Title - An Introduction to the CableHome Networking Protocol


CableHome™ is a new home networking architecture that has the potential to be an important step in the evolution of communications for the home. It was sponsored by CableLabs® which is an organization that supports the common development interests of the cable operators. As with the cable modem standard DOCSIS (Data Over Cable Service Interface Specification), the operators commitment to only buy and support equipment that is certified by CableLabs is a powerful influence in determining the direction for the industry.

The objective of the CableHome™ architecture is to provide a variety of new services to residential customers, simplify the management the home network, and protect copyrighted information from being diverted to other uses. Within the residence, the CableHome architecture is designed to be independent of the underlying network technologies such as Ethernet, wireless, HomePNA, and HomePlug. While not all services can operate on all LAN technologies, the desire to let market forces decide on the most cost effective systems allows for an evolution of the network.


The initial version of CableHome architecture, CableHome 1.0, describes the functions needed to implement the security, management, and network protection services of the architecture. This should enable the benefits of networks used in businesses to migrate into the home environment The CableHome functions are implemented in the Home Access (HA) device shown in Figure 1. The HA device is a combination of a Cable Modem (CM) and additional features known as Portal Services (PS). The HA device can be a single unit or allows for using existing DOCSIS 1.1 Cable Modems and a new PS unit connected together.


Figure 1. Overview of CableHome 1.0


The HA device is a combination of a Cable Modem (CM) and additional features known as Portal Services (PS). The HA device can be a single unit or it can be implemented using an existing DOCSIS 1.1 Cable Modem and a new PS unit connected together. The functions that are implemented in the Portal Service are:

  • Address Translation

  • Address Management

  • Remote management

  • Support Quality of Service for the user's data

  • Security features to protect the user

  • Security of the content providers data to the user

Today, people wanting to implement a home network with these features buy units that implement Network Address Translation (NAT), address servers (DHCP), firewalls, and routers. It is normally very difficult for people not familiar with networking to install this equipment and diagnose problems when they occur. In a business environment, specialists are employed to perform these functions but this is not feasible for the average home network.


The CableHome architecture implements the desirable features mentioned without the complexity for the user. It also overcomes many of the limitations of the traditional NAT while ensuring the needs of content providers are protected.

In order to offer new services, the provider must be assured:

  • that the data is going to the specified user and not somebody masquerading as that user.

  • that the data is protected (copied by the user or someone listening to the data)

  • that the quality of the data stream is acceptable (low error rate, not visibly jerky, not too long a delay if it interactive).

  • that the user cannot later say they don't want to pay for the service because the data stream did not arrive or was of poor quality.

CableHome helps achieve these objectives because it gathers information about the network that is attached to the HA. The HA collects information on error rates, manages bandwidth on home networking technologies that support Quality-of-Service (QoS) delivery such as HomePNA and the IEEE 802.11 wireless technology.


The complete CableHome 1.0 specification can be found at www.cablelabs.com/cablehome/specifications.html.

Portal Services

The CableHome Portal Services feature is composed of several different functions. A block diagram of the various PS functions is shown in Figure 2. A brief explanation of each function is described below.

Figure 2. Block diagram of the PS functions

CAP = CableHome Address Portal
CDP = CableHome DHCP Portal
CTP = CableHome Testing Portal
CNP = CableHome Naming Portal
CQP = CableHome Quality of Service Portal
CMP = CableHome Management Portal

CableHome Address Portal (CAP)

The CAP creates address translation mappings between the External Network and Home Network. Its basic functions are:

  • Shares a single external IP address with many home devices (Network Address Translation - NAT).

  • Decides which packets from the home network need to go to the external network which helps to preserve upstream bandwidth.

  • Provides a Passthrough function, which allows packets for a particular device not to be translated by the NAT or altered by Application Layer Gateway (ALG) functions.

CableHome DHCP Portal (CDP)

The CDP provides the address assignment functions (e.g. those transmitted via DHCP) including a server for the devices on the home network and a client for the external network. This allows home devices to get an IP address, the address of the gateway to get out of the home, an the address of a DNS server which translates a site name into an IP address, and a variety of other network configuration parameters.

CableHome Naming Portal (CNP)

The CableHome Naming Portal is a service running in the PS that provides a simple DNS server for LAN IP Devices in the home. Its basic functions are:

  • Provide Domain Name Service (DNS) from a server in the PS to DNS clients within home devices, even during cable connection outages.

  • Enable subscribers to refer to local devices via intuitive device names rather than by IP address.

  • Refer LAN DNS clients to Headend DNS servers, for resolution of non-local hostnames.

  • Provide easy DNS service recovery upon re-establishment of cable connectivity after an outage.

CableHome Quality of Service Portal (CQP)

CQP provides the environment to enable home networking applications to utilize PacketCable and DOCSIS QoS resources. These resources provide a management mechanism that prioritizes data session to support real-time application traffic, such as VoIP, A/V streaming, and video gaming, by reducing packet latency and jitter delays. Its basic functions are:

  • Enable home networking applications to establish prioritized data sessions between the CMTS and HA device using PacketCable compliant messaging.

