time waiting for the DTEs to respond to the transmitted data user's input of waiting for the user to send more data.

Generically refers to the user access channel across which frame relay data travels. Within a given T1 or E1 physical line, a channel can be one of the following, depending of how the line is configured.

The entire T1/E1 line is considered a channel, where:
The T1 line operates at speeds of 1.536 Mbps and is a single channel consisting of 24 T1 time slots.
The E1 line operates at speeds of 1.984 Mbps and is a single channel consisting of 20 E1 time slots.

The channel is any one of N time slots within a given line, where:
The T1 line consists of any one or more channels. Each channel is any one of 24 time slots. The T1 line operates at speeds in multiples of 56/64 Kbps to 1.536 Mbps, with aggregate speed not exceeding 1.536 Mbps.
The E1 line consists of one or more channels. Each channel is any one of 31 time slots. The E1 line operates at speeds in multiples of 64 Kbps to 1.984 Mbps, with aggregate speed not exceeding 1.984 Mbps.

The T1/E1 channel is one of the following groupings of consecutively or nonconsecutively assigned time slots:
N T/1 time slots (NX56/64Kbps where N = 1 to 23 T1 time slots per FT1 channel).
N E1 time slots (NX64Kbps, where N = 1 to 30 time slots per E1 channel).

Channel Service Unit (CSU)
An ancillary device needed to adapt the V.35 interface on a F.R. DTE to the T1 (or E1) interface on a frame relay switch. The T1 (or E1) signal format on the frame relay switch is not compatible with the V.35 interface on the DTE: therefore, a CSU or similar device, placed between the DTE and the frame relay switch, is needed to perform the required conversion.

Committed Burst Size (Bc)
The maximum amount of data (in bits) that the network agrees to transfer, under normal conditions, during a time interval Tc. See also Excess Burst Size (Be).

Comite Consultatif International Telegraphique et Telephonique (CCITT)
International Consultative Committee for Telegraphy and Telephony, a standards organization that devises and proposes recommendations for international communications. See also American National Standards Institute (ANSI).

Committed Information Rate (CIR)
The committed rate (in bits per second) at which the ingress access interface trunk interfaces, and egress access interface of a frame relay network transfer information to the destination frame relay end system under normal conditions. The rate is averaged over a minimum time interval Tc.

Committed Rate Measurement Interval (Tc)
The time interval during which the user can send only Bc-committed amount of data and Be excess amount of data. In general, the duration of Tc is proportional to the "burstiness" of the traffic. Tc is computed (from the subscription parameters of CIR and Bc) as Tc = Bc/CIR. Tc is not a periodic time interval. Instead, it is used only to measure incoming data, during which it acts like a sliding window. Incoming data triggers the Tc interval, which continues until it completes its commuted duration. See also Committed Information Rate (CIR) and committed Burst Size (Bc).

Cyclic Redundancy Check (CRC)
A computational means to ensure the accuracy of frames transmitted between devices in a frame relay network. The mathematical function is computed, before the frame is transmitted, at the originating device. Its numerical value is computed based on the content of the frame. This value is compared with a recomputed value of the function at the destination device. See also Frame Check Sequence (FCS).

Data Communications Equipment (DCE)
Term defined by both frame relay and X.25 committees, that applies to switching equipment and is distinguished from the devices that attach to the network (DTE). Also see DTE.

Data Link Connection Identifier (DLCI)
A unique number assigned to a PVC end point in a frame relay network. Identifies a particular PVC endpoint within a user's access channel in a frame relay network and has local significance only to that channel.

Dual Tone Multi Frequency (DTMF)
A fancy term for describing push button or Touch Tone dialing. (Touch Tone is a registered trademark of AT&T.) In DTMF, when you touch a button on a push-button dial, it makes a tone, which is actually the combination of two tones, one high frequency and one low frequency. Thus the name Dual Tone Multi Frequency. In U.S. telephony, there are actually two types of DTMF signaling -- one that is used on normal business or home push-button/Touch Tone phones, and one that is used for signaling [T103] within [T101] the telephone network itself. When you go into a central office, look for the test-board. There you will see what looks like a standard Touch Tone pad.Next to the pad there will be a small toggle switch that allows you to choose the sounds the Touch Tone pad will make -- either normal Touch Tone dialing or the network version.

