1. Zhenyun Zhuang Sriram Lakshmanan
DOCSIS® Overview
Zhenyun Zhuang
Sriram Lakshmanan

2. CATV networks with DOCSIS
Services
Video
Single Integrated Pipe
High-Speed
Data
HFC (Cable) Network using DOCSIS transport
ATM or IP Over xDSL Lines
Fixed Wireless
Voice/Fax
“A network where all kinds of information (voice, fax, video, data) are transported uniformly using packet-based transport and switching media.”

3. Operator Aggregation network Operator Core Backbone
Cable Architecture
New Services Opportunities
PacketCable
DOCSIS
CableHome
HVAC control
Fire sense & control
Security
Air quality monitoring
Child monitoring
Energy management, etc. CM CM
CMTS
MPEG Services CM CM CM
Operator Aggregation network
Operator Core Backbone
CMTS CM CM
Remote file sharing
Shared calendar
Unified messaging
Managed services CM CM
IP Services
CMTS CM CM
Core Network
Aggregation Network
Access Network
Operator administered
Backend
Headend
CPE

4. What is a CMTS?
A CMTS is a Cable Modem Termination System, or router, which is a device located in the cable headend that allows cable television operators to offer high-speed Internet access to home computers.
A cable modem termination system (CMTS) is a component that exchanges digital signals with cable modems on a cable network. A cable modem termination system is located at the local office of a cable television company. A data service is delivered to a subscriber through channels in a coaxial cable or optical fiber cable to a cable modem installed externally or internally to a subscriber's computer or television set. One television channel is used for upstream signals from the cable modem to the CMTS, and another channel is used for downstream signals from the CMTS to the cable modem.
When a CMTS receives signals from a cable modem, it converts these signals into Internet Protocol (IP) packets, which are then sent to an IP router for transmission across the Internet. When a CMTS sends signals to a cable modem, it modulates the downstream signals for transmission across the cable to the cable modem. All cable modems can receive from and send signals to the CMTS but not to other cable modems on the line.

5. What does Cable Modem mean?
“CABLE” is short for Cable TV (CATV) Network
“MODEM” is MOdulator-DEModulator
Actually more like a network adapter than a modem
CMTS (Head-End)
Cable Modem
Upstream Demodulator
QPS K/16-QAM
F: 5-65 MHz BW: eg 2 MHz
Rate: eg. 3 Mbit/s
Upstream Modulator
QPS K/16-QAM
F: 5-65 MHz BW: eg 2 MHz
Rate: eg. 3 Mbit/s
Downstream Modulator
64-QAM/256-QAM
f: MHz BW: 6/8 MHz
Rate: Mbit/s
Downstream Demodulator
64-QAM/256-QAM
f: MHz BW: 6/8 MHz
Rate: Mbit/s

6. Inside the Cable Modem Tuner Demodulator Modulator
Media Access Control (MAC) device
Microprocessor

7. Cable Modems
What’s Inside?

8. Cable TV Frequency Allocation
MHz Sub band Upstream Cable
MHz VHF-Lo Ch. 2 – 6
88 – 108 MHz
FM Radio FM radio
90 – 174 MHz Mid band Downstream Cable
174 – 216 MHz VHF-Hi Ch. 7-13
216 – 300 MHz Super band Downstream Cable
300 – 1002 MHz Hyper band Downstream Cable
MHz UHF Ch

9. CM- Up Stream What the Cable Modem transmits
Frequency 5-42 MHz (5-42 MHz)
Bandwidth e.g., 2 MHz
Modulation QPSK or 16-QAM
Data-rate e.g., 3 Mbit/s (~400 KB/s)
Transmit bursts of data in timeslots (TDM)
Reserved and contention timeslots

10. CM- Down Stream What the Cable Modem receives Frequency 50-750 MHz
Bandwidth 6 MHz (USA); or 8 MHz (EU)
Modulation 64-QAM (or 256 QAM)
Data-rate Mbit/s (4-7 Mbyte/s)
Continuous stream of data
Received by all modems

11. DOCSIS Data Over Cable Service Interface Specification.
Developed by Cable Labs and approved by the ITU in March 1998, defines the communications and operation support interface requirements for a data over cable system.
It permits the addition of high-speed data transfer to an existing cable TV (CATV) system. It is employed by many cable television operators to provide Internet access over their existing hybrid fibre coaxial (HFC) infrastructure.

