The ADSL Super Frame Schemes for Accessing Network Service Providers

THE CABLE-BASED ACCESS NETWORK 273 CMTS … Data Video Video Tx Rx … ONU Fiber Coax CM CM CM MANWAN network Data Figure 11.12 The DOCSIS reference architecture. CMTS DS TC DOCSIS MAC Link security 802.2 LLC Bridge 802.3 MAC 802.2 LLC IP CM 802.3 10Base TC Cable PMD DOCSIS MAC Link security 802.2 LLC Bridge Data link layer IP PHY layer Cable PMD Cable PMD US cable PMD Cable network Figure 11.13 The protocol stacks of CMTS and CM. carrier ranging from 5 MHz to 42 MHz. Multiple carriers can also be used. A specially designed MAC, referred to as the DOCSIS MAC, assures that there are no collisions in the upstream direction. Both CM and CMTS support the IP protocol and other IP-related protocols. The pro- tocol stacks of the CMTS and CM are shown in Figure 11.13. The protocol stack of the CM at the data-over-cable interface, i.e., at the side of the cable network, consists of the physical layer, the DOCSIS MAC protocol, the link security layer, and the IEEE 802.2 LLC. The physical layer consists of the transmission convergence TC sublayer and the cable physical medium dependent PMD sublayer. The TC sublayer is only present in the downstream direction DS – that is, from the CMTS to the CM. It is not present in the upstream direction US – that is, from the CM to CMTS. The DOCSIS MAC protocol controls the upstream transmission of the CMs, and provides QoS and other fea- tures. The CM communicates with the customer premises equipment CPE via Ethernet. The CM could also be part of a router that forwards packets to CPEs using MPLS label switching or classical IP-based forwarding. The protocol stack of the CMTS at the data-over-cable interface i.e., at the side of the cable network is similar to that of the CM’s protocol at the side of the cable network. The stack of the CMTS on the interface to the MANWAN network i.e., at the network 274 ACCESS NETWORKS side consists of the appropriate LLC and physical layer necessary to communicate with the MANWAN network, to which the CMTS is connected. Packets can be forwarded between the data-over-cable interface and the MANWAN network interface via Ethernet. However, it is more likely that the CMTS is also a router that forwards packets to the MANWAN network using MPLS label switching or classical IP-based forwarding. In this case, the IP packets are recovered from the LLC frames and sent to the MPLS or IP protocol, from where they are routed out to the network.

11.2.1 The Physical Layer

As mentioned above, the physical layer comprises of the sublayers: transmission conver- gence TC and physical media dependent PMD. The TC sublayer is present only in the downstream direction. The upstream PMD sublayer uses an FDMATDMA mode referred to as the TDMA mode or an FDMATDMAS-CDMA mode referred to as the S-CDMA mode. Using frequency division multiple access FDMA , multiple radio frequency RF channels can coexist in the upstream band. A CM transmits on a single RF channel until it is recon- figured by the CMTS to change. Time division multiple access TDMA permits the upstream transmission on a channel to be slotted. Access to the slots, referred to in DOCSIS as mini-slots, is controlled by the DOCSIS MAC protocol. In the S-CDMA mode, multiple CMs can transmit simultaneously on the same RF channel and during the same TDMA mini-slot. In TDMA mode, a mini-slot is a power-of-two multiple of 6.25 µs; that is, it is equal to T × 6.25 µs, where T = 2 n , and n = 0, 1, . . . , 7. That is, T = 1, 2, 4, 8, 16, 32, 64, 128. In S-CDMA mode, a mini-slot is not restricted to be a power-of-two multiple of 6.25 µs increments. Instead, a mini-slot is a unit of capacity that is dependent on the modulation rate, number of spreading codes, and number of spreading intervals configured for the upstream channel. The length of the mini-slot is unrelated to the length of the MAC frames, so that transmitting one MAC frame might require several contiguous mini-slots. The CM upstream transmission speed typically ranges between 500 Kbps and 2.5 Mbps, although it can go up to 10 Mbps. The downstream PMD uses a 6-MHz channel in the range of 91 MHz to 857 MHz frequencies, and it conforms to the ITU-T standard J.83, Annex B, for low-delay video applications. The downstream transmission speed typically ranges between 1 Mbps and 3 Mbps, but it can reach up to 27 Mbps. The downstream TC sublayer was defined in order to provide a common receiving hardware at the CM for both video and data. This permits future video services to be offered in addition to the data services. The TC sublayer receives MAC frames from the DOCSIS MAC layer and produces a continuous bit-stream of 188-byte MPEG packets. This continuous stream is passed on to the PMD sublayer which is responsible for trans- mitting it out. The MPEG packet format is shown in Figure 11.14. The following fields have been defined: MPEG header Pointer_field 1 byte DOCSIS payload 183 or 184 bytes Figure 11.14 The MPEG packet format. THE CABLE-BASED ACCESS NETWORK 275 • MPEG header : Consists of four bytes and contains various fields, such as: an 8-bit sync byte field; a 1-bit payload unit start indicator PUSI that is used to indicate the presence of a pointer field in the first byte of the DOCSIS payload; and a 13-bit packet identifier PID field that carries the DOCSIS data over cable well-known PID 0x1FFE. • DOCSIS payload : This field carries DOCSIS MAC frames. It might contain a pointer field; if it does, then the DOCSIS payload is 183 bytes. Otherwise, it is 184 bytes. The standard defines a stuff-byte pattern that is used within the DOCSIS payload to fill gaps between DOCSIS MAC frames. The value of this pattern is 0xFF, which cannot be used as the first byte of a DOCSIS MAC frame. A DOCSIS MAC frame can begin anywhere within an MPEG packet and span over several MPEG packets. Also, several frames can potentially exist within the same MPEG packet. The pointer field is used to correctly recover MAC frames from the MPEG packets. The pointer field is present in the fifth byte of the MPEG packet, whenever the PUSI in the MPEG header is set to one. It contains the number of bytes in the MPEG packet that immediately follow the pointer field, and which the CM decoder must skip before looking for the beginning of a DOCSIS MAC frame. The pointer field gives the beginning of the first DOCSIS MAC frame in the MPEG packet or the first stuff-byte preceding a DOCSIS MAC frame.

