Specifications

ShoreTel Architecture Media
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DTAS makes the distributed TMS architecture transparent to applications and reduces the number of
network connections needed if every application were required to connect to every TMS instance.
Voice switches are assigned to specific TMS servers through ShoreTel Director. Assignment of a voice
switch to a TMS server is restricted to TMS servers at its own site, or if there is no TMS server at that
site, to the nearest TMS servers in the site hierarchy above.
ShoreTel applications that use ShoreTAPI, such as CAS/IPDS and workgroups, communicate directly
with TMS. Other applications that use TAPI use ShoreTel Transport to communicate with TMS. TMS
uses Network Call Control to exchange commands and events with ShoreTel voice switches so that
TMS can present extensions and trunks as TAPI lines to the applications.
Media
Media travels through the ShoreTel system using Real-Time Protocol (RTP) and Secure Real-Time
Protocol (SRTP). After call setup, media flows directly between IP phones via RTP. For ShoreTel 400-
Series IP phones, media flows between phones using SRTP if it has been enabled through the Media
Encryption option in the Call Control > Options page in ShoreTel Director. The ShoreTel voice switch is
involved only when setting up or tearing down a call (Figure 6).
A voicemail message is normal RTP traffic, unless it is a recorded voicemail message moving from
one server to another. Voicemail media streams conform to the G.711 codec. If a switch or IP phone is
configured to use G.729 or ADPCM (for example, for an intersite call), a media server proxy is used to
transcode between G.729/ADPCM and G.711. Because the media server proxy is a switch resource,
there are a limited number of G.729 proxies. If there are insufficient G.729 proxies, then ADPCM is
used instead.
IP Phone to IP Phone
Media between IP phones or other IP endpoints such as SoftPhones, passes directly point to point.
IP Phone to Analog Phone
Analog phones depend on the ShoreTel voice switch to which they are connected. Media from IP
endpoints must pass though the ShoreTel voice switch supporting the analog phone.
Analog Phone to Analog Phone
Media between analog phones passes though the switches supporting the analog phones.
Analog Phone to Voicemail
Voicemail media from analog phones passes through the switch supporting the analog phone before
going to voicemail via the server’s media driver. When the analog phone is located on the same LAN
as the host server, the ShoreTel voice switch connects to the server using a G711 ulaw codec. If the
analog phone is connected via a WAN, and there are ShoreTel voice switch resources available, the
ShoreTel voice switch uses an intersite codec (G729 or ADPCM). If ShoreTel voice switch resources
are not available, the call reverts to the G711 ulaw codec.