Simple Network Time Protocol Support; Ip Qos Using Differentiated Services (Diffserv) - 3Com VCX v7111 User Manual

Simple Network Time Protocol Support

Simple Network Time Protocol (SNTP) client functionality generates requests and reacts to
the resulting responses using the NTP version 3 protocol definitions (according to RFC
1305). Through these requests and responses, the NTP client is able to synchronize the
system time to a time source within the network, thereby eliminating any potential issues
should the local system clock 'drift' during operation. By synchronizing time to a network time
source, traffic handling, maintenance, and debugging actions become simplified for the
network administrator.
The NTP client follows a simple process in managing system time; the NTP client requests
an NTP update, receives an NTP response, and updates the local system clock based on a
configured NTP server within the network.
The client requests a time update from a specified NTP server at a specified update interval.
In most situations this update interval should be every 24 hours based on when the system
was restarted. The NTP server identity (as an IP address) and the update interval are
configurable parameters that can be specified either in the ini file (NTPServerIP,
NTPUpdateInterval respectively) or using an SNMP MIB object.
When the client receives a response to its request from the identified NTP server it must be
interpreted based on time zone, or location, offset that the system is to a standard point of
reference called the Universal Time Coordinate (UTC). The time offset that the NTP client
should use is a configurable parameter that can be specified either in the ini file
(NTPServerUTCOffset) or using an SNMP MIB object.
If required, the clock update is performed by the client as the final step of the update
process. The update is done in such a way as to be transparent to the end users. For
instance, the response of the server may indicate that the clock is running too fast on the
client. The client slowly robs bits from the clock counter in order to update the clock to the
correct time. If the clock is running too slow, then in an effort to catch the clock up, bits are
added to the counter, causing the clock to update quicker and catch up to the correct time.
The advantage of this method is that it does not introduce any disparity in the system time,
that is noticeable to an end user, or that could corrupt call timeouts and timestamps.

IP QoS Using Differentiated Services (DiffServ)

DiffServ is architecture providing different types or levels of service for IP traffic. DiffServ
(according to RFC 2474) offers the capability to prioritize certain traffic types, depending on
their priority, thereby accomplishing a higher-level QoS at the expense of other traffic types.
By prioritizing packets, DiffServ routers can minimize transmission delays for time-sensitive
packets such as VoIP packets.
The V7111 gateway can be configured to set a different DiffServ value to IP packets
according to their class-of-service (Network, Premium Media, Premium Control, Gold and
Bronze).
For the mapping of an application to its class-of-service, see Table 83.
The DiffServ parameters are described in Table 50 .
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3Com VCX V7111 VoIP Gateway User Guide