11.2.2 The DOCSIS MAC Frame Format

The DOCSIS MAC protocol controls the upstream transmission of the CMs, and it pro- vides QoS and other features. For simplicity, we will refer to it as the MAC protocol. In our discussion below, we will assume that there is a single downstream and a single upstream channel between the CMTS and the CMs. Multiple upstream and downstream channels between the CMTS and the CMs can be also used. The MAC frame format consists of the MAC header, followed by an optional data PDU. The MAC header format is shown in Figure 11.15. The following fields have been defined: • Frame control FC : An 8-bit field used to identify the type of MAC header. It is broken down to the following subfields: ◦ FC TYPE: This 2-bit field specifies one of the following four possible MAC frame formats: MAC header with packet PDU, MAC header with ATM cells, MAC header reserved for future PDU types, and MAC header used for specific control functions. FC LENSID EHDR optional MAC_PARM HCS FC_TYPE EHDR_ON FC-PARM Figure 11.15 The MAC header format. 276 ACCESS NETWORKS ◦ FC PARM: A 5-bit field that contains parameters dependent on the value of the FC TYPE. ◦ EHDR ON: A 1-bit field indicating whether or not an extended header is present. • MAC-PARM : This 1-byte field contains parameters whose use depend on the value of the FC field. • LEN SID : A 2-byte field that gives the length of the extended header if present, plus the number of bytes that follow after the HCS field. In certain cases, this field is used to carry the value of a SID see Section 11.2.4. • Extended header EHDR : This is an optional variable-size extended header. • Header check sequence HCS : The integrity of the MAC header is ensured by a CRC. The HCS is a 2-byte field that contains the FCS, which is obtained using the pattern x 16 + x 12 + x 5 + 1. The HCS covers the entire header i.e. it starts from the FC field and includes the extended header. As stated above, the following MAC frame formats are possible: MAC header with packet PDU, MAC header with ATM cells, reserved MAC header for future PDU types, and MAC header used for specific control functions. The MAC header with packet PDU is used to transport Ethernet packets. In this case, the MAC header is followed by a data PDU with the following fields: • Destination address DA : A 48-bit field populated with the destination address. • Source address SA : A 48-bit field populated with the source address. • Typelen : A 16-bit Ethernet type length field. • User data : A variable-length field that contains user data of up to 1500 bytes. • CRC : This 32-bit field contains the FCS obtained using the Ethernet-prescribed CRC used to protect the data PDU. The MAC header with ATM cells is to be defined in the future. The following MAC headers are used for specific control functions: • MAC header for timing : Used in the downstream direction to transport the global timing reference, to which all of the cable modems synchronize. In the upstream direction, it is used as part of the ranging message needed for the cable modem’s timing and power adjustment. • MAC header for requesting bandwidth : This header is the basic mechanism that a cable modem uses to request bandwidth. It is only used in the upstream direction. There is no data PDU following this MAC header. Because there is no data PDU, the LEN field is not required and is replaced with a SID. This is the SID allocated to a CM by the CMTS for the upstream service flow. • MAC management header : Used to transport all management messages. • MAC header for fragmentation : In the upstream direction, a large data PDU can be split into a number of smaller pieces, and each piece can be transmitted individually. The pieces are then reassembled into the original data PDU at the CMTS. This MAC header provides the mechanism to do this fragmentation. • MAC header for concatenation : Used to transport several MAC frames MAC header plus optional data PDU in a single transmission.