MT90883 Zarlink Semiconductor, MT90883 Datasheet - Page 55

• Manufacturer Part Number: MT90883
• Description: (MT90880 - MT90883) TDM to Packet Processors
• Manufacturer: Zarlink Semiconductor

Drop Thresholds
The context queues are used to buffer the constant bit rate TDM interfaces from the variable delays
experienced in the packet network (see the section on "Jitter buffer Operation", on page 28). The initial size of
this jitter buffer can be pre-programmed, but in cases where packets arrive very late across the network, the
jitter buffer can grow. For instance, if there was a link failure somewhere in the network, it could result in a large
backlog of packets being stored up at a network node. When the link is restored, these packets may be
released suddenly, creating a rush of very late packets. These packets will build up a large jitter buffer and a
consequent increase in end to end latency.
Drop thresholds can be used to limit the size of this jitter buffer to a maximum value. Each of the queues to the
TDM interfaces has a drop threshold that can be programmed in units of the number of packet in the queue.
Packets arriving once this threshold has been reached are dropped, preventing the size of the buffer and hence
the end to end latency from increasing further.
Different applications have different requirements for where to set the drop thresholds. Applications such as
packet backplanes generally operate with tightly controlled networks with small packet delay variations.
Therefore the drop thresholds for this type of application need to be small, and the MT9088x allows the drop
thresholds to be set in increments of one packet, between 0 and 15 packets.
However, for applications such as circuit emulation, while end to end latency is still important, the network is
much larger and the packet delay variation cannot be as tightly controlled. Therefore the MT9088x provides a
second range, where the thresholds can be set to increment by steps of 16 packets, between 15 and 255
packets. Where a packet contains a single TDM frame of data, this represents a step size of 2 ms. The two
ranges are known as "backplane mode" and "network mode", and are programmed by a bit in the queue
manager control register.
An overall threshold can also be set for the total amount of memory used by the queues to the TDM interfaces.
This is set in units of granules, and has a maximum size of 65535 granules.
6.6
On transmission, a user defined static header is added to the packet payload. This contains the Ethernet
header, the context descriptor, and the header required for the higher level protocols to be used (if any), e.g. IP,
UDP. Since the header is completely user-defined, any high level protocol can be used provided the header
does not change from packet to packet (e.g. contains a sequence number or a checksum across the payload).
6.6.1
The MT9088x family can be used with a number of different protocol stacks. The common element is the use of
the context descriptor for determination of the TDM context that the packet relates to. Examples of the most
common expected stacks are (CDP = Context Descriptor Protocol):
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•
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Ethernet - CDP
Ethernet - VLAN - CDP
Ethernet - IPv4 - CDP
This is the most basic expected stack. Where the TDM traffic is only passed over a switched network, no
upper layer headers are required. The Ethernet destination address is used to switch the packet to the cor-
rect destination, and the context descriptor used to determine the context.
An Ethertype needs to be allocated to CDP to enable the traffic type to be determined in the packet classifi-
cation process.
It is also possible to use a VLAN tag to switch the packets to the correct destination in the network. Again,
an Ethertype needs to be allocated to CDP to enable the traffic type to be determined in the packet classifi-
cation process.
Packet Transmit
Protocol Stacks
Zarlink Semiconductor Inc.
MT90880/1/2/3
55
Data Sheet