12. EuroDOCSIS The European version of DOCSIS is called EuroDOCSIS.
The main difference is that in Europe cable channels are 8 MHz wide (PAL), whereas in North-America cable channels are 6 MHz wide (NTSC).
This translates to permitting more bandwidth to be allocated to the downstream data path (taken from a user's point of view, downstream is used to download data, while upstream is used to upload data).
There are also different DOCSIS flavors in Japan.

13. DOCSIS Evolution So Far
DOCSIS 1.0 (High-Speed Internet Access) March 26, 1997
Specification first issued on 30-million DOCSIS modems shipped worldwide as of Q3-2003
5Mbps
DOCSIS 1.1 (Telephony, Gaming, Streaming Media) 1999
Quality of Service (QoS), service security, operations tools
Backward compatible with DOCSIS 1.0
10Mbps
The DOCSIS specifications have evolved through three revisions (1.0, 1.1, and 2.0)
DOCSIS 1.0 targets High-Speed Internet Access
DOCSIS 1.1 targets Telephony, Gaming, and Streaming Media
DOCSIS 2.0 targets Capacity for Symmetric Services
DOCSIS 2.0 (Capacity for Symmetric Services) 2001
More upstream capacity (30Mbps)
Improved robustness against interference (A-TDMA and S-CDMA)
Backward compatible with DOCSIS 1.0 and 1.1

14. Continued DOCSIS Evolution
DOCSIS Set-top Gateway (DSG – set-top signaling path)
Specification first issued on February 28, 2002
Currently being updated, I02 version later this year
Interoperability testing in Q3 2004
Certification/Qualification in Q4 2004
eDOCSIS (DOCSIS in embedded devices)
Specification first issued on March 12, 2003
Details how DOCSIS cable modems are integrated into multifunction devices, for example:
Embedded Media Terminal Adaptor (E-MTA) – PacketCable VoIP
Embedded Portal Services (E-PS) – CableHome Residential Gateway
Embedded Set-top Terminal
Plant and network health monitoring devices
The DOCSIS specifications have evolved through three revisions (1.0, 1.1, and 2.0)
DOCSIS 1.0 targets High-Speed Internet Access
DOCSIS 1.1 targets Telephony, Gaming, and Streaming Media
DOCSIS 2.0 targets Capacity for Symmetric Services

15. DOCSIS® Roadmap DOCSIS Version DOCSIS 1.0 DOCSIS 1.1 DOCSIS 2.0
DOCSIS 2.X
DOCSIS 3.0
Services
Broadband Internet
Tiered Services
VoIP
Video Conferencing
Commercial Services
Roaming Services
Entertainment Video X
Consumer Devices
Cable Modem
VoIP Phone (MTA)
Residential Gateway
Video Phone
Mobile Devices
IP Set-top Box
Downstream Bandwidth
Mbps/channel
Gbps/node 40 5
200
6.3
Upstream Bandwidth
Mbps/node 10 80 30
170
100
450
[1] Assumes 750MHz of available downstream spectrum (125 channels)
[2] Aggregation of four 6MHz channels. With 256QAM = 160 Mbps, with 1024QAM = 200Mbps.
[3] Assumes ~25MHz of useable upstream spectrum
[4] Assumes ~35MHz of useable upstream spectrum
[5] Assumes ~100MHz of useable upstream spectrum (change in upstream/downstream split)

