DOCSIS Quality of Service

DOCSIS 1.0 was optimized to provide Internet Access. When used in this way, data is sent and received without concern about how often it arrives since the images eventually get displayed. There are other types of data that do require the accurate delivery of data at specified time intervals. This feature, known as Quality of Service (QoS) was added in DOCSIS 1.1 and this is the primary difference between DOCSIS 1.0 and DOCSIS 1.1.

Telephone conversations are an example of a data stream that requires the accurate delivery of the data and is shown graphically in Figure 1. In such an application, the data must arrive at some constant time interval t and if the data does not arrive at the required time, it becomes useless and the sound quality suffers. When the telephony data is not being sent, other data may be sent.

Figure 1. Data needs to arrive at uniform time intervals

QoS in technical terms is a commitment by the network to provide the characteristics required to support an application. Some of the important commitments applications require are:

· Bandwidth - the average amount of data required per unit time
· Jitter - the variation in the arrival time
· End-to-End delay - the amount of time it takes to go from the sender to the receiver.
· Peak Bandwidth - the maximum amount of information that can be sent in some unit time.

In order to implement Quality of Service, the following features where added to DOCSIS 1.0.

· Packet Priorities - this allows the system to know which packets are to be given priority.
· The CMTS had to understand how much bandwidth was going to be given to each application. A given modem may have QoS data along with non-QoS data operating simultaneously.
· Unsolicited Grants - A mechanism to gain access to the shared bus.
· Packet Fragmentation - Allow large packets from one computer to be broken into small packets so important packets from another computer and interrupt the transmission.
· Payload Header Suppression - Information in packets that are always the same are not transmitted. Source and destination address are examples of this redundant information.

Unsolicited Grant Service

In DOCSIS 1.0, when a computer has data to send, the Cable Modem (CM) sends a request to the Cable Modem Termination System (CMTS) in the headend. The CMTS then provides the CM with the time it should transmit the data and how much data it is allowed to transmit. It is possible that many CMs may issue these requests simultaneously and the requests would collide and the CMTS would not be able to understand any of the requests. This is analogous to a person on the stage in an auditorium listening to 100 people all yelling simultaneously.

To avoid this type of problem, DOCSIS 1.1 implements a method called Unsolicited Grant Service (UGS). (UGS is one the inventions of the author). Figure 2 is useful in understand how this works.

Figure 2. Understanding UGS

Before a CM starts the session, the CM tells the CMTS the requirements of the session and the CMTS can accept or reject the session if the commitment can be ensured. Some of the parameters the CM identifies are:

· Nominal Grant interval - the time between CM transmissions
· Maximum jitter - how much variation in this time interval the application can tolerate.
· Grant size - How much data is going to be transmitted in a single transmission
· Grants per interval - How many grants are required in each interval.

The CMTS then schedules each grant for the CM without the CM having to request a transmission.

This avoids the problem of several CMs sending requests simultaneously and having those requests collide.

Payload Header Suppression

Although not strictly required to support QoS, Payload Header Suppression (PHS) allows more data to be transmitted over the channel by removing duplicate data. DOCSIS 1.1 has a very unique way of compressing the packet headers.

Figure 3. Payload Header Suppression

PHS is achieved by having the CM identify information that will not change from one transmission to another transmission. The information may be on layer 2 (Ethernet), layer 3 (IP), or layer 4 (TCP). Source and destination addresses, flags, time-to-live counters, and protocol identifiers are examples of information that will remain consistent between transmissions.

The CM identifies the fields that remain consistent, the length of the field and the value of the field. It then sends this information known as the Payload Header Suppression Mask (PHSM) to the CMTS.

From that point on, the CM strips these consistent fields from the packet and sends the modified packet to the CMTS. When the packet is received at the CMTS, the consistent information is then reinserted into the packet and forwarded to the destination.

Packet Fragmentation

As was seen in one of the previous examples, when needed, a packet is scheduled for transmission automatically by the CMTS. It is possible that during this time interval that another packet from a different CM is in the process of being transmitted. If the entire packet was transmitted, the priority packet would miss its transmit opportunity.

DOCSIS 1.1 provides a mechanism for a packet to be interrupted, a second packet from a different CM to be transmitted and then continuing with the first packet. This is shown in Figure 4 below and is accomplished by using packet fragmentation.

Figure 4. Packet Fragmentation

As shown, a large packet overlaps the time interval of the priority packet. The CMTS when controls this process by informing the CM of the first packet that it is to transmit part the first part of the first packet and then to stop transmitting. The CM for the second packet is told to transmit its priority packet and then the CM for the first packet is told to continue transmitting.

More Information

The DOCSIS specifications are available at: www.cablemodem.com/specifications.html .

Additional DOCSIS seminars:

Overview - An overview of the components of a cable TV system.

The DOCSIS Protocol - A description of the messages between the CMTS and the CM. This includes the mechanism to share the coax, ranging and registration.

The DOCSIS Physical Layer - The downstream/upstream modulation and data rates.

In Summary:

• The primary difference between DOCSIS 1.0 and DOCSIS 1.1 is Quality of Service.

• Data can be accurately delivered by using the Unsolicited Grant Service.

• Payload Header Suppression operates on all of the communication layers to reduce the amount of data to be transmitted.

• Packet Fragmentation allows maximum utilization of the upstream bandwidth by allowing large packets to be interrupted for priority packets.