Direct Inward Dialing (DID)
The ability for a caller outside a company to call an internal extension without having to pass through an operator or attendant. In large PBX systems, the dialed digits are passed down the line from the CO (central office). The PBX then completes the call. Direct Inward Dialing is often proposed as Centrex's major feature. But automated attendants (a specialized form of interactive voice response systems) also provide a similar service

Discard Eligibility (DE)
A user-set bit indicating that a frame may be discarded in preference to other frames if congestion occurs, to maintain the committed quality of service within the network. Frames with the DE bit set are considered Be excess data. See also Excess burst Size (Be).

Frame relay frames leaving a frame relay network in the direction toward the destination device. Contrast with Ingress.

End Device
The ultimate source or destination of data flowing through a frame relay network sometime referred to as a Data Terminal Equipment (DTE). As a source device, it sends data to an interface device for encapsulation in a frame relay frame. As a destination device, it receives de-encapsulated data (i.e., the frame relay frame is stripped off, leaving only the user's data) from the interface device. Also see DCE.
NOTE: An end device can be an application program or some operator-controlled device (e.g., workstation). In a LAN environment, the end device could be a file server or host.

A process by which an interface device places an end device's protocol-specific frames inside a frame relay frame. The network accepts only frames formatted specifically for frame relay; hence, interface devices acting as interfaces to an frame relay network must perform encapsulation. See also Interface device or Frame-Relay-Capable Interface Device.

Excess Burst Size (Be)
The maximum amount of uncommitted data (in bits) in excess of Bc that a frame relay network can attempt to deliver during a time interval Tc. This data (Be) generally is delivered with a lower probability than Bc. The network treats Be data as discard eligible. See also Committed burst Size (Bc).

Transmission rate of 2.048 Mbps on E1 communications lines. An E1 facility carriers a 2.048 Mbps digital signal. See also T1 and channel.

File Server
In the context of frame relay network supporting LAN-to-LAN communications, a device connecting a series of workstations within a given LAN. The device performs error recover and flow control functions as well as end-to-end acknowledgment of data during data transfer, thereby significantly reducing overhead within the frame relay network.

Forward Explicit Congestion Notification (FECN)
A bit set by a frame relay network to notify an interface device (DTE) that congestion avoidance procedures should be initiated by the receiving device. See also BECN.

Frame Check Sequence (FCS)
The standard 16-bit cyclic redundancy check used for HDLC and frame relay frames. The FCS detects bit errors occurring in the bits of the frame between the opening flag and the FCS, and is only effective in detecting errors in frames no larger than 4096 octets. See also Cyclic Redundancy Check (CRC).

Frame-Relay-Capable Interface Device
A communications device that performs encapsulation. Frame-relay-capable routers and bridges are examples of interface devices used to interface the customer's equipment to a frame relay network. See also Interface Device and Encapsulation.

Frame Relay Frame
A variable-length unit of data, in frame-relay format that is transmitted through a frame relay network as pure data. Contrast with Packet. See also Q.922A.

Frame Relay Network
A telecommunications network based on frame relay technology. Data is multiplexed. Contrast with Packet-Switching Network.

Ground Start
A way of signaling on subscriber trunks in which one side of the two wire trunk (typically the "Ring" conductor of the Tip and Ring) is momentarily grounded to get dialtone. There are two types of switched trunks one can typically lease from a local phone company -- ground start and loop start. PBXs work best on ground start trunks, though many will work -- albeit intermittently -- on both types. Normal single line phones and key systems typically work on loop start lines. You must be careful to order the correct type of trunk from your local phone company and correctly install your telephone system at your end -- so that they both match. Technically, a ground start trunk initiates an outgoing trunk seizure by applying maximum local resistance of 550 ohms to the tip conductor. See Loop Start.

High Level Data Link control (HDLC)
A generic link-level communications protocol developed by the International Organization for Standardization (ISO). HDLC manages synchronous, code-transparent, serial information transfer over a link connection. See also Synchronous Data Link Control (SDLC).

A single trunk line between two switches in a frame relay network. An established PVC consists of a certain number of hops, spanning the distance from the ingress access interface to the egress access interface within the network.

Host Computer
A communications device that enables users to run applications programs to perform such functions as text editing, program execution, access to data bases, etc.

Frame relay frames from an access device toward the frame relay network. Contrast with Egress.

Interface Device
Provides the interface between the end device(s) and a frame relay network by encapsulating the user's native protocol in frame relay frames and sending the frames across the frame relay backbone. See also Encapsulation and Frame-Relay-Capable Interface Device.

Link Access Procedure Balanced (LAPB)
The balanced-mode, enhanced, version of HDLC. Used in X.25 packet-switching networks. Contrast with LAPD.

