MT92220 Zarlink Semiconductor, MT92220 Datasheet - Page 206

• Manufacturer Part Number: MT92220
• Description: 1023 Channel Voice Over IP/AAL2 Processor
• Manufacturer: Zarlink Semiconductor

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Appendix B
HDLC Format, Including Zero-Insertion and Extraction
The MT92220 supports 2 types of HDLC over the TDM bus: the first, bit-wise form of HDLC uses a control flag of
"01111110" and inserts a '0' after every 5 '1's of payload. When using this form of HDLC, each HDLC packet must
begin with a flag and end with a flag, although a single flag may represent both the end of a packet and the
beginning of another. If neither flags nor data are being transmitted onto the bus, the idle code must be used: it is
simply an endless string of '1's. Note that the idle code must be at least 7 bits long (7 '1's) to be valid
The second form is a byte-wise HDLC format, which also uses "01111110" (7Eh) as a control flag. An actual 7Eh
within the payload is replaced by 7Dh - 5Eh, while a 7Dh is replaced by 7Dh - 5Dh. On the SONET interface, a 7Dh
- 7Eh control code indicate the abortion of the packet. This form of HDLC is easier in terms of computation
(because it looks at each byte individually instead of each bit) and thus is easier for neighboring DSP to use. It does
not contain an idle code: instead, the flag character is repeated endlessly until valid data is ready to be transmitted
onto the bus.
In both cases, the MT92220 can accept or generate an HDLC header that can contain 0, 1 or 2 address bytes, as
well as a possible control byte. There is also an optional 16-bit CRC that may be added at the end of the packet.
When using HDLC streams, the low 9 bits of the address are used to select the channel number. Finally, the
payload of the mini-packet may range from 1 to 1500 bytes. Note that the payload of the mini-packet may include
an RTP header.
The HDLC packet formats for RTP and AAL2 have slight difference. In RTP HDLC format, a complete RTP packet
including RTP header is encapsulated, while as in AAL2 HDLC format, only CPS-packet payload is encapsulated.
CID is indicated by address byte(s), UUI is carried before the first byte of payload, and LI is not transported.
1. Address bytes can be 0, 1 or 2 byte(s).
2. Control byte is optional.
3. CRC bytes are optional
4. Complete RTP packet starts from D0
1. Address bytes can be 0, 1 or 2 byte(s).
2. Control byte is optional.
3. CRC bytes are optional
4. UUI is placed in MSB of D0
5. CPS-packet payload starts from D1
Figure 112 - Supported AAL2 HDLC Packet Format (after zero extraction)
FLAG
FLAG
Figure 111 - Supported RTP HDLC Packet Format (after zero extraction)
H0
H0
H1
H1
Cntrl
Cntrl
D0
D0
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D1
D1
D2
D2
D3
D3
D4
D4
CRC0
CRC0
CRC1
CRC1
FLAG
FLAG