16. Cable Bandwidth Raw Data Rate (Mbps) Upstream Rates Downstream Rates
DOCSIS 2.0
Raw
Data
Rate
(Mbps)
DOCSIS 1.0/1.1
Maximum Channel BW
Spectral Efficiency
Maximum Data Throughput
DOCSIS 1.0
3.2 MHz
1.6 bps/Hz* (QPSK)
5.12 Mbps
DOCSIS 1.1
3.2 bps/Hz (16QAM)
10.24 Mbps
DOCSIS 2.0
6.4 MHz
4.8 bps/Hz (64QAM or 128QAM/TCM)
30.72 Mbps

17. Features
DOCSIS uses the TDMA/SCDMA access method. This is different from the Ethernet system, in that DOCSIS systems experience few collisions.
DOCSIS 2.0 brings more options to the upstream, including higher orders of modulation (64-level QAM) and wider channels (6.4 MHz).
DOCSIS 2.0 also introduces Ingress Cancellation, which greatly improves throughput.
All three versions of the DOCSIS standard support a downstream throughput of up to 38 Mbit/s per channel with 256-level QAM.
The EuroDOCSIS standard supports downstream throughput of up to 51 Mbit/s per channel (due to the 8MHz channel width).

18. DOCSIS 3.0
Currently under development, DOCSIS 3.0 is expected to feature "channel bonding", which enables multiple downstream and upstream channels to be used together at the same time by a single subscriber.

19. Equipments
Before a cable company can run DOCSIS, it must upgrade its HFC network to support a return path for upstream traffic.
The customer PC and associated peripherals are termed Customer Premise Equipment (CPE). It's connected to the cable modem, which is in turn connected through the HFC network to the CMTS.
The CMTS will then route traffic between the cable network and the Internet.
Cable operators (sometimes called Multiple Service Operators - MSOs) have full control of the cable modem's configuration.

20. Current Market
Comcast, the largest cable provider in the United States, caps downstream bandwidth at 4 Mbit/s and upstream bandwidth at 384 kbit/s for standard home connections.
In some areas they are phasing in 6 Mbit/s and 8 Mbit/s downstream and 768 kbit/s upstream.

21. Cont’
Another major cable provider, Cox Communications recently upgraded their base package to 4 MBit/s downstream and 512 kbit/s upstream. Their premier package allows downstream rates of 9 Mbit/s and upstream rates of 1 Mbit/s. In select markets (such as Northern Virginia) they have increased these speeds further still, allowing 15 Mbit/s downstream and 2 Mbit/s upstream for the premier package.
The operator UGC (locally known as UPC) in Sweden offers the service Chello in ranges up to 24 Mbit/s downstream and 8 Mbit/s upstream.

22. Architectural Framework

23. Interface Categories Data Interfaces CMCI-Cable Modem CPE Interface
CMTS-NSI CMTS-Network side Interface
Operations Support Systems Interfaces
Network Management,billing,business processes
RF Interfaces
Between Cable modem and cable network
Between CMTS and Cable network(downstream)
Between CMTS and cable network (upstream)
Security Interfaces

24. Communication Protocols
SNMP
TFTP
DHCP
UDP
IP,ICMP
LLC
Security
MAC
PHY

25. Physical Layer specifications
Supports TDMA/FDMA and FDMA/Synchronous-CDMA
What’s Synchronous-CDMA?
Why is it better than just CDMA?
Data Transmitted in two dimensions code,time.Data to be transmitted into 2 D frames prior to transmission
A burst from a CM can be transmitted on 2 or more codes in one or more frames
In normal operation the MAC requests the PHY to transmit a burst of length n minislots starting at the minislot – m (which it has been assigned by the CMTS)

26. S-CDMA Channel

27. Physical Layer (continued)
Modulations- high spectral efficient (upto 128 QAM
Improvement over Previous spec.)
Reed Solomon and Trellis Coded Modulation for Forward Error Correction(also contributes to the improvement)
Interleaver and scrambler for noise immunity
Synchronization using SYNC bytes
-provides upto 6.25us accuracy-TDMA Minislot duration
Sublayers:
Downstream Transmission Convergence sublayer(additional services like Digital video and new multimedia services)
Physical Media Dependent Sublayer(defines all the RF and media related constituents)