Link Access Procedure on the D-channel (LAPD)
A protocol that operates at the data link layer (layer 2) of the OSI architecture. LAPD is used to convey information between layer 3 entities across the frame relay network. The D-channel carries signaling information for circuit switching. Contrast with LAPB.

Local Area Network (LAN)
A privately owned network that offers high-speed communications channels to connect information processing equipment in a limited geographic area.

Loop Start
You "start" (seize) a phone line or trunk by giving it a supervisory signal. That signal is typically taking your phone off hook. There are two ways you can do that -- ground start or loop start. With loop start, you seize a line by bridging through a resistance the tip and ring (both wires) of your telephone line. See Ground Start.

LAN Protocols
A range of LAN protocols supported by a frame relay network, including Transmission Control Protocol/Internet Protocol (TCP/IP), Apple Talk, Xerox Network System (XNS), Internetwork Packet Exchange (IPX), and Common Operating System used by DOS-based PCs.

LAN Segment
In the context of a frame relay network supporting LAN-to-LAN communications, a LAN linked to another LAN by a bridge. Bridges enable two LANs to function like a single, large LAN by passing data from one LAN segment to another. To communicate with each other, the bridged LAN segments must use the same native protocol. See also Bridge.

A group of fixed-length binary digits, including the data and call control signals, that are transmitted through an X.25 packet-switching network as a composite whole. The data, call control signals, and possible error control information are arranged in a predetermined format. Packets do not always travel the same pathway but are arranged in proper sequence at the destination side before forwarding the complete message to an addressee. Contrast with Frame Relay Frame.

Packet-Switching Network
A telecommunications network based on packet-switching technology, wherein a transmission channel is occupied only for the duration of the transmission of the packet. Contrast with Frame Relay Network.

A numerical code that controls an aspect of terminal and/or network operation. Parameters control such aspects as page size, data transmission speed, and timing options.

Permanent virtual Circuit (PVC)
A frame relay logical link, whose endpoints and class of service are defined by network management. Analogous to an X.25 permanent virtual circuit, a PVC (often referred to as a PVC) consists of the originating frame relay network element address, originating data link control identifier, terminating frame relay network element address, and termination data link control identifier. Originating refers to the access interface from which the PVC is initiated. Terminating refers to the access interface at which the PVC stops. Many data network customers require a PVC between two points. Data terminating equipment with a need for continuous communication use PVCs. See also Data Link Connection Identifier (DLCI).

Q.922 Annex A (Q.922A)
The international draft standard that defines the structure of frame relay frames. Based on the Q.922A frame format developed by the CCITT. All frame relay frames entering a frame relay network automatically conform to this structure. Contrast with Link Access Procedure Balanced (LAPB).

Q.922A Frame
A variable-length unit of data, formatted in frame-relay (Q.922A) format, that is transmitted through a frame relay network as pure data (i.e., it contains no flow control information ). Contrast with Packet. See also Frame Relay Frame.

Ringing Voltage
In addition to talk battery, a Central Office provides ringing signaling. Ring Voltage is generally 70 to 90 volts at 17 Hz to 20 Hz.

A device that supports LAN-to-LAN communications. Routers may be equipped to provide frame relay support to the LAN devices they serve. A frame-relay-capable router encapsulates LAN frames in frame relay frames and feeds those frame relay frames to a frame relay switch for transmission across the network. A frame-relay-capable router also receives frame relay frames from the network, strips the frame relay frame off each frame to product the original LAN frame, and passes the LAN frame on to the end device. Routers connect multiple LAN segments to each other or to a WAN. Routers route traffic on the Level 3 LAN protocol (e.g., the Internet Protocol address). See also Bridge.

Statistical Multiplexing
Interleaving the data input of two or more devices on a single channel or access line for transmission through a frame relay network. Interleaving of data is accomplished using the DLCI.

Synchronous Data Link Control (SDLC)
A link-level communications protocol used in an International Business Machines (IBM) Systems Network Architecture (SNA) network that manages synchronous, code-transparent, serial information transfer over a link connection. SDLC is a subset of the more generic High-Level Data Link Control (HDLC) protocol developed by the International Organization for Standardization (ISO).

Transmission rate of 1.544 Mbps on T1 communications lines. A T1 facility carriers a 1.544 Mbps digital signal. Also referred to as digital signal level 1 (DS-1). See also E1 and channel.

Trunk Line
A communications line connecting two frame relay switches to each other.

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