ESI 40 Business Phone Spezifikationen PDF herunterladen (Seite 16)

ESI Communications Servers Product Overview

Quality of Service (QoS) support

Quality of Service is an important component in any converged or pure IP telephone system. It increases the

likelihood for IP voice communications to be clear, and free of dropped calls and delayed audio.

QoS is defined as providing the means for specific data streams in a network to be prioritized over other types of

traffic. In the case of a voice over IP application, the IP packets carrying the voice conversation are given priority

over data packets. When using the built-in two-port data switch to connect the IP Phone and customer’s computer

to the same Ethernet port, it is highly advisable for the customer’s network to support QoS so that large downloads

do not affect the quality of voice communications to the IP phone.

Benefits of QoS

Networks that are designed to support QoS are best suited for IP deployment since quality of voice is judged by

the end-to-end experience of the user. It is not sufficient for ESI’s IP applications to support QoS if all network

components used in the transport of voice over the customer’s LAN are not properly configured for QoS support.

The benefits of end-to-end QoS in any IP telephony application are many, and when absent, quite noticeable to

the user:

• Available bandwidth is optimized by ensuring that voice packets are sent and delivered at a higher priority than

“regular” data traffic on the LAN. This may allow the customer to delay upgrading the speed of transmission of

his network. He may be able to defer this expense until other applications are added or IT changes in the

business dictate it is necessary.

• The quality of the IP conversation is improved by ensuring that voice packets are delivered and “reassembled”

at the other end of the conversation in order. This eliminates garbled conversation, hollowness, and noticeable

gaps in speech.

• Unlike data packets, voice packets cannot be resent if they are dropped. Jitter

is reduced for voice packets by

QoS. This improves the likelihood that all voice packets will not be dropped before being delivered at the other

end of the IP conversation, as happens when the amount of jitter of a packet exceeds an acceptable level.

• The latency with which voice packets are delivered is minimized in a network employing QoS. This results in

more natural-sounding speech patterns for both sides of an IP conversation.

802.1p and 802.1q standards for VLANs

Virtual LANs (VLANs) provide a method of separating data streams to make a local area network appear to be two

or more networks. A VLAN is likely to be implemented in a business where IP telephony is heavily used. The

VLAN segregates the voice packets onto their own network to prevent the degradation of voice quality, loss of

packets, and late delivery of voice packets (latency).

Two standards are concerned with VLAN. Both are required to be supported in order to adequately support VLAN

operation. These are:

• 802.1p — Provides for the prioritization of voice packets. This standard establishes eight levels of priority,

0 through 7, with 7 being the highest priority. Level 7 is reserved for those applications and packets that are

considered network-critical. Levels 5 and 6 identify packets that are delay-sensitive. Priority levels below 5 are

used for “loss-eligible” data, meaning that if a packet is lost and must be retransmitted, nothing is affected. This

is not the case with voice, where if a packet is lost, portions of words will be missing or unintelligible. ESI

defines its prioritization field at 5.

• 802.1q — Dictates how the prioritization level (or “tag”) is attached to each packet. Without this tagging of voice

packets, prioritization would not be possible because there would be no differentiation between types of packets.

By compliance to the 802.1p and 802.1q standards, ESI’s local IP phones have built-in prioritization to simplify

managing traffic and QoS over a LAN.

The variation from packet-to-packet in transit time, expressed in milliseconds. For a more detailed explanation, see the Esi-Link Product Overview

(ESI # 0450-0214).

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