Abstract 

The ubiquity of broadband connectivity and the advancements in home networking technologies introduces into market a new breed of consumer premises equipment termed as home gateways or residential gateways. Home gateway device provides the consumer, benefits such as broadband Internet connection sharing, Firewall security, VPN connectivity, IP telephony, audio/video streaming, Wireless LAN connectivity etc.

Although the home gateway device is a consumer communication equipment with a low price point, engineering wise it is a highly sophisticated device demanding a lot of design, integration, development and testing challenges both on the hardware and on the software front. This paper details the hardware, software technical architecture and specifications of the home gateway devices and lists out the engineering design challenges (along with the possible solutions) faced by a home gateway product designer or a home gateway silicon designer.

Table of Contents

2     Home Gateway Device Personalities

2.1      SOHO Router Home Gateway

2.2      Cable Router Home Gateway

2.3      Cable MTA Home Gateway

2.4      DSL IAD / Router Home Gateway

2.5      Digital Settop Box Home Gateway

3     Technical Architecture of Home Gateway

3.1      Hardware Architecture

3.2      Software Architecture

3.3      Home Gateway Engineering Design Challenges

4     References

5     About the Author

6     Glossary

1         Home Gateway

Home gateway, also called as residential gateway is defined as an intelligent network interface device located at the consumer premises. It provides the means for the residential user to access the Internet services delivered to home and also to access the different services offered by the various smart devices located within home. Essentially the home gateway device provides the necessary connectivity features to enable the consumer to exploit the advantages of a networked home.

In technical terms, a home gateway device does the bridging/routing, protocol and address translation between external broadband network and the internal home networks. It acts as a secure firewall, and also is the focal point for applications such as Voice/Video Over IP, home automation etc. Home gateway device allows the residential users to access their home networks and to control various devices from a remote location through Internet.

Figure 1: Home Gateway Usage Scenario

The consumer benefits provided by the Home Gateway device include:

• Internet connection sharing and simultaneous Internet access

• In-home file/print sharing

• In-home Wireless LAN connectivity

• VPN connectivity to work place

• Firewall security, parental protection

• Broadband telephony (Voice over IP)

• IP Video on Demand

• In-home audio & video streaming

• Remote health monitoring

• Security surveillance

• Home automation & home control

• Remote meter reading

For the Service Provider, the home gateway device provides the following benefits:

• New revenue streams due to new and differentiating services/applications

• Ability to deliver multiple services through a single consumer premises equipment

• Ability to reduce the expensive truck roll costs

2         Home Gateway Device Personalities

At the technical functionality level the home gateway device performs the activities such as bridging, routing, security processing (Firewall, VPN), address management, quality of service etc. However the home gateway device may take various physical device personalities or form factors. 

The various physical device personalities of a home gateway is described in Figure-2. Examples of the home gateway device personalities available in the market is provided in  Table-1  and the Consumer benefits offered by them are summarized in Table-2.

The traditional broadband modem devices (cable modem, DSL modem, fixed wireless modem) could be added with the home networking hardware and software functionality so as to perform the role of a home gateway. Most of the new broadband modems in the market have the home networking functionality and hence classify as home gateway devices. The popular home networking options include the Wireless LAN, Home PNA, 4 Port Ethernet Switch, Home Plug etc. and the desired software functionality include bridging, routing, Voice Over IP, VPN, and Firewall Remote Management capabilities.

The traditional Digital Settop Box (STB) is a consumer device providing services like HDTV, content decryption, Personal Video Recorder, electronic programming guide etc.  Optionally it provides Web browsing and interactive television features. The Digital Settop Box is fast evolving in to a Settop Box home gateway, where in it caters to the data, voice and entertainment connectivity needs of all the devices located in the household and does not limit the connectivity to the TV set alone. 

The other device personalities that can be classified as home gateways include the home automation and security gateways. These devices are not full-fledged home gateways. They only provide a subset of the home gateway functionality and serve the niche need of home security surveillance and automation.

Figure 2: Home Gateway Device Personalities

Table 1: Home Gateway Device Personalities in the Market

Table 2: Summary of Consumer Benefits offered by Home Gateway Devices

2.1      SOHO Router Home Gateway

SOHO Router is a consumer communication device that is connected to the external broadband modem (Cable or DSL modem) via Ethernet or USB and supports Internet connectivity sharing and several other consumer benefits.  It is essentially a two box solution (SOHO Router works in conjunction with a Cable or DSL broadband modem) and it could optionally contain the Wireless LAN (IEEE802.11) Access Point functionality. The typical usage scenario of SOHO Router home gateway is shown in Figure-3.

