4.5 Wired Solutions - PLC Technology

Power Line Communications comprises all technologies which attempt to transmit data over the power lines. The tests performed assess the technology for home networking, considering specifically IPTV data transmission.

The tests have been performed using both current existing standards: the one from the HomePlug Powerline Alliance (HomePlug) and the one from the Universal Powerline Association (UPA).

For the UPA standard, the tested equipment has been Comtrend devices with DS2 chipsets. For the HomePlug, Intellon chipsets inside Thomson and Devolo devices have been tested.

Performance

Both technologies have reached a 200 Mbps performance at the physical level. This means that actual throughput in a typical scenario could be up to 100 Mbps.

Performance in PLC is always conditioned by:

• Noise

Any domestic appliance (air conditioning systems, blender …) may alter the PLC signal. PLC equipment and signal is prone to be interfered by certain types of noise. Depending on the chipset used (DS2 or Intellon), certain types of noise are more harmful, creating a dependency chipset-noise type. PLC vendors solve this problem by using adaptation.

• Interference

Devices working in surrounding bands may affect PLC signal transmission. For example, interferences with bands used by amateur radio, cordless keyboard/mouse … prevent an optimal transmission. PLC vendors solve this problem by using adjustable notches.

• Electrical installation

There are several factors connected to the home electrical installation which have an influence on the performance:

1. Installation quality: cabling type, shielding, installation age …
2. Distance
3. Intermediate elements: electricity interrupter or electricity meter …
4. External elements: it could be a problem to performance if PLCs would be placed just to an electrical building infrastructure, another PLCs devices sharing the same electrical wiring, etc…

Solving these problems is not easy since it would involve a lot of effort and a high cost, which would discard this technology as an inhome networking solution.

During the lab tests, it has been observed that Intellon chipsets perform slightly better in non optimal scenarios, obtaining a higher throughput.

In optimal conditions, PLC technology can provide up to 4-6 SD MPEG2 channels. However, as shown in the next figure, actual throughput depends on several factors.

Throughput results on different scenarios (UPA)

*Bad. With electricity interrupter and poor quality electrical infrastructure.

**Intermediate. With electricity interrupter and standard quality electrical infrastructure.

***Optimal. Without electricity interrupter and standard quality electrical infrastructure.

Throughput results on different scenarios (HomePlug)

*Bad. With electricity interrupter and poor quality electrical infrastructure.

**Intermediate. With electricity interrupter and standard quality electrical infrastructure.

***Optimal. Without electricity interrupter and standard quality electrical infrastructure.

Quality of Service

Class of Service (CoS)

Class of Service (CoS) is a way of managing traffic in a network by grouping similar types of traffic (for example, e-mail, streaming video, voice, large document file transfer) together and treating each type as a class with its own level of service priority.

CoS woks with 3 bits within a Layer2 Ethernet frame header of the IEEE 802.1Q protocol. These bits specify a priority value of between 0 (signifying best-effort) and 7 (signifying priority real-time data) that can be used by Quality of Service disciplines to differentiate traffic.

Unlike Quality of Service (QoS) traffic management, Class of Service technologies do not guarantee a level of service in terms of bandwidth and delivery time; they offer a "best-effort." On the other hand, CoS technology is simpler to manage and more scalable as a network grows in structure and traffic volume.

One can think of CoS as "coarsely-grained" traffic control and QoS as "finely-grained" traffic control.

Type of Service (ToS)

The Type of Service is used to indicate the quality of the service desired. The type of service is an abstract or generalized set of parameters which characterize the service choices provided in the networks that make up the internet. This type of service indication is to be used by gateways to select the actual transmission parameters for a particular network, the network to be used for the next hop, or the next gateway when routing an internet datagram.

The TOS facility is one of the features of the Type of Service octet in the IP datagram header. The Type of Service octet consists of three fields:

Type of Service (ToS)

The first field, labeled "PRECEDENCE" above, is intended to denote the importance or priority of the datagram. The second field, labeled "TOS" above, denotes how the network should make tradeoffs between throughput, delay, reliability, and cost. The last field, labeled "MBZ" (for "must be zero") above, is currently unused.

