Specifications
Chapter 1 Introduction Installation and Operation Manual
1-16 Functional Description MPW-1 MP-4100 Ver. 2.0
• The accuracy of the original timing source, in accordance with the standard
SDH/SONET terminology (Stratum 1, 2, 3, 3E, or 4/unknown)
• The type of PSN that transports the traffic: router-based network (for
example, UDP/IP) versus switch-based network (for example, MPLS/Ethernet).
• Handling of transient conditions: even after the adaptive clock recovery
mechanism reaches a stable state, there may still be temporary changes in
the network delay, which may occur on a timescale that does not allow for
the mechanism to fully readjust. To provide the best possible user experience,
the user can specify how to handle such transient conditions (a capability
referred to as
delay sensitivity
):
By disabling delay sensitivity, performance is optimized for accurate clock
recovery. This selection is optimal for data transmission applications.
By enabling delay sensitivity, performance is optimized for constant
delay. This selection is optimal for voice transmission applications.
For HDLCoPSN pseudowires, it is not necessary to restore the original data rate,
because only useful HDLC payload (extracted from some of the HDLC frames
reaching each endpoint) is transferred through the pseudowire, as explained in
the
HDLCoPSN Processing
section. Therefore, the payload, which requires only a
fraction of the available bandwidth, can be reinserted in timeslots at the receiving
endpoint rate, without requiring any clock adaptation mechanism.
OAM Protocol
The OAM protocol, supported only by packet payload version V2, is used to check
for a valid pseudowire connection: this includes checks for compatible
configuration parameters at the packet processors at the two endpoints of a
pseudowire, and detection of inactive pseudowire status. Therefore, the use of
OAM must always be enabled when redundancy is used on MPW-1 modules.
The information regarding the pseudowire state is collected by the continuous,
periodic handshake between the two endpoints of a pseudowire, which
generates little traffic, but ensures that each endpoint recognizes the
connection, and that it is enabled. In case no response is received to OAM
packets within a predefined interval (a few tens of seconds), the pseudowire is
declared inactive.
When the use of the OAM protocol is enabled, little traffic flows until the
connection between the two pseudowire endpoints is established: only after the
connection is confirmed by the OAM exchange is transmission at the normal (full)
rate started, and the pseudowire starts carrying traffic. In case the connection is
lost, the transmitted traffic is again significantly decreased (several packets per
second per connection). Therefore, the OAM connectivity check also prevents
network flooding in case the connection is lost.
OAM packets sent by MPW-1 are identified in accordance with the source port:
the OAM packets run over a UDP port number (see the
Determining UDP Port
Numbers Used by Pseudowires
section) that is assigned only to OAM traffic, but
use the VLAN ID and ToS of the originating connection.