Etic Telecom XLAN+ Series, Setup Manual

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SETUP
DOC_DEV_XS_Setup guide_C
page 45
13 Quality of service (Qos) - DiffServ
13.1 Overview
The IP protocol can multiplex different services on the same media (video, command and control, html ...).
The advantages are well known; However, if a service transmits an excessive IP traffic, the network is
congested and the latency becomes important.
By default, each SHDSL port has a transmit FIFO buffer of 10 packets (this value can be modified). When
activating the "Quality of Service" (QoS), the FIFO is replaced by a SFQ buffer (Stochastic Fairness Queuing).
SFQ automatically classifies incoming frames into streams according to their addresses and their source and
destination ports. Each stream alternately send a frame. This method limits the latency and reserve a
bandwidth for each traffic. This method is sufficient in most cases and requires no further setting.
If the result is not satisfactory, it is possible to manually classify and prioritize different traffics using the
DiffServ algorithm that will mark each IP frame in the DSCP field.
Principle of operation :
A traffic is the couple formed by an IP address and a service (ftp, html, Modbus etc ...).
Moreover, the bandwidth available is shared into 5 parts called "classes"
Each traffic defined is assigned to one of the first 4 classes: Platinum, Gold, Silver, Bronze. .
The undefined traffic is assigned to the class « Default ».
The "Platinum" class has the highest priority; That traffic is routed first whatever the traffic in the other classes.
Classes "Gold, Silver, Bronze" share the available bandwidth:
Exemple Minimum bandwidth Maximum bandwidth
% of the whole bandwidth % of the whole bandwidth
Gold 50 % 80 %
Silver 30 % 80 %
Bronze 15 % 80 %
Default 5 % Unlimited
Total 60 %
Once the traffic of one of these 4 classes fills the minimum band allocated, the additional traffic of this class
can take more bandwidth provided that some unused band by other classes is available. The attribution rule
of that unused band depends of the class priority: Additional traffic of the Gold class has the highest priority
and the one of the Default class has the lowest priority.
Care should be taken not to affect too much traffic to the "Platinum" class. Indeed, the traffic of this class has
the highest priority; if it is too large it prevents traffic from other classes to flow.
"Platinum" class should be reserved for example, to “Control and Command” traffic.
The DiffServ classification is effective in the area of SHDSL links. Within this domain IP frames keep their
DSCP classification mark. By cons, IP frames that exit the Ethernet LAN port lose their classification (DSCP
0). Sometimes it is useful to extend the DiffServ domain to Ethernet LAN, for instance, to communicate with
another XS on the Ethernet LAN. In this case IP frames that exit the Ethernet LAN port keep their classification,
the DSCP field is not set to 0.