Guidelines for providers offering VOIP service over Amplex’s network

Amplex provides service to customers using a fixed position wireless technology.  The network is capable of providing excellent performance for voice services – provided that the guidelines in this post are followed.  Failure to follow these recommendations will likely result in poor call quality and significant customer dissatisfaction.

#1. Before selling or purchasing voice services please check with Amplex to determine if the existing Internet service is appropriate.   We do not recommend deploying VoIP services to locations served by 900Mhz or 802.11 equipment.  In some cases we may be able to upgrade service to accommodate VoIP services.   Significant costs may be encountered if  additional height is needed for the antenna.  The type of equipment in use can not be determined by looking at our invoices – you must contact us.

#2. Notify Amplex of the IP address(es) of the service providers VOIP gateway.  Amplex runs a ‘Quality of Service’ (QoS) enabled network.  This means that we classify and prioritize traffic flowing over the network.   For traffic entering the network at our external borders (from the Internet) any existing QoS is reset to default priority. Traffic is then classified into several categories.  For the purpose of this discussion RTP (the voice part of a IP call) from KNOWN PROVIDERS is set to high priority (DSCP 46).   If you notify Amplex of the VOIP gateway address we will mark traffic as high priority. As of the publication date Buckeye Telesystems is the only provider to have supplied this information.

#3. Notify Amplex of the amount of traffic at the customer location that is needed for the number of call sessions * codec bandwidth.  Based on this information we will enable and set a high priority CIR (committed information rate) for the desired amount of bandwidth. Do not request more bandwidth than is needed as excessive bandwidth reservations will negatively affect other network traffic.

#4). Set the correct DSCP headers in outgoing RTP traffic.   Outgoing traffic to be handled as high priority is determined by the CPE (Amplex’s Customer Premise Equipment) based on the low latency bit of the DSCP header – specifically bit 3 of the 6 bit DSCP header.  We suggest using DSCP code point 46 (or 101110 in binary, some equipment may refer to this as ‘EF’ for Expedited Forwarding).  Note that DSCP is a 6 bit field that is part of an 8 bit IP header.  Equipment that requests a 8 bit value should use 10111000 as a binary value.  Either the customer VIOP gateway or the SIP phones must set DSCP appropriately.

#5)  The customer router MUST clear, at a minimum, the low latency bit of the DSCP field of all non-voice traffic. Failure to accomplish this step will allow other common traffic (SCP, SSH, video) to consume high priority upload bandwidth resulting in poor call quality.  Manipulation of the DSCP values can be handled by many business class routers and firewalls. We have found the Juniper SRX series to be cost effective and capable.

#6.  Check with Amplex to verify that we are seeing traffic flowing in the high priority queue of the radio, in both directions, during a VOIP call.

Following these guidelines should result in a quality VOIP experience. If you are considering either purchasing or selling voice service over Amplex’s network we strongly encourage you to ensure that the VOIP provider and your internal networking team (or consultant) is capable of understanding and following the recommendations in this whitepaper.

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