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
· Bandwidth - the average amount of data required per unit
· 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
· 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
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.
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
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.
The DOCSIS specifications are available
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.
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
Packet Fragmentation allows maximum utilization
of the upstream bandwidth by allowing large packets to be
interrupted for priority packets.