G.hn specifies a single physical layer based on fast Fourier transform (FFT) orthogonal frequency-division multiplexing (OFDM) modulation and low-density parity-check code (LDPC) forward error correction (FEC) code. G.hn includes the capability to notch specific frequency bands to avoid interference with amateur radio bands and other licensed radio services. G.hn includes mechanisms to avoid interference with legacy home networking technologies and also with other wireline systems such as VDSL2 or other types of DSL used to access the home.
OFDM systems split the transmitted signal into multiple orthogonal sub-carriers. In G.hn each one of the sub-carriers is modulated using QAM. The maximum QAM constellation supported by G.hn is 4096-QAM (12-bit QAM).
The G.hn Media Access Control is based on a time division multiple access (TDMA) architecture, in which a “domain master” schedules Transmission Opportunities (TXOPs) that can be used by one or more devices in the “domain”. There are two types of TXOPs:
- Contention-Free Transmission Opportunities (CFTXOP), which have a fixed duration and are allocated to a specific pair of transmitter and receiver. CFTXOP are used for implementing TDMA Channel Access for specific applications that require quality of service (QoS) guarantees.
- 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. In G.hn, a series of consecutive CFTS is allocated to a number of devices. The allocation is performed by the “domain master” and broadcast to all nodes in the network. There are pre-defined rules that specify which device can transmit after another device has finished using the channel. As all devices know “who is next”, there is no need to explicitly send a “token” between devices. 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. In general, CSMA systems cannot completely avoid collisions, so CBTS are only useful for applications that do not have strict Quality of Service requirements.
Optimization for each medium
Although most elements of G.hn are common for all three media supported by the standard (power lines, phone lines and coaxial cable), G.hn includes media-specific optimizations for each media. Some of these media-specific parameters include:
- OFDM Carrier Spacing: 195.31 kHz in coaxial, 48.82 kHz in phone lines, 24.41 kHz in power lines.
- FEC Rates: G.hn’s FEC can operate with code rates 1/2, 2/3, 5/6, 16/18 and 20/21. Although these rates are not media specific, it is expected that the higher code rates will be used in cleaner media (such as coaxial) while the lower code rates will be used in noisy environments such as power lines.
- Automatic repeat request (ARQ) mechanisms: G.hn supports operation both with and without ARQ (re-transmission). Although this is not media specific, it is expected that ARQ-less operation is sometimes appropriate for cleaner media (such as coaxial) while ARQ operation is appropriate for noisy environments such as power lines.
- Power levels and frequency bands: G.hn defines different power masks for each media.
- MIMO support: Recommendation G.9963 includes provisions for transmitting G.hn signals over multiple AC wires (phase, neutral, ground), if they are physically available.
G.hn uses the Advanced Encryption Standard (AES) encryption algorithm (with a 128-bit key length) using the CCMP protocol to ensure confidentiality and message integrity. Authentication and key exchange is done following ITU-T Recommendation X.1035.
G.hn specifies point-to-point security inside a domain, which means that each pair of transmitter and receiver uses a unique encryption key which is not shared by other devices in the same domain. For example, if node Alice sends data to node Bob, node Eve (in the same domain as Alice and Bob) will not be able to easily eavesdrop their communication.
G.hn supports the concept of relays, in which one device can receive a message from one node and deliver it to another node further away in the same domain. Relaying provides extended range for large networks. To ensure security in scenarios with relays, G.hn specifies end-to-end encryption, which means that if node Alice sends data to node Bob using node Mallory as an intermediate relay, the data is encrypted in such a way that Mallory cannot decrypt it or modify it.
The G.hn architecture includes the concept of profiles. Profiles are intended to address G.hn nodes with significantly different levels of complexity. In G.hn the higher complexity profiles are proper supersets of lower complexity profiles, so that devices based on different profiles can interoperate with each other.
Examples of G.hn devices based on high complexity profiles are Residential Gateways or Set-Top Boxes. Examples of G.hn devices based on low complexity profiles are home automation, home security and Smart Grid devices.
G.hn protocol stack
G.hn specifies the physical layer and the Data Link Layer, according to the OSI model.
- The G.hn Data Link Layer (Recommendation G.9961) is divided into three sub-layers:
– The Application Protocol Convergence (APC) Layer, which accepts frames (usually in Ethernet format) from the upper layer (Application Entity) and encapsulates them into G.hn APC protocol data units (APDUs). The maximum payload of each APDU is 214 bytes.
– The Logical Link Control (LLC), which is responsible for encryption, aggregation, segmentation and Automatic repeat-request. This sub-layer is also responsible for “relaying” of APDUs between nodes that may not be able to communicate through a direct connection.
– The Medium Access Control (MAC), which schedules Channel Access.
- The G.hn physical layer (Recommendation G.9960) is divided into three sub-layers:
– The Physical Coding Sub-layer (PCS), responsible for generating PHY headers.
– The Physical Medium Attachment (PMA), responsible for scrambling and forward error correction coding/decoding.
– The Physical Medium Dependent (PMD), responsible for bit-loading and OFDM modulation.
The PMD sub-layer is the only sub-layer in the G.hn stack that is “medium dependent” (i.e., some parameters may have different values for each media – power lines, phone lines and coaxial cable). The rest of sub-layers (APC, LLC, MAC, PCS and PMA) are “medium independent”.
The interface between the Application Entity and the Data Link Layer is called A-interface. The interface between the Data Link Layer and the physical layer is called Medium Independent Interface (MII). The interface between the physical layer and the actual transmission medium is called Medium Dependent Interface (MDI).