Embedded Electronic Systems Blog

Introduction

G.hn a worldwide standard, addresses technology for wired home-networks. The G.hn technology goal is to unify connectivity of digital content and media devices by providing a network over three popular types of wiring found in homes today: coax cable, phone lines, and AC power wiring.

G.hn will offer the consumer the simplicity and ease of having a single unified network for any wiring in the home, allowing for the delivery of high bandwidth, error free media such as streaming video/TV, streaming audio for entertainment or surveillance. The network  easily handle lower bandwidth application for home automation.

The three standards discussed in this article are ITU G.9960 (G.hn), ITUG.9970 (G.hnta) and IEEE P1901.

Home Networking Standards

There are several existing technologies that arecurrently deployed as home wiring technology in the home. These technologies are represented by organizations such as Consumer Electronics Powerline Communication Alliance™ (CEPCA™), the HomePNA™Alliance, Multimedia over Coax Alliance, HDPLC Alliance, HomePlug Alliance, and the Universal Powerline Association (UPA). The advancement for compatibility and convergence of G.hn with other home networking technologies is performed thru forums and committees who work closely with the ITU-T and other standards bodies. The ITU group which is responsible for the G.hn standard has decided that the interoperability between G.hn and other home networking transceivers is outside the scope of the G.hn definition, but it has decided that G.hn transceivers must coexist with existing home wired network technologies and standards. The further work by committees and forums, working with the ITU and other standards bodies, may allow compatibility beyond coexistence to emerge in the near future.

Home Networking ITU.G.hn ITU.hnta and IEEE1901 Standards

The ITU G.hn standard’s goal is to specify the physical (layer 1) and link (layer 2) layers for home wired networks. This work defines physical transceivers which operates over existing home wiring such as coaxial cable, telephone wiring, AC power line wiring. This work culminated in Recommendation ITU G.9960 specifying G.hn’s Physical Layer.

The IEEE P1901 group adopted a baseline that includes three non-interoperable options for PHY and MAC. One of the three PHY/MAC options is compatible with G.hn/G.9960. On power line wiring,G.hn can use the same coexistence scheme adopted by the IEEE P1901 project for sharing of the power line medium between G.hn devices, IEEE Access PLC systems,and existing home networking technologies.

The ITU G.hnta standard’s goal is to specify the network layer (Layer 3) for home wired networks. This work defines the generic architecture for home networks and their interfaces which operates over existing home wiring such as coaxial cable, telephone wiring, AC power line wiring. This work culminated in Recommendation ITU-T G.9970 specifying G.hnta Network Layer.

G.hn and Network Co-existence

When G.hn technologies are deployed onto home wiring that already supports previously installed home networking technologies,they should, as a part of a migration path toward a fully compliant G.hn solution, be able to coexist with these existing technologies to provide an acceptable user experience. A core requirement for G.hn is to provide spectral compatibility with home networking transmission with DSL telecom WAN connectivity.

Overview

G.hn specifies a Physical Layer based on fast Fourier transform (FFT) Orthogonal frequency-division multiplexing (OFDM) modulation and Low-Density Parity-Check (LDPC) FEC code. G.hn includes the capability to notch specific frequency bands to avoid interference with licensed radio services. G.hn includes mechanisms to avoid interference with legacy home networking technologies and also with other wireline systems used to access the home.

OFDM systems split the transmitted signal into multiple orthogonal carriers. These carriers are modulated using Quadrature amplitude modulation (QAM).

The G.hn Medium Access Control in the link layer is based on a TDMA architecture, in which a domain master arbitrates by scheduling Transmission Opportunities (TXOPs) used by devices in the domain. There are two types of TXOPs:

• Contention-Free Transmission Opportunities (CFTXOP) fixed duration, allocated to a specific pair of transmitter and receiver. Used to implement QOS.

• Shared Transmission Opportunities (STXOP), which are shared among multiple devices in the network. STXOP are divided into Time Slots (TS). There are two types of TS:
  • Contention-Free Time Slots (CFTS), which are used for implementing “implicit” Token passing Channel Access. The process of “passing the token” is implicit and ensures that there are no collisions during Channel access.
  • Contention-Based Time Slots (CBTS), which are used for implementing CSMA/CARP Channel Access. CSMA systems cannot completely avoid collisions, so CBTS are used for applications that do not have strict QOS requirements.

G.hn Protocol stack

Image from wikipedia

G.hn Physical Layer and the Data Link Layer

• The G.hn Data Link Layer is divided into three sub-layers:
  • The Application Protocol Convergence (APC) Layer, which receives frames from the application layer or another upper layer and encapsulates them into G.hn MAC Service Data Units (MSDUs).
  • The Logical Link Control (LLC), which is responsible for encryption, aggregation, segmentation and Automatic repeat-request.
  • The Medium Access Control (MAC), which schedules Channel Access. This Layer is similar to bus arbitration on a electrical shared node bus. Scheduling helps avoid collisions from multiple devices accessing the shared medium or bus.

• The G.hn Physical Layer is divided into three sub-layers:
  • The Physical Coding Sub-layer (PCS), for generating PHY headers.
  • The Physical Medium Attachment (PMA), for scrambling and FEC coding/decoding.
  • The Physical Medium Dependent (PMD), for bit-loading/stuffing and OFDM modulation. This sub-layer is the only sub-layer in the G.hn stack that is “medium dependent”.

The Physical interface between the Data Link Layer and the Physical Layer is called Medium Independent Interface (MII). The interface is similar to the interface used between a Ethernet controller and the ethernet PHY.

The standardization, performance, and co-existence with current devices make G.hn the ideal solution for networking digital media to consumer devices within homes. Come Back to the Vas-N-Mac blog to read more posts on home networking and G.hn.