Figure 3: SOHO Router Home Gateway Usage Scenario

2.2      Cable Router Home Gateway

Cable Router (also called as cable home gateway) device is a one box solution integrating the DOCSIS cable modem device and the SOHO Router device. Cable Router facilitates the cable broadband Internet connection sharing, VPN and other applications all in one box. Application scenario of cable Router home gateway is shown in Figure-4.

Figure 4: Cable Router Home Gateway Usage Scenario

2.3      Cable MTA Home Gateway

Cable MTA (Multimedia Terminal Adaptor) home gateway device provides the Voice and IP Video functionality over cable broadband and is based on the PacketCable Standards from Cablelabs. There are two flavors of cable MTA:

• EMTA (Embedded Multimedia Terminal Adapter) is a one-box solution providing both Voice Over Cable and High Speed Data Over Cable Broadband.  A DOCSIS 1.1 Cable modem providing the Voice Over Cable functionality is an EMTA.

• SMTA (Standalone Multimedia Terminal Adapter) is a two-box solution. SMTA device providing the Voice Over Cable functionality is externally connected to a DOCSIS 1.1 cable modem through the USB or Ethernet port.

The application/usage scenario of Cable MTA home gateway is shown in Figure-5.

Figure 5: Cable MTA Home Gateway Usage Scenario

2.4      DSL IAD / Router Home Gateway

DSL IAD (Integrated Access Device) is a single box consumer premises equipment solution, providing for the simultaneous high speed Internet data access and toll quality voice calls over DSL broadband connection. DSL IAD could additionally incorporate the SOHO Router functionality (hence called a DSL Router or DSL residential gateway) and provides for the Internet connection sharing, VPN and Wireless LAN access point benefits to the consumer.

DSL broadband is based on the ATM technology. Data over DSL is transported as AAL5 cells and IP packets are encapsulated in the AAL5 cells. Voice calls (called as derived voice lines) can be made over DSL broadband in two ways :

• Voice Over DSL: Voice is transported as AAL2 cells. BLES (Broadband Loop Emulation Service) spec TR-036 of DSL forum defines the guidelines.

• Voice Over IP Over DSL: Voice is transported as IP packets (RTP Packets) which are encapsulated in AAL5 cells. MGCP is commonly used for signaling. However H.323 and SIP signaling are also used based on the investment decisions of TELCO.

Figure 6: DSL IAD / Router Home Gateway Usage Scenario

2.5      Digital Settop Box Home Gateway

Digital Settop Box (STB) is a consumer device connected to a TV set to provide the services like Digital High Definition Television, Content Decryption, Personal Video Recorder, Electronic Programming Guide etc.  Optionally it provides Web browsing and interactive television features. In addition to such traditional functionality of digital/HDTV, the STB home gateway device provides the below mentioned additional consumer benefits. The application/usage scenario of an STB home gateway device is shown in Figure-7

• High broadband connectivity to the multiple PCs, and the information appliances in the house hold. high speed Wireless LAN access to anywhere in the house

• In-home file/print/device sharing

• Firewall security, parental protection and VPN connectivity

• Show the content programming (directly received through satellite, cable, terrestrial or stored in the STB PVR hard disk) on to multiple TVs and display devices located in various rooms of the house

• Streaming content and IP Video on Demand (received through broadband Internet connectivity or stored in the STB PVR hard disk) on to multiple TVs and display devices located in various rooms of the house

• Enables remote health monitoring and security surveillance applications

Figure 7: Digital Settop Box Home Gateway Usage Scenario

3         Technical Architecture of Home Gateway

3.1      Hardware Architecture

The home gateway device hardware could be implemented as a single chip solution or a single board solution or a multi board solution depending on the perceived device volume and BOM cost trade offs. The hardware architecture for a home gateway device (implemented as a custom communication ASIC) is illustrated in Figure-8.