28. MAC Sublayer Bandwidth allocation controlled by CMTS
A stream of mini-slots in the upstream
Dynamic mix of contention and reservation based transmit opportunities
Bandwidth efficiency through support of variable length packets
Extensions provided for future support of ATM
QOS
Security
Wide range of data rates

29. Logical Upstream Channel
A CM can register to operate on only one logical upstream channel
Four types of logical upstream (designed for better allocation and compatibility with existing CMs)
Type 1:DOCSIS 1.x upstreams that do not support DOCSIS Features
Type 2:Mixed upstreams that support DOCSIS 1.x and DOCSIS 2.0 TDMA Bursts
Type 3A:DOCSIS 2.0 TDMA bursts only
Type 3B:S-CDMA upstreams supporting CMs operating in S-CDMA mode

30. MAC Frame
MAC Header
Payload

31. Note that compatibility with ATM and future schemes is envisaged
MAC FRAME (Contd.)
Note that compatibility with ATM and future schemes is envisaged

32. MAC Operation

33. Bandwidth allocation
Each CM has one or more short (14 bit) service identifiers) as well as 48 bit addresses
Upstream bandwidth is divided into number of minislots numbered relative to a master reference maintained by the CMTS and communicated through SYNC byte
CMs may issue requests to the CMTS for bandwidth
CMTS transmits allocation MAPs downstream defining the usage of minislots

34. Allocation MAP MAC management message
Is a varying length MAC management message transmitted by the CMTS to define transmission opportunities on the upstream channel
Consists of a fixed length header followed by a variable number of information Elements
Request IE specifies a time interval during which requests may be made for upstream bandwidth

35. A typical Scenario
At time t1,the CMTS transmits a MAP whose effective starting time is t3.Within this MAP is a request IE which will start at t5.
At t2, the CM receives this map and scans it for request opportunities.In order to minimize request collisions, it calculates t6 as a random offset based on a Backoff start value in the MAP
At t4, the CM transmits a request for as many minislots as needed to accommodate the PDU.Time T4 is based on calculating the ranging offset so that the request will arrive at the CMTS at t6

36. A typical scenario
At t6 the CMTS receives the request schedules it for service in the next MAP.
At t7, the CMTS transmits a MAP whose effective starting time is t9.Within this MAP a data grant for the CM starts at t11
At t8, the CM receives the grant and scans for its data grant
At t10, the CM transmits its data so that it will arrive at the CMTS at t11.
Other cases
If the request collides and is lost, it can be identified from the next MAP that there is no ACK or allocation and a backoff is performed and retried
The CMTS may not allocate bandwidth in the next MAP , when it indicates a grant length of 0.As long as the CM receives a zero length grant it must not retry and keep waiting

37. Quality of Service
QOS is achieved by providing a number of flows with a range of bandwidth,delay parameters using service flows and classifiers
A service flow is a unidirectional flow of packets that is provided a particular QOS
The CM and CMTS provide QOS by shaping,policing and prioritizing traffic according to the QOS parameter set defined for each service flow.

38. Mechanism
The CMTS assigns one or more service flow Ids to each CM corresponding to the service flows required by it.
The mapping is negotiated between the CM and CMTS during registration or can also be done dynamically using the dynamic service establishment mechanism
In a basic implementation two service flows (one upstream,one downstream) are used.(which can be used for IP traffic )
Incoming packets are matched to a classifier to determine to which service flow the packet is forwarded

39. Other layers Network layer - uses the semantics of IP and ICMP
Higher layer protocols such as SNMP,TFTP,DHCP ,Time of Day protocol are also to be supported(mandatory)

40. Summary
DOCSIS uses advanced modulation and coding to improve spectral efficiency
Required QOS is provided by the CMTS by appropriately provisioning resources and granting slots for transmission using service flows
The decision on the channel allocation and grant is made by the CMTS to satisfy the QOS requirements
Support for IP,ATM and possible future extensions is also provided
Thus transfer of IP traffic through cable is accomplished with additional overhead at the CMTS