The DOCSIS MAC Protocol Operation

284 ACCESS NETWORKS is shared by all of the ONUs. The G.983.1 standard also permits the use of additional unidirectional fibers, operating at 1310 nm. Two transmission options can be used: symmetric and asymmetric. In the symmetric option, both the upstream and downstream transmission rate for ATM traffic is 155.52 Mbps. In the asymmetric option, the downstream and upstream transmission rate for ATM traffic is 622.08 Mbps and 155.52 Mbps, respectively. The maximum fiber distance from an ONU to an OLT is 20 km, the minimum supported number of splits for a passive splitter is 16 or 32, and the minimum number of ONUs supported in an APON is 64. These specifications will probably change as the technology evolves. APON can provide high-speed access for Internet traffic, voice over ATM, voice over IP and video services. APON can be deployed in new neighborhoods and municipalities. In a new neighborhood, the fiber can be laid at the same time as the infrastructure. Municipalities have an interest in providing high-speed connectivity to their residents, and they can easily deploy APONs by passing the fiber through existing underground conduits that lead close to the homes. Also, power companies are a potential provider since they can deploy the fiber using the existing poles that support the electrical cables

11.3.1 Frame Structures for Downstream and Upstream Transmission

The downstream transmission for both 155.52 Mbps and 622.08 Mbps consists of a con- tinuous stream of time slots. Each time slot consists of 53 bytes, and it contains either an ATM cell or a 53-byte physical layer OAM PLOAM cell. PLOAM cells are only used every 28th time slot. Their function is explained in the following section. Groups of time slots are organized into frames. The frame structure for 155.52 Mbps and 622.08 Mbps transmission rates is shown in Figure 11.22. For the 155.52-Mbps transmission rate, the frame consists of 56 time slots, two of which are PLOAM cells and the remaining 54 are ATM cells. As can be seen, the first time slot of the frame carries a PLOAM cell and the remaining 27 time slots contain ATM cells. This group of 28 time slots repeats once more within the frame. That is, the 29th time slot is a PLOAM cell, and the remaining 27 time slots carry ATM cells. The frame for 622.08-Mbps transmission rate consists of 244 time slots, of which eight are PLOAM cells, and the remaining 216 are ATM cells. The first time slot of a frame contains a PLOAM cell and the next 27 time slots contain ATM cells. This group of 28 time slots repeats seven more times within the frame. The frame structure for the upstream transmission is shown in Figure 11.23. The upstream frame consists of 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 a Frame structure for 155.52 Mbps—56 time slots . . . PLOAM Cell 1 Cell 27 PLOAM Cell 28 Cell 54 . . . b Frame structure for 622.08 Mbps—224 time slots PLOAM Cell 1 Cell 27 PLOAM Cell 189 Cell 216 . . . . . . . . . Figure 11.22 Frame structures for downstream transmission. THE ATM PASSIVE OPTICAL NETWORK 285 3 Bytes . . . Cell 1 Cell 2 Cell 53 Figure 11.23 Frame structure for upstream transmission – 53 time slots. or a PLOAM cell, or a divided-slots cell the function and structure of this type of cell will be explained below in Section 11.3.3. The overhead bytes carry the following information: a guard time, a preamble, and a delimiter. The guard time provides enough distance between two consecutive cells or mini-slots in a divided-slot cell to avoid collisions. The minimum guard time is 4 bits. The delimiter is a unique pattern used to indicate the start of an ATM or a PLOAM cell or the start of a mini-slot in a divided-slots. It is used to acquire bit synchronization. The upstream and downstream frames are synchronized at the OLT. Figure 11.24 shows the frame alignment for the symmetric option i.e. 155.52 Mbps downstream and 155.52 Mbps upstream. Upstream cells are aligned to the frame using the ranging proce- dure. Figure 11.25 shows the frame alignment for the asymmetric option i.e. 622.08 Mbps downstream and 155.52 Mbps upstream.

11.3.2 The PLOAM Cell

PLOAM cells are used to convey physical layer OAM messages in the downstream and upstream direction. In addition PLOAM cells can carry grants which are used by the ONUs to transmit in the upstream direction. PLOAM Cell 1 Cell 27 PLOAM Cell 28 Cell 54 Cell 1 Cell 2 Cell 53 56 Time slots 53 Time slots . . . PLOAM Cell 1 Cell 27 PLOAM Cell 28 Cell 54 . . . . . . Cell 1 Cell 2 Cell 53 56 Time slots 53 Time slots Figure 11.24 Frame alignment for the symmetric option. . . . Cell 1 Cell 2 Cell 53 PLOA M Cell 1 Cell 27 PLOAM Cell 189 Cell 216 . . . . . . . . . 224 Time slots 53 Time slots Figure 11.25 Frame alignment for the asymmetric option. 286 ACCESS NETWORKS 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 frame’s first PLOAM cell, and to 0 for the all of the frame’s 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 down- stream 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: x 8 + x 2 + 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 ith upstream frame received their grants in the i − 1 downstream frame. 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