SatLink VSAT User Guide
Publication no. 101557
Copyright © 2009 – STM Group, Inc.
Page 63 (160)
11. Quality of Service
QoS is used to provide differentiated handling of different types of IP traffic. The aim is to be able to
satisfy traffic flows for applications with different requirements, while at the same time minimizing the
required bandwidth on the return link. For example, to assure a given return link bandwidth with minimal
delay jitter for VoIP or Video real time traffic by avoiding influence from traffic that is not so sensitive to
delay jitter, e.g. Web browsing and FTP.
The STM SatLink VSAT does QoS differentiation on the return link based on the DiffServ architecture,
and can work in networks that offer performance as per hop behaviour (PHB) as defined in the DiffServ
architecture. QoS on the forward link is handled by the DVB-RCS Hub alone, and not discussed further in
this manual.
The STM SatLink VSAT supports use of the following PHBs and PHB groups on the return link
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:
• Best Effort (BE) PHB
• Critical Data (CD) PHB
• Real Time Video Conferencing (RT-ViC) PHB group
• Real Time Voice over IP (RT-VoIP) PHB group
Internally the STM SatLink VSAT classifies the traffic that shall be sent on the return link into different
QoS groups. One or more QoS groups will then be mapped to a PHB / PHB group in the network. The
VSAT internally supports the following QoS groups:
QoS Group ID QoS Group Name Maps to PHB
0 Best Effort Best Effort (BE)
1 VoIP Audio Real Time Voice over IP (RT-VoIP)
2 VoIP Signalling Real Time Voice over IP (RT-VoIP)
3 ViC Video Real Time Video Conferencing (RT-ViC)
4 ViC Audio Real Time Video Conferencing (RT-ViC)
5 ViC Signalling Real Time Video Conferencing (RT-ViC)
6 Critical Data Critical Data (CD)
The QoS implementation in the VSAT is based on a multi-field classifier, i.e. the DSCP/TOS field and
other fields in the IP header are used to classify which QoS group each IP packet belongs to. IP packets
belonging to different QoS groups are then treated differently by the VSAT. The VSAT can be configured
to support traffic differentiation based on DSCP values as used in DiffServ based networks.
Associated with each QoS group is a QoS policy. Parameters that impact the QoS policy are:
• Capacity request algorithm and parameters
- The capacity request algorithms for real-time traffic are tailored to request a continuous rate
capacity,
- The capacity request algorithms for best effort and critical data are designed for more bursty and
delay insensitive traffic such as Web browsing and FTP.
• Transmission PID or VPI/VCI
- Used to give precedence to real-time traffic when interleaving traffic from different QoS groups
at the MPEG/ATM level
• Drop policy
- Real-time traffic packets are dropped at the head of the transmission queue if the queue
overflows, while best effort traffic packets are dropped at the tail of the queue.
- The transmission queue length are tailored for each QoS class
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Critical Data and Real Time Video Conferencing are only available on the STM SatLink 1000, 1900,
1901 and 2000, when the software license key for 4 QoS classes has been set (see section 18.3).
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