User guide

EDA System Services
56 1/1551-HSC 901 35/3 Uen C 2005-12-02
Such small delays are in contrast to other VoIP based systems, often
imposing one-way delays of 100 ms or more, and they are very close to
what can be achieved with ATM based systems implemented according to
the BLES model (Broadband Loop Emulation Service), as recommended
by the DSL Forum.
The delay figures for Telephony over IP can also be validated in a
comparison with the recommended acceptable one-way delay of less than
150 ms, as specified by ITU.
Due to propagation delay a transatlantic call from San Francisco to London,
or any call over a distance of approximately 10,000 km, would introduce a
delay of the order of 50 ms. A call that both originates and terminates at
EDA Telephony over IP subscribers would consequently experience a total
one-way delay of approximately 130 ms (40+50+40 ms). This is well below
the recommended value of 150 ms.
6.3 QoS and Bandwith Control in the IP DSLAM
Like the rest of the EDA network, differentiated services are also used to
prioritize transmission on the ADSL line. However, on this line the
prioritization is not sufficient to ensure low latency.
The bandwidth of the ADSL line is often significantly lower than the
capacity of other links in the system. Due to low transmission capacity,
considerable time is used to transfer a video or voice-carrying frame over
the ADSL line. Transmission of a number of large data frames may also
cause long delays for real time service traffic if QoS mechanisms are not
imposed.
Two techniques are used to secure adequate QoS for different service
types on the ADSL line:
• Using different ATM PVCs with different service classes
• Packet based queuing which ensures different treatment of Ethernet
frame according to their classification (the packet based queuing is
described in further details in section 3 on page 59)
The two techniques coexist and are always active. Note that the IP DSLAM
can only ensure the QoS on the downstream direction. QoS handling in the
upstream direction must be done in the CPE.
In cases where the IP DSLAM is overloaded with traffic, it must be ensured
that the high priority packets are always processed first. This is done using