The originator of a datagram sets this field to zero Routers and recipients of datagrams ignore the value of this field. This field is copied on fragmentation. The TOS field as a four bit field defined as following values expressed as binary numbers):

1000 -- minimize delay

0100 -- maximize throughput

0010 -- maximize reliability

0001 -- minimize monetary cost

0000 -- normal service

The TOS field value 0000 is referred to as the "default TOS". Although the semantics of values other than the five listed above are not expressly defined, they are perfectly legal TOS values, and hosts and routers must not preclude their use in any way.

The equipments will accommodate this TOS value according the service Level need into the home network.

Layer 4 QoS

Layer 4 QoS is a method of QoS classification that allows data streams to be classified based upon IP-Address and Port number. However, it does bring challenges in the real world in implementing this kind of classification reliably.

Security

Hardware-supported encryption is a very important feature, especially when using AES 128-bit or 256-bit. This feature is supported by DS2 chipsets while Intellon chipsets perform software encryption. This results in performance lowering when a robust encryption is used in Intellon chipsets.

For splitting networks belonging to different homes, a network ID and a unique key are used for data transmission. This way, any neighbor node which is able to physically detect another network won’t be able to decrypt data since the key is only known by the devices which the network comprises.

Management

There is considerable difference between features offered by DS2 and Intellon chipset for device management. The next figure shows main options available in both chipset equipments.

Management options available in PLC equipment

Advanced Features

DS2 offers a set of advanced functionalities which are not available in Intellon chipset equipment:

• Advanced tools which allow to collect data about the physical layer and to change settings in the device to improve the device performance. Among these tools, we have:

1. SNR Viewer. It allows to graphically controlling physical layer behavior when traffic is being sent between PLC devices.
2. Notcher. It allows enabling spectral notches in the desired frequencies which may be interfering with other devices (amateur radio, cordless keyboards…)

• It is also possible to develop applications at operator’s request. These applications specifically developed will allow for more flexibility during the deployment stage, for example, helping the installer when performing the installation at the customer premises. Portugal Telecom has developed a tool for the installers where a small piece of software tells them if electrical installation is suitable or not for PLC deployment. It just shows a red or green light.
• Another available feature is Automatic Multicast, which allows the automatic configuration of the multicast for the inhome network. IGMP snooping is configured at the same time that PLC receivers search automatically the emitter PLC device. This allows automatic provisioning during deployment stage or when substituting damaged devices.
• Smart Routing functionality optimizes throughput between devices since it chooses the best physical path based on the link quality. It is also possible to use a PLC device as a repeater in scenarios where interference or distance makes it impossible that two devices communicate directly in the network.
• Automatic Power Saving Mode switches the device off when no traffic is being sent through the devices. Once the device detects link activity, it switches on automatically. Now, new products based Intellon chipset are arising with that functionality.
• New DS2 PLC devices are fully backwards compatible with existing ones, allowing joint use, and being managed with the same tools.
• Devices include an embedded filter in the chassis, whose main advantages are:

1. There is no loss of an electric socket, indirectly avoiding the use of a multiple outlet for the connection of all nearby devices. This reduces noise caused by power supplies.
2. The use of the socket included in the PLC device for connecting the associated STB assures that the embedded filter eliminates any possible existing noise.

Roadmap

DS2 roadmap plans to launch low cost chipsets (100 Mbps) by the end of 2008. During 2009, launching of the 400 Mbps chipset is planned, which will be an evolution of the existing 200 Mbps Aitana chipset.

DS2 Roadmap

Also, Intellon roadmap plans to launch 400 Mbps chipset during 2009.

All planned DS2 chipsets to be launched will be backwards compatible with existing ones.

Backward Comapibility

Actually, Intellon chipsets are not backward compatibility.

In year 2009, DS2 will offer a functionality which will allow PLC devices the automatic management of transmitted power. This will prevent devices in neighboring networks from interfere among them. Each network will transmit with a certain power so that not to flood the neighbor physical layer, improving medium sharing and obtaining a higher performance.