Download (direct link):
Table 11.3 The structure of the downstream PLOAM cell.
Byte Description Byte Description
1 IDENT 25 GRANT 20
2 SYNC 1 26 GRANT 21
3 SYNC 2 27 CRC
4 GRANT 1 28 GRANT 22
5 GRANT 2 29 GRANT 23
6 GRANT 3 30 GRANT 24
7 GRANT 4 31 GRANT 25
8 GRANT 5 32 GRANT 26
9 GRANT 6 33 GRANT 27
10 GRANT 7 34 CRC
11 CRC 35 MESSAGE PON ID
12 GRANT 8 36 MESSAGE ID
13 GRANT 9 37 MESSAGE FIELD 1
14 GRANT 10 38 MESSAGE FIELD 2
15 GRANT 11 39 MESSAGE FIELD 3
16 GRANT 12 40 MESSAGE FIELD 4
17 GRANT 13 41 MESSAGE FIELD 5
18 GRANT 14 42 MESSAGE FIELD 6
19 CRC 43 MESSAGE FIELD 7
20 GRANT 15 44 MESSAGE FIELD 8
21 GRANT 16 45 MESSAGE FIELD 9
22 GRANT 17 46 MESSAGE FIELD 10
23 GRANT 18 47 CRC
24 GRANT 19 48 BIP
The structure of the PLOAM cell in the downstream direction is shown in Table 11.3. The following 1-byte fields have been defined:
IDENT: Bits 1 to 7 are set to 0. Bit 8 is set to 1 for the frames first PLOAM cell, and to 0 for the all of the frames subsequent PLOAM cells.
SYNC 1, SYNC 2: A 1 KHz reference signal provided by the OLT is transported to the ONUs using these two bytes. It is used by the ONUs to synchronize to the downstream frame.
GRANT fields: Provide grants to the ONUs for upstream transmission.
MESSAGE fields: Transport alarms and threshold-crossing alerts.
Bit interleaving parity (BIP): Monitor the bit-error rate (BER) on the downstream link.
CRC: Groups of seven grants are protected with a CRC with pattern: x8 + x2 + x + 1. No error correction is done.
The grants are permits issued by the OLT to the ONUs to transmit in the next upstream frame. Recall from Section 11.3.1 that a downstream frame is synchronized with an upstream frame. Grants transmitted in downstream frame i are used by the ONUs to transmit in the next upstream frame i + 1. The ONUs transmitting during the i th upstream frame received their grants in the i 1 downstream frame.
THE ATM PASSIVE OPTICAL NETWORK
Only 53 grants are required to be sent in a downstream frame, since an upstream frame has only 53 time slots. Each grant is a permission for a particular ONU to transmit in a specific time slot in the next frame. Each PLOAM cell contains only 27 grants (see Table 11.3). Therefore, the first two PLOAM cells in a frame (whether it is a 56 or 224 time-slot frame) suffice to carry all 53 grants. In fact, they carry a total of 54 grants, but the last one is an idle grant; that is, it is ignored by the ONUs. Idle grants are also carried in the remaining PLOAM cells if the frame has 224 time slots. The following types of grants have been defined:
Data grant: During the ranging protocol (see Section 11.3.5), an ONU is assigned a PON-ID number between 0 and 63. A data grant contains the PON-ID number of an ONU. This is an indication to the ONU to send an ATM cell, or an idle cell if it has no data to send, in the next upstream frame in the time slot that corresponds to the position of the grant relative to the set of all of the grants in the frame.
PLOAM grant: Indicates the PON-ID of an ONU. The ONU sends a PLOAM cell in the next upstream frame in the time slot that corresponds to the position of the grant relative to the set of all of the grants in the frame.
Divided-slots grant: Indicates a group of ONUs. Each ONU in the group sends a minislot in the next frame (see Section 11.3.3).
Reserved grants: Reserved for future grants.
Ranging grants: Used in the ranging protocol.
Unassigned grants: Indicates an unused upstream slot.
Idle grant: Ignored by the ONUs.
The following fields have been defined for the message field:
MESSAGEPONJD: Contains the PON-ID number of an ONU for which the message is addressed to. The field can also be set to 0 x 40 for broadcasting to all ONUs.
MESSAGEID: Indicates the type of message.
MESSAGE Jield: Contains the message.
CRC: The message fields are protected with a CRC with pattern: x8 + x2 + x + 1. No error recovery is done.
The PLOAM cell in the upstream direction is used to convey physical layer OAM messages. Its structure is shown in Table 11.4. The fields IDENT (it is set to 0), MESSAGE PONJD, MESSAGE ID, MESSAGE FIELD, CRC, and BIP are the same as those defined above in the downstream PLOAM cell. The LCF (laser control field) bytes and RXCF (receiver control field) bytes are used by the physical layer. A PLOAM cell is issued by an ONU in response to a PLOAM grant transmitted in a PLOAM cell in the downstream frame.
11.3.3 The Divided-slots Cell
Recall that the upstream frame carries 53 time slots. Each slot consists of 56 bytes, of which the first 3 bytes are used for overheads and the remaining 53 bytes carry either an ATM cell or a PLOAM cell, or a divided-slots cell. The structure of a divided-slots cell is shown in Figure 11.26. We see that it consists of a number of mini-slots, with each mini-slot consisting of a 3-byte overhead and a variable-length payload. The payload can