At the functional level the home gateway device SOC contains a RISC processor core, memory, DMA controllers and peripheral I/O interfaces. The I/O interfaces could be embedded with in the home gateway ASIC or they can be located as an external chip on the home gateway hardware board.  The popular home networking interfaces include Ethernet (as a 4 port Ethernet switch) , Home PNA (phone line), Home Plug (power line), USB Device, USB Host, IEEE 1394, Wireless LAN, and Bluetooth. The popular broadband Interfaces include cable and DSL. In general all the MAC's could be implemented in the ASIC itself and the PHYs can be connected externally to the home gateway ASIC.

In the case of a voice enabled home gateway, DSP and SLIC are needed for the voice compression and for analog phone connectivity. Some home gateway designs use an external DSP, whereas some implement the DSP instruction set in the custom ASIC through either the usage of a RISC/DSP combo processor core or through the usage of a DSP processor core. There are also implementations, where the Real Time Protocol (RTP) packetization of the voice samples could be done within the hardware itself. To improve the performance of the VPN functionality, the security (Crypto) algorithm functionality could be implemented in the hardware itself though a dedicated security coprocessor.

Figure 8: Home Gateway Hardware Block Diagram

3.2      Software Architecture

The software building blocks of a home gateway device is shown in Figure-9.  The software for home gateway devices can be classified into three levels: firmware, OS and application software stacks. The firmware level software includes the diagnostics, boot loader, debug interface drivers and operating system board support package. The home gateway device needs an embedded operating system, with the device drivers for all the on-board and on-chip physical interfaces like the cable, DSL, Wireless LAN, Ethernet, USB, 1394, Bluetooth, Home PNA, Home Plug etc. The Popular embedded operating systems include Embedded Linux, VxWorks, Nucleous etc.

All the software components for home gateway devices are based on the open standards like IEEE specifications, IETF RFCs, ITU specifications and industry specific forums like Cablelabs, DSL/ATM Forum, UPNP Forum, OSGi Consortium etc. The home gateway software should have well defined interfaces to integrate with the underlying hardware and should hold finely defined and documented APIs for the application development.

Figure 9: Home Gateway Software Building Blocks

The home gateway application software stacks shown in Figure-10 and explained in Table-3,  provide the core functionality of the home gateway. The home gateway application stacks are comprise of the following software components:

• Communication protocol stacks for Routing, Bridging, Address Management (DNS, DHCP, NAT), quality of service (class based queuing, RSVP), security (VPN, IPSEC, Firewall), Remote and System Management (SNMP, Software Upgrades, HTML UI, UPnP IGD)

• Voice Over Cable (Cablelabs PacketCable) and Voice Over DSL (DSL Forum BLES Specification) and Voice Over IP (H.323, SIP, MGCP) infrastructure software stacks

• Audio/video streaming (RTP, RTCP, SDP, RTSP) and application service delivery (UPnP, OSGi) software stacks.

Figure 10: Home Gateway Software Stack

Table 3: Home Gateway Software Specifications

3.3      Home Gateway Engineering Design Challenges

Since the home gateway device is a consumer premises equipment, with low price point, it poses a number of engineering design challenges and constraints for the product manufacturers and silicon vendors. Explained below are a few of the software challenges and the proposed solutions for these challenges.

Low BOM cost: It would be a design challenge to manage the BOM cost within the prescribed limits, without compromising on the feature set and functionality. The home gateway software should occupy minimal FLASH and RAM. To accommodate this, the code should be optimized, pruned and should include only those features that are required. The software components should be well integrated to reduce redundancy of code. Home gateway usually runs on a low powered processor. To accommodate this, the software is separated into control and data path. The data path is frequently traversed, hence the source code for it has to be highly profiled and finely tuned, and code segments have to be optimized for performance. Data path can also utilize the services of hardware accelerators (e.g. DES, SHA-1, Bridging/NAT etc.).  

Interoperability: Home gateway device has to be interoperable with other devices and network elements. Interoperability gives confidence to the end user, that he can communicate to any body in the world over his broadband network. This is a very time consuming and capital-intensive process and can be achieved through the participation in the interop and dry-run events organized by the standards forums. On the data side, VPNC, UPnP, ICSA CableHome certifications help home gateway to be interoperable. A Cable MTA home gateway device has to be interop tested with other MTA devices and with multiple Call Management, Gateway and Provisioning Servers deployed at the cable head end. DSL IAD home gateway device has to be interop tested with other DSL IAD devices and with multiple voice gateway and Provisioning Servers deployed in the field, which is achieved through the participation in the interop and dry-run events organized by the organizations like OpenVoB CallFests, CIRPACK, NH-IOL.