Comtech EF Data CTOG-250 - Manual (Page 224)

CTOG-250 Comtech Traffic Optimization Gateway Revision 1
Appendix E MN-CTOG250
E–2
transport efficiency and b) a strong, packet-based Quality of Service (QoS) engine. Advanced
VSAT provides a very strong and tightly coupled solution for both.
Advanced VSAT Outbound ACM offers significant operational enhancements:
• ACM turns fade margin into increased link capacity – gains of 100% or more are possible
when compared to traditional Constant Coding and Modulation (CCM). This is
accomplished by automatically adapting the modulation type and FEC code rate to give
highest possible throughput.
• ACM maximizes throughput regardless of link conditions (noise, clear sky, rain fade,
inclined orbit, antenna pointing error, or other impairments). Initial setup is easy and
requires no further user intervention.
• With Point-to-Multipoint CCM systems, the network wide transmission efficiency
(bits/Hz) must be set to support the most disadvantaged terminals. This results in
tremendous inefficiencies and either higher than necessary OpEx (satellite operating
expense/lease cost) or ‘advantageous terminal starving’ (i.e., terminals that could be
receiving much great data (bits/Hz) are constrained to the CCM efficiency.
• With a CCM system, severe rain fading can cause total loss of the link – the result being
zero throughput. ACM can keep the link active, with lower throughput during heavy
fades, and can yield much higher system availability.
E.1.1 Background
The Advanced VSAT Outbound carrier is based on DVB-S2 standards. The DVB-S2 standardized a
suite of 28 ModCods that work over an Es/No range of -2.3 dB to 16.0 dB (ideal). They have also
defined much of the system for both ACM and a related feature called Variable Coding and
Modulation (VCM). Because DVB-S2 did not specify the means of passing necessary information
between system components, the ACM schemes adopted by different manufacturers are
generally not interoperable.
During ACM operation, control parameters between modems are transferred over the satellite
link along with user traffic in what is usually referred to as Inband Signaling. DVB-S2 also
describes an ACM method using “out-of-band” ACM controllers (per EN 302 307 Annex D.1) that
is not supported with Advanced VSAT.
In ACM mode, the symbol rate remains constant and the ModCod changes to preserve the data
integrity. Most links are designed with enough Es/No margin to provide error-free performance
under faded conditions when there is higher attenuation in the uplink path to or the downlink
path from the satellite.