2THE HomePlug AV NETWORKARCHITECTURE

2.1 INTRODUCTION

This chapter presents an overall description of the HomePlug AV network and its

associated architecture. Special attention is given to the function and role of the

HomePlug AV PHY, Medium Access Control (MAC), and Convergence layers as

well as the grouping of protocol entities into the Data Plane and the Control Plane.

The chapter also defines the HomePlug AV network topology, station roles, and the

HomePlug AV Local Network (AVLN) and its associated Central Coordinator

(CCo). The chapter then uses these definitions to outline the essentials of peer-

to-peer communication, bridging, network membership, and channel access in

HomePlug AV networks.

2.2 PROTOCOL LAYERS

At the highest level of abstraction, the HomePlug AV system consists of the protocol

layers shown in Figure 2.1. The functions at the transmitter are also implemented in

reverse order at the receiver. The PHY layer performs forward error correction

(FEC), mapping data onto OFDM symbols, and the generation of requisite time-

domain waveforms. The MAC layer determines the correct position of transmission,

formats the data frames into fixed-length entities for transmission on the channel, and

12

HomePlug AV and IEEE 1901: A Handbook for PLC Designers and Users, First Edition.

Haniph A. Latchman, Srinivas Katar, Larry Yonge, and Sherman Gavette.

� 2013 by The Institute of Electrical and Electronics Engineers, Inc. Published 2013 by John Wiley & Sons, Inc.

ensures timely and error-free delivery through Automatic Repeat Request (ARQ).

The MAC and PHY layers are separated by a logical PHY interface. The Conver-

gence layer performs bridging, classification of traffic into Connections, and data

delivery smoothing functions. The Convergence and MAC layers are separated by a

logical M1 (MAC) interface. The logical H1 (Host) interface exposes the services

provided by HomePlug AV to higher layer entities (HLE).

In relation to the International Standards Organization’s (ISO) Open System

Interconnect (OSI) model, the HomePlug AV specification covers the lower two

layers, namely, the PHY layer and the data link layer.

Figure 2.2 shows the protocol entities defined in the HomePlug specification.

Protocol entities that are directly involved in the transfer of user Payload make up the

Data Plane of the protocol stack, while protocol entities that are involved in creating,

managing, and terminating the flow of data are defined in the Control Plane. The

HomePlug AV specification further divides the Control Plane into a Central

Coordinator component and a Connection Manager (CM) component. In each

AV Logical Network (AVLN), defined in greater detail shortly, one station (STA)

is designated as the CCo. The CCo is responsible for setting up and maintaining the

logical network, managing the communication resource on the wire and coordinating

with neighbor networks (NNs). The control functions associated with the CCo are

treated as part of the CCo component of the Control Plane, while functions

associated with each local station fall within the CM component of the Control Plane.

HPAV PHY

HPAV MAC

Convergence layer (CL)

Higher layer en��es (HLE)

PHY interface

M1 (MAC) interface

H1 (Host) interface

FIGURE 2.1 System block diagram.

PROTOCOL LAYERS 13

2.3 NETWORK ARCHITECTURE

A HomePlug AV Powerline Network consists of a set of HomePlug stations

connected to the AC powerline. From the physical layer perspective, stations in

one dwelling might be able to communicate with stations in another dwelling.

However, HomePlug AV enables stations to be logically separated by a privacy

mechanism based on a 128-bit AES encryption scheme associated with a unique

Network Encryption Key (NEK). An AV Logical Network is the set of STAs,

typically used in a home environment, that possess the same Network Identifier

(NID) and Network Membership Key (NMK). In certain situations, the CCo may

deploy multiple NEKs (possibly using multiple NMKs), thus forming several logical

subnetworks of the AVLN. These are called sub-AVLNs. Coordination, clock

reference, and scheduling are performed on the basis of an AVLN. Cryptographic

isolation is provided at the level of the sub-AVLN.

Each AVLN is managed by a single controlling station, the CCo, introduced

earlier (Figure 2.3). The CCo performs network management functions such as

authentication and association of new stations joining the AVLN, AC line cycle

synchronization of transmission intervals, and admission control and scheduling for

Time Division Multiple Access (TDMA), and Carrier Sense Multiple Access

(CSMA) sessions and allocations.

