Quality And Configuration Of The Switches; Traffic Priority - SpectraLink IP-DECT Server 400 Deployment Manual

Synchronization and Deployment Guide
If the traffic load causes problems for the base station synchronization, it may be necessary to
separate the base stations from the data network. Be aware that separation via VLAN may not
help as it is still using the same physical link.

Quality and Configuration of the Switches

The LAN based synchronization is highly dependent on the quality and configuration of the
deployment network. The single most important property of the switches in the network is their
ability to forward multicast Ethernet packets with low jitter, i.e. close to a constant delay. The total
forwarding jitter added by switches on any path through the deployment network should be less
than one microsecond and preferably less than 100 nanoseconds.
Unfortunately, it is usually difficult to find the forwarding jitter specified for a given switch. Lab
tests indicates that enterprise level switches generally has adequately low forwarding jitter,
whereas SOHO and unmanaged switches often do not meet the requirements and thus must
not be used.
When configuring the deployment network, multicast setup is critical for LAN synchronization to
work. Multicast is usually either blocked, forwarded as broadcast to all ports, forwarded according
to static configuration or forwarded to selected ports learned by IGMP snooping. The simplest
option is to forward as broadcast to all ports, but this might create unwanted traffic on unrelated
network parts. When using static configuration, the relevant multicast addresses listed earlier
must be forwarded to the ports forming the deployment network. Enabling IGMP snooping on the
switches allow them to automatically configure which ports the multicast packet should be
forwarded to, minimizing the network load caused by the LAN synchronization. In order to keep
the multicast configuration updated, a IGMP querier must be present in the network – this
functionality can be enabled in many enterprise class switches.

Traffic Priority:

All time critical PTPv2 packets sent by the LAN synchronization software is by default marked
with either an Expedited Forwarding (EF) (46/0x2e) priority for IPv4 and IPV6 packets or a
Class of Service value of 7 for VLAN encapsulated Ethernet packets. This is to allow the
switches to give preference to the LAN synchronization packets.
Since the Expedited Forwarding priority on IP packets is shared with voice RTP packets, this is
not sufficient to ensure strict priority over all other traffic for the PTPv2 events packets. There
are two possible solutions to this:

Give the highest priority to a custom IP priority and configure the server to apply this IP priority to
PTPv2 traffic.

Give the highest priority to multicast UDP packets on port 319 with the destination address
224.0.1.129 (IPv4) or FF02::181 (IPv6).
Configuration and Administration
A few configuration settings are used to control base station synchronization via LAN.
DECT/IP-DECT Server System Settings
The system wide settings for synchronization via LAN are located under Configuration -> Wireless Server
-> Base stations:
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