Alcatel Carrier Internetworking Solutions Omni Switch/Router - Manual (Page 941)

Page 30-3
Multilink Modes of Operation
Multilink PPP supports combinations of both permanent and switched connections. This
results in two possible modes of operation:
• permanent connection only
• switched connection only
Note
One important thing to remember when setting up
multilinks is that all links to be bundled must exist on
the same slot.
Permanent Connection Only
This mode allows multiple links to be joined into a single bundle. Permanent connections can
be universal serial ports or fractional T1/E1 ports.
Switched Connection Only
This mode supports only switched connections. The only switched connections currently
supported are ISDN calls. This allows multiple switched connections to be joined into a single
bundle. In this mode, the first call is initiated as a demand connection, if a frame is available
for the peer, or a backup connection, if the primary link becomes inactive, according to the
configuration of the ISDN link.
♦ Note ♦
ISDN MLPPP bundles are limited to 2 B-channels
PPP Fragmentation Interleaving
The PPP Fragmentation/Interleaving functionality creates two prioritized virtual streams within
a single PPP connection. The lower priority stream uses an MLPPP header to sequence the
frames while the higher priority stream uses a standard PPP header without MLPPP sequence
numbers. The lower priority stream is fragmented according to maximum delay parameters so
that a higher priority frame can be injected in the middle of the low priority frame and not
have to wait for the entire low priority frame to be transmitted.
On the transmitting side, when low priority frames are being transmitted they are divided into
multiple fragments. The size of each fragment is determined by the configured maximum
delay parameter and the speed of the physical interface. The fragments are encapsulated with
a standard MLPPP header, which contains a sequence number to identify lost fragments and
beginning and ending flags to identify frame boundaries. When high priority frames are
transmitted they are sent whole with standard PPP headers.
The delay of the high priority frame is the time it takes to finish transmitting the current frame
or fragment plus the time it would take to transmit any other high priority frames in queue.
On the OA-512, which has a hardware based high and low priority transmit queues, the high
priority frame would be sent as soon as the current fragment/frame is finished. On the WSX,
which has software based high and low priority transmit queues, it depends on how many
frame/fragments have been committed to transmit buffer descriptors ahead of it. As part of
the WSX transmit data flow improvements, the number of frames/fragments committed to
buffer descriptors will be kept to a minimum, but because the queues are software based, will
not be able to match the delays of the OA-512.