The authentication of new stations is based on the knowledge of a shared secret,

namely, the NMK. A user can provide the new station with the NMK or use a push-

button approach for enabling it to join the AVLN. Successful authentication will

enable the station to associate with the AVLN. During the association process, the

CCo provides the new station with a Terminal Equipment Identifier (TEI). The TEI is

used to identify a station uniquely within the AVLN. All transmissions in HomePlug

Convergence (CL)

Media Access Control (MAC)

Physical (PHY)

Connection Manager

(CM)

Higher layer entity (HLE)

CentralCoordinator

(CCo)

H1 interface

M1 interface

PHY interface

Control SAP ETH SAPData PlaneControl Plane

FIGURE 2.2 Protocol layer architecture.

14 THE HomePlug AV NETWORK ARCHITECTURE

AV carry the source and destination TEIs for addressing. It should be noted that

although the CCo is used to manage the AVLN as describer earlier, HomePlug AV

stations normally communicate directly with one another without having to go

through the CCo. Communication with the CCo is needed only to manage TDMA

allocations and certain other infrequent control functions. This is in contrast with

popular technologies like Wi-Fi in which all transmissions go through the Access

point in Infrastructure mode.

Figure 2.3 shows the organization of HomePlug AV devices into different classes

of networks. The CCo and the devices in the logical network that can directly

communicate with it form the Central Network (CN). The attenuation and noise

characteristics on the powerline channel may give rise to situations where certain

devices that belong to the same home network may not be able to communicate with

the CCo. A Proxy Network (PN) is instantiated in such scenarios to allow control of

the “hidden devices” through a relay of communications between a Proxy Coordi-

nator (PCo) and a CCo. Direct peer-to-peer communications are still enabled

between devices in a PN and devices in the CN with which the PN is associated.

The PN concept improves coverage by enabling communications for hidden

devices. Due to the robust physical layer used by HomePlug AV, proxy networks

are very rare.

FIGURE 2.3 HomePlug AV network architecture.

NETWORK ARCHITECTURE 15

While PNs always depend on and are associated with a CN, a Neighbor Network

(NN) is an entirely autonomous association of HPAV devices. Neighbor networks are

independent networks that can exist in neighboring homes. The HPAV system

provides for coordination among the neighboring networks so that access to the

medium is shared fairly by the various networks and Quality of Service (QoS) is

preserved for communications within a network.

2.3.1 Station Roles

Each HomePlug AV station is capable of operating as a CCo of the AVLN. The

selection of the station in the AVLN to assume the role of a CCo is typically done in

an automated manner based on the station capabilities and network topology. The

user may also appoint a specific station to act as the CCo. For example, it may be

prudent to assign the Home Gateway/Router with HPAV capabilities as the default

CCo. HomePlug AV defines three different levels of CCo capability:

� Level-0 CCo: A basic CCo that can only support CSMA-based channel access

(i.e., supports CSMA-Only mode).

� Level-1 CCo: A CCo that can support TDMA-based channel access when

there are no neighbor networks (i.e., supports Uncoordinated mode).

� Level-2 CCo: ACCo that can support TDMA-based channel access even in the

presence of neighbor networks (i.e., supports Coordinated mode).

All HomePlug AV stations are required to, at a minimum, support Level-0 CCo

functionality. Furthermore, all HomePlug AV stations are required to be capable of

operating under Level-0, Level-1, and Level-2 CCos.

HomePlug AV stations may also assume the role of a Proxy Coordinator to enable

hidden stations to join the AVLN. Proxy Coordinators are selected by the CCo of the

AVLN.

All other HomePlug AV stations in the AVLN operate as normal stations and rely

on stations that assumed the role of CCo and PCo for managing the AVLN.

2.3.2 Bridging

One or more stations in the AVLN may also act as bridges to other networks. The

bridge is responsible for routing traffic between the AVLN and other networks based

on the list of MAC addresses of devices it is bridging for. The bridge also provides

this list to other stations in the AVLN so that other stations can efficiently deliver

traffic within the AVLN using unicast transmissions.

2.3.3 Channel Access

HomePlug AV is designed to provide high-quality multimedia streaming within the

home network. To enable strict guarantees on QoS (i.e., bandwidth, latency, jitter,

and packet loss probability), the MAC layer is based on a hybrid TDMA and CSMA

16 THE HomePlug AV NETWORK ARCHITECTURE

protocol. The CCo is responsible for coordinating medium access and providing

TDMA allocations based on QoS requirements by periodically generating a Central

Beacon with information on TDMA and CSMA allocations.

2.4 SUMMARY

The HomePlug AV network architecture and protocols introduced in this chapter are

at the core of the HomePlug AV specification and the associated IEEE 1901 standard.

The detailed operation of the various elements in this architecture will be developed

in the rest of the book.

Chapter 3 examines the overall philosophy and reasoning that guided the

technology selection in overcoming the challenges of high-speed multimedia

communications over the powerline channel.

SUMMARY 17