Specifications

Mediatrix 4100 163
C HAPTER
16
Quality of Service (QoS)
QoS (Quality of Service) features enable network managers to decide on packet priority queuing. The
Mediatrix 4100 supports the Differentiated Services (DS) field and 802.1q taggings. There are three variables
– one variable for signalling (MGCP/NCS) and one variable for each of voice and T.38 media.
The Mediatrix 4100 supports the Real Time Control Protocol (RTCP), which is used to send packets to convey
feedback on quality of data delivery.
The Mediatrix 4100 does not support RSVP (Resource Reservation Protocol).
Differentiated Services (DS) Field
Differentiated Services (DiffServ, or DS) is a protocol for specifying and controlling network traffic by class so
that certain types of traffic – for example, voice traffic, which requires a relatively uninterrupted flow of data,
might get precedence over other kinds of traffic.
DiffServ replaces the first bits in the ToS byte with a differentiated services code point (DSCP). It uses the
existing IPv4 Type of Service octet.
It is the network administrator’s responsibility to provision the Mediatrix 4100 with standard and correct values.
 To enable the DS field configuration:
1. In the qosDiffServ group of the qosMIB, locate the following variables:
• qosSignalingDiffServ
• qosVoiceDiffServ
• qosT38FaxDiffServ
What are Differentiated Services?
Differentiated Services avoids simple priority tagging and depends on more complex policy or rule
statements to determine how to forward a given network packet. An analogy is made to travel services, in
which a person can choose among different modes of travel – train, bus, airplane – degree of comfort,
the number of stops on the route, standby status, the time of day or period of year for the trip, and so
forth.
For a given set of packet travel rules, a packet is given one of 64 possible forwarding behaviors – known
as per hop behaviors (PHBs). A six-bit field, known as the Differentiated Services Code Point (DSCP), in
the Internet Protocol header specifies the per hop behavior for a given flow of packets. The DS field
structure is presented below:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| DSCP | CU |
+---+---+---+---+---+---+---+---+
MSB LSB
• DSCP: Differentiated Services CodePoint.
• CU: Currently Unused. The CU bits should always be set to 0.
For both signalling and media packets, the DSCP field is configurable independently. The entire DS field
(TOS byte) is currently configurable.
Standards Supported RFC 2475 – An Architecture for Differentiated Services