Specifications

MBG Engineering Guidelines, Release 8.0
Note: The frame size override only affects the streams to and from devices. The ICP-side streaming is always
auto-negotiated. On SIP trunks, both WAN and ICP sides can be specified separately.
11.5 TFTP Block Size
MiNet devices use the TFTP protocol to fetch their firmware from the MBG server. The Mitel TFTP server is
slightly non-standard – it uses symmetric UDP to traverse NAT devices, and a “sliding window” to improve
performance – but is otherwise RFC-compliant.
The default block size in the TFTP protocol is 512 bytes, and with large firmware loads in a lock-step protocol
like TFTP
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this can take a prohibitively long amount of time to download. MBG employs the TFTP “blksize” option
to attempt to transfer 4096 byte blocks, if possible. Depending on one’s network this may or may not be possible:
the large packets will require fragmentation on a standard Ethernet network with a 1500 byte MTU, and some
ISPs do not permit this.
Set the global TFTP block size to the largest value that works. Only the options in the pulldown (512, 1024, 2048
or 4096) are permitted.
11.6 Set-side Codec
MBG allows transcoding from G.711 to G.729 or from G.729 to G.711, if Compression licenses have been
purchased. The usual use of this is to force a lower-bandwidth codec for remote devices. However, it can also be
used to force G.711 for devices that do not support G.729 (e.g. voicemail systems) and that register to MCD as a
MiNet IP device.
The global default setting is found under Configuration; Settings; MiNet Options, but the codec can also be set
for each individual device.
11.7 SRTP Port Range
Each active call on the MBG requires two UDP ports for the RTP stream. Some SIP calls (video, T.38) may
require four or more ports. (Two ports per SDP media line are required.) Ports are only used while calls are
actually active, and are released upon the end of the call, or, for MiNet, end of the stream (which may be a hold
or transfer, not actually the end of the “call”).
The default SRTP port range of 20000-31000 provides enough ports for over 5000 simultaneous voice calls or
2500 video calls. For smaller sites, this range can be reduced if desired. However, be sure any third-party
firewalls have the matching range programmed. Mismatches between a DMZ firewall and the MBG will result in
some calls having no audio (or one-way audio). This problem appears after many successful calls and is cleared
by a reset of the MBG.
11.8 DSCP
MBG allows configuration of a Differentiated Services Code Point (DSCP) to be inserted into the header of ip
packets. A separate value can be configured for signaliing and media packets. This is one of the tools available
to help improve voice quality in some congested managed network environments.
As an example, consider a deployment with a managed WAN on the icp side of MBG and the Internet on the
sets side of MBG. Now consider a situation where a managed WAN Service Level Agreement (SLA) is
purchased that divides traffic into classes, including an Expedited Forwarding (EF) queue with priority over other
traffic classes. In this situation, MBG can be configured to insert an EF value (46 decimal) for voice packets as
they pass from the Internet to the icp side so that they can be protected on the managed WAN during periods of
4 Receipt of each block must be acknowledged before the next block is sent
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