Home' RTCA Documents for Review : DO-230H FRAC Contents 256
© 2017, RTCA, Inc.
required for 802.11a access). Data collision compensation using Carrier Sense Multiple Access/ Collision
Avoidance policy incur further delays for Short Inter-Frame Space, a DCF Inter-Frame Space, and Back
Off, which typically run 10 μsec, 50 μsec, and 310 μsec, respectively, in an 802.11b environment. At the
end of the day, a network rated at 11 M bit/sec of bandwidth is reduced to an effective transmission capacity
of approximately 6M bit/sec. This reduction must be factored into network bandwidth calculations.
Oversubscription is inherent in the design of hierarchical networks, as a common means of maximizing the
number of customers served while minimizing the hardware cost – a practice carried over from
telecommunication networks which typically provisioned one telephone circuit for each 10 telephone
subscribers. Oversubscription lowers cost by sharing common components, such as network processor units
and optimizes their utilization.
The user interface can range from 10/100 Ethernet to 1 Gbit and 10 GbE. To minimize the degradation of
network performance in cases of congestion and to ensure that critical traffic is transmitted, intelligent over-
subscription must be implemented. Intelligent oversubscription uses:
Ingress frame classification to sort traffic into queues based on assigned Class of Service (CoS).
Programmable methods, suitable to traffic type, to drop traffic as queues fill.
Rate metering to police the traffic as it leaves the network processing unit.
Oversubscription by itself, however, is insufficient. When full system-side bandwidth is consumed, the tail
drop method – where the last traffic into the system is the first traffic dropped – is insufficient for traffic
management. If the last traffic into the system is voice, it is sitting behind email and web traffic, the voice
traffic will be dropped and/or voice quality will degrade significantly or the call may be dropped.
The requirements for programmed drop of traffic at 10/100/1000-Mbit and 10-GbE rates drive the need for
systems and devices with enhanced capabilities across all physical interface types. The network designer
will need to address means to offer the same type of capabilities - quality of service (QoS), bandwidth
guarantees, traffic shaping, etc. - regardless of port speed or whether the port faces the customer or the
Quality of Service (QoS) Issues
Quality of Service addresses the ability of a network to guarantee different levels of service to selected
traffic. Its goal is to prioritize certain traffic flows without making other flows fail - ensuring consistent,
guaranteed performance. QoS provisioning is essential for a network carrying ISSA voice and video traffic
as well as data traffic because it protects critical streams against packet losses and delays by monitoring
and prioritizing traffic, and by managing LAN and WLAN bandwidth.
Data traffic is often tolerant of delays, e.g., most users are not sensitive to brief email delays. Voice and
video traffic, which are time-critical streams, have different requirements for quality performance.
By adding QoS, critical applications such as voice, video, and business systems receive priority queuing,
so the traffic is shaped before being transmitted over the network. QoS should be a major design
consideration to establish priorities that ensure important traffic gets the required level of Service.
Voice-over-Internet Protocol (VoIP) has become a common application on modern networks. Properly
configured, and properly secured, VoIP offers many advantages for an ISSA including duplex voice
communications with mobile security personnel using PDAs (personal digital assistants) and interfacing
Links Archive Navigation Previous Page Next Page