  • Facilitate design and field-testing leading to the manufacture and interoperability of conforming hardware and software by multiple vendors.

CableHome Management Portal (CMP)

The PS is managed from the external network via the CMP, and to a large degree, this involves access to the information in the PS Database. Management is used for initialization and provisioning of the network elements, and diagnostics or status of the LAN. Its basic functions are to:

  • Provide cable operators with visibility to LAN IP Devices.

  • Provide cable operators with a minimum set of remote diagnostic tools that will allow the cable operator to verify connectivity between the Portal Services element and any home networking device.

  • Provide cable operators with access, via the MIBs, to internal data in the PS element and enable the cable operator to monitor CableHome-specified parameters and to configure or reconfigure CableHome-specified capabilities as necessary.

  • Provide a means for reporting exceptions and other events in the form of SNMP messages to a local log or to the cable network.

The PS Database

The PS Database stores a myriad of data relationships. The CMP provides the management interface (SNMP) to the PS database. The CableHome functions within the PS enter and revise data relationships in the PS Database. Additionally, the CableHome Functions within the PS may retrieve information from the PS Database that is maintained by other CableHome Functions within the PS.

CableHome Security Portal (CSP)

The CSP acts as a security portal for other HA elements. One of its primary functions is to forward security messaging between Cable Operator's servers (including the security server) and PacketCable applications. Its basic functions are to:

  • provide security services, such as authentication and key management for the PS element.

  • provide firewall functions to protect the home user from network attacks

  • provide screening functions to prevent children from getting to unauthorized sites.

CableHome Testing Portal (CTP)

The CTP (CableHome Test Portal) contains the "remote tools" with which the NMS can collect further LAN device information. The CTP is a local proxy used to interpret and execute the remote fault/diagnostic class of SNMP messages it receives from the cable operator. Its basic functions are to:

  • Enable LAN IP Device fault diagnostics

  • Enable visibility to LAN IP Devices

  • Enable LAN IP Device performance monitoring

Future Versions

The CableHome 1.0 specification is just the initial version of the overall CableHome Architecture. While this specification focuses on the Home Access device, there is much more to the architecture. The complete CableHome Architecture Framework Technical Report can be found at www.cablelabs.com/cablehome/specifications.html.


Shown in Figure 3 are some of the additional elements that will be included in future versions of CableHome. The HB device, known as a gateway, joins two of the home networks both of which are compliant to the CableHome specification. For networks that are not complaint with CableHome, an HC device, known as a Boundary Point, performs the necessary buffering and monitoring functions for the non-compliant networks.

Figure 3. Overview of the CableHome Architecture


Thought has been given as to how a home network may evolve and the architecture is flexible enough to handle a variety of changes. As in today's business environment, when LAN segments start being overloaded, there is point were it makes sense to subdivide them. When that happens, typically bridges or routers are used to separate these segments. In the CableHome architecture, the generic device that isolates networks is known as Gateway (GW) or HB device.


The GW may contain a bridge, router, NAT, and other functions. It is more than a standard bridge or router because it can be managed by the operator, can perform network monitoring and testing. An HB can have one or more LAN segments connected to it. In the above figure, LAN 4 is a network consisting of AV devices and its large bandwidth requirements for local transfer does not affect the data transfer of the PC or other devices not on that segment.


Another CableHome device is a Boundary Point (BP) or HC device. Normally the devices attached to LANs managed by the operator have the TCP/IP protocol installed and can participate in bandwidth reservation and provisioning. This assumption is valid for many intelligent devices such as a PC but there may be low cost devices that can't support the economics of such a protocol. For this situation, the BP isolates these low cost / low sophistication devices from CableHome compliant networks.


The BP essentially acts like a translator taking CableHome commands and translates them to appropriate commands for that type of device. The desire is to support all types of end devices, so the BP is defined only on the CableHome side, and a BP manufacturer does whatever is necessary to act as an agent for devices on non-compliant networks.


In Figure 3, inexpensive communications technologies are required for "smart" appliances (LAN 5) or non-IP aware devices such as a security camera. If the user chooses to have the home network configured and maintained by the operator, these devices can be identified, and problems diagnosed because of the intelligence of the BP.

More Information

CableHome uses DOCSIS (the cable modem standard) to transmit information between the headend and the home.


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.

Quality of Service (DOCSIS 1.1) - The changes to DOCSIS 1.0 to implement QoS.


In Summary:

  • CableHome will be an important home networking technology because the Cable Operators will require its use for customers to get enhanced content.

  • CableHome will accommodate existing and future LAN technologies.

  • Existing NATs, Routers, and Bridges will need to be enhanced to work with CableHome services.

  • CableHome 1.0 is the first version of the overall CableHome architecture.


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