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In case of contention-based transmission, a CM does not know if its transmission has collided with a transmission from another CM. Therefore, the CM expects to receive a confirmation from the CMTS. This confirmation varies depending on what the CM transmitted. For instance, if it transmitted a bandwidth request, then it expects to receive a data grant. If it transmitted a data PDU, then it expects to receive an acknowledgement. If it does not receive a confirmation in a subsequent MAP message, the CM repeats the above process by doubling its back-off window. This re-try process is repeated i6 times, which is the maximum number of retries, after which the CM discards the MAC frame.
11.2.4 Quality of Service (QoS)
In the MAC protocol, a QoS scheme has been defined which provides different priorities for the transport of different flows of packets across the cable network. The QoS scheme uses the concept of service flow. This is a unidirectional flow of packets from a CM to the CMTS, or from the CMTS to a CM. The CMTS and CM provide QoS by shaping, policing, and scheduling the transmission of packets associated with a service flow.
In a basic CM implementation, only two service flows are used: an upstream flow and a downstream flow. These two service flows are known as primary service flows. These two primary service flows are used to implement best effort service. In more complex modems, multiple service flows can be implemented in addition to the primary ones. The additional service flows can be established to provide different service classes. For the network to function properly, all CMs must support at least the primary service flows.
A service flow could be in different states, such as provisioned, admitted, and active. To keep things simple, we will only discuss active service flows, service that is, flows for which data packets are being transmitted across the cable network. An active upstream service flow is assigned a unique Service Id (SID).
Packets delivered to the MAC protocol from the upper-level bridge or router are classified by a classifier into different service flows. The classifier can use the packet’s IP address, for instance, to decide which service flow the packet belongs to. There is a downstream classifier in the CMTS, and an upstream classifier in a CM. Classifiers and service flows can be provisioned into the access network via a provisioning server, or they can be set up dynamically by a CM or the CMTS.
A service flow is associated with QoS parameters (e.g. delay and throughput) and with a scheduling service. Based on the QoS parameters and the scheduling service, the CMTS can provide opportunities to the CM to send up bandwidth requests for the service flow and receive data grants, so that the QoS associated with the service flow is achieved. The following scheduling services have been defined:
• Unsolicited grant service (UGS)
• Real-time polling service (rtPS)
• Unsolicited grant service with activity detection (USG-AD)
• Non-real-time polling service (nrtPS)
• Best effort (BE) service
The unsolicited grant service (UGS) is designed to support real-time service flows that generate fixed-size data packets on a periodic basis, such as voice over IP. The CMTS provides fixed size data grants to the flow on a periodic basis, so that to meet the flow’s real-time needs. This eliminates the need for the CM to send bandwidth requests to the CMTS.
The real-time polling service (rtPS) was designed to support real-time service flows that generate variable size data packets on a periodic basis, such as MPEG video. The CMTS solicits periodically bandwidth requests from the CM and allows the CM to specify the size of the data grant.
The unsolicited grant service with activity detection (USG-AD) was designed to support real-time service flows, such as those supported by the unsolicited grant service (UGS), such as voice over IP with silence suppression, which can become idle for substantial portions of time (i.e., tens of milliseconds or more). The CMTS provides periodic data
THE ATM PASSIVE OPTICAL NETWORK
grants when the flow is active. When the flow is inactive, it solicits bandwidth requests from the CM for this flow.
The non-real-time polling service (nrtPS) was designed to support non-real time flows that require variable size data grants on a regular basis, such as high-bandwidth file transfers. The CMTS solicits bandwidth requests from the CM for this flow on a periodic or non-periodic time interval on the order of one second or less.
Finally, the best effort (BE) service provides an efficient service for best effort traffic. The CMs send bandwidth requests on a contention basis in order to receive data grants for the flow.
11.3 THE ATM PASSIVE OPTICAL NETWORK
An APON is a cost-effective alternative to the telephone-based and cable-based access networks. An APON consists of an optical line terminator (OLT), an optical distribution network (ODN) and optical network units (ONU). The OLT, which resides at the premises of the APON operator, is responsible for transmitting and receiving traffic to and from the ONUs, which reside at the customer site. Also, the OLT has interfaces to a packet-switching backbone network. The OLT is connected to multiple ONUs via an optical distribution network. An APON, as its name implies, was designed with a view to carrying ATM traffic.