MT92220BG Zarlink Semiconductor, MT92220BG Datasheet - Page 36

• Manufacturer Part Number: MT92220BG
• Description: Description = 1023 Channel Voice Over IP/AAL2 Processor ;; Package Type = Epbga ;; No. Of Pins = 608
• Manufacturer: Zarlink Semiconductor

MT92220
Data Sheet
4.5
Packet Reassembly
When communicating only IP packets, the MT92220 can use Ethernet, Packet over SONET or ATM as its primary
network interface. When using AAL2 (and potentially IP at the same time), the primary interface must be a UTOPIA
bus. This is used to carry AAL2 cells, other ATM cells that can carry signaling, and packets broken down and
carried over AAL5. Any packets transmitted or received over AAL5 on the link will be done so one cell at a time,
contrarily to Ethernet and Packet over SONET, which both transfer and receive entire IP packets. On the
transmission side, this is not a problem: each packet is broken down into as many AAL5 cells as necessary and the
cells are then sent one at a time over UTOPIA.
The reception side is a little trickier: as ATM cells are received, they must be targeted to a connection in order to
determine if they are AAL2, AAL5 carrying IP or another protocol, or "raw" cells which will be routed to another port
or to software. To do so, the chip consults an ATM header look-up table. The look-up table is contained in external
SSRAM and can use up to 16 header bits to determine the destination of the cell. The result of this look-up points to
a destination field for non-OAM cells as well as one for OAM cells (note that OAM cells cannot be AAL2 or AAL5).
The cells are then sent to their appropriate destinations. If the cells are AAL2 or AAL5, an AAL2/AAL5 VC structure
base number is also listed.
The AAL5 reassembly structure contains all information about the current packet and the connection to which it
belongs, such as the number of cells contained in the packet so far, and diagnostic information indicating how many
bytes, cells and packets have been received on this connection since start-up. It also contains a Flow Table Pointer:
this pointer uniquely identifies a Flow, which corresponds to a logical subnet number. The Flow Table Pointer can
be modified later in the packet's life according to any MPOA tags, MPLS labels or ELAN-IDs it may contain. If none
of these modifications have taken place, then the Flow Table Pointer corresponds directly to the VC number.
When the last cell is received and the packet is reassembled successfully, the complete packet is checked for
correct length and correct CRC: an error in either of these will cause it to be discarded. If the packet passes these
tests, it will be sent to the packet identification queue, where it will be routed to its final destination.
When the packet is ready to be treated, the look-up table will indicate whether it is a data packet, or it needs to be
sent through the regular IP packet flow. It is to be noted that, to save overhead, some implementations eliminate
IP/UDP headers to save bandwidth. Like all other network interfaces, AAL5 can also support voice packets both
with and without RTP headers. If the IP/UDP packet routing is chosen and the IP header is present, the look-up
proceeds using the IP and UDP headers (and possibly the RTP synchronization source) to identify the packet.
However, if the IP and UDP headers are absent, the 24-bit Flow Table Pointer is added to the binary tree look-up
key. Only the RTP SSRC can be used along the Flow Table Pointer in the search, as long as RTP is present in the
packet. Otherwise, the packet will be looked-up using only the Flow Table Pointer.
IP packets carried over AAL5 can be encapsulated using Classical IP over ATM or LANE version 1 or 2. Classical
IP over ATM uses an 8-byte SNAP/LLC header at the beginning of the packet to identify the type of the packet (e.g.
IP). Packets using LANE have an Ethernet-compatible header before the IP header, containing the Destination and
Source MAC addresses corresponding to the packet, as well as an Ethernet Length/Type field that, in the same
way as the SNAP/LLC type, identifies the protocol above it (IP or other). LANE v2 also uses LLC encapsulation and
contains an ELAN-ID that may be used to resolve a logical subnet number. LANE headers may be compatible to
Ethernet p/Q. The Destination MAC address in the LANE header will be checked in the same way as it would be in
an Ethernet packet: the chip will accept MAC addresses corresponding to its own, as well as broadcast MAC
addresses. MAC address checking can also be disabled and all packets will be accepted.
When the primary network interface is configured as Ethernet or Packet over SONET, a single Reassembly
structure is used and all packets are routed to this structure. Since Ethernet and Packet over SONET do not break
down packets, interleave them, or carry several packets over different VC, the packets always arrive contiguously,
thus not requiring more than 1 structure.
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Zarlink Semiconductor Inc.