Quality of Service QoS

THE ATM PASSIVE OPTICAL NETWORK 287 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 mini- slot 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: • MESSAGE PON ID : Contains the PON-ID number of an ONU for which the message is addressed to. The field can also be set to 0 × 40 for broadcasting to all ONUs. • MESSAGE ID : Indicates the type of message. • MESSAGE field : Contains the message. • CRC : The message fields are protected with a CRC with pattern: x 8 + x 2 + 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 PON ID, 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 288 ACCESS NETWORKS Table 11.4 The structure of the upstream PLOAM cell. Byte Description Byte Description 1 IDENT 25 LCF 11 2 MESSAGE PON ID 26 LCF 12 3 MESSAGE ID 27 LCF 13 4 MESSAGE FIELD 1 28 LCF 14 5 MESSAGE FIELD 2 29 LCF 15 6 MESSAGE FIELD 3 30 LCF 16 7 MESSAGE FIELD 4 31 LCF 17 8 MESSAGE FIELD 5 32 RXCF 1 9 MESSAGE FIELD 6 33 RXCF 2 10 MESSAGE FIELD 7 34 RXCF 3 11 MESSAGE FIELD 8 35 RXCF 4 12 MESSAGE FIELD 9 36 RXCF 5 13 MESSAGE FIELD 10 37 RXCF 6 14 CRC 38 RXCF 7 15 LCF 1 39 RXCF 8 16 LCF 2 40 RXCF 9 17 LCF 3 41 RXCF 10 18 LCF 4 42 RXCF 11 19 LCF 5 43 RXCF 12 20 LCF 6 44 RXCF 13 21 LCF 7 45 RXCF 14 22 LCF 8 46 RXCF 15 23 LCF 9 47 RXCF 16 24 LCF 10 48 BIP ONUx ONUy . . . . . . Upstream time slot Mini-slot payload 1 to 53 bytes 3-byte overhead Figure 11.26 The divided-slot cell. vary from 1 byte to 53 bytes. The three-byte overhead is the same as the one used with each cell in the upstream frame. Each divided slots cell is associated with a group of ONUs. The OLT assigns one divided-slots cell to group of ONUs using the grant field in the PLOAM cell. Each mini- slot contains information from one ONU belonging to the group, such as the size of its