4.2 Wireless solutions - WiFi

WiFi

Traditional WiFi based technologies have difficulties delivering multimedia traffic due to range limitations, unpredictable performance, inadequate quality of service (QoS), and for IPTV in particular, the gratuitous handling of multicast traffic.

As an alternative to standard WiFi solutions, I have tested Ruckus Wireless multimedia equipment. This has been the chosen option according to actual performance and new features (QoS and multicast managing, directional antenna system) of the devices and its current deployment in some IPTV operators both in Europe and America.

Ruckus Wireless covers the next Wireless bands; 802.11a/b/g/n. Each one of them has been tested like IPTV inhome networking solution. The test and results are explained below.

Performance

Performance is mainly conditioned by two factors:

• Technology limitations

WiFi solutions are not ready for a multicast environment. Packets are not retransmitted since link layer ACKs are not supported. Ruckus solves this problem by using a proprietary technology (patent pending) where ACKs which are needed for retransmission of lost packet at link level are sent. By doing this, anytime a problem at physical layer stops the transmission, the packets will be resent. This functionality is called SmartCast and has been developed by Ruckus, achieving good performance when using the WiFi transmission.

Ruckus equipment may reach a throughput up to 20 Mbps in 802.11a/b/g and up to 60 Mbps in 802.11n when used in optimal scenarios. This is equivalent to eight MPEG4 SD (2,5 Mbps) channels or one MPEG4 HD channel (8-10 Mbps) plus four MPEG4 SD channels in 802.11a/b/g and overhead of twenty MPEG4 SD channels or 6 MPEG4 HD channels.

• Environment limitations

The widespread use of 802.11b/g networks is flooding the WiFi radio band. In terms of data transmission (Internet traffic), the problem is affordable, but in video transmission, this is a tough problem.

There are two approaches in order to solve this issue, a short term one and a mid term one. The first one would involve using 802.11a since the 5 GHz band is much less used and comprises more available channels to be used (8 channels vs. 3 channels in 802.11b/g). The second approach will use 802.11n, which will unite the advantages of both 802.11b/g and 802.11a, resulting in better performance and less interference. Using 802.11n (once the standardization is finished) will allow to automatically regulate the transmission power and antenna direction (MIMO) in order to obtain a better coverage range and reduce neighboring interference.

Some problems have been found in some functionalities regarding channel selection. Both automatic channel selection (when the device is started) and CCS (Continuous Channel Selection) are not working as expected. This issue prevents a performance improvement.

Both in 802.11b/g and 802.11a, physical data rate is 54 Mbps; in 802.11n the physical rate is 200Mbps actually. However, actual throughput is scenario dependent. The next figure shows the results obtained from performance tests over different scenarios.

Throughput results on different scenarios (802.11a/b/g)

*Bad. High level of interference

**Intermediate. Mid-low level of interference

***Optimal. No interference

Throughput results on different scenarios (802.11n)

*Bad. High level of interference

**Intermediate. Mid-low level of interference

***Optimal. No interference

¹Only optimal scenarios have been tested with 802.11a since this is considered to be a “clean” band. If interference happens on the band, the performance would be as with 802.11b/g.

²Only optimal scenarios have been tested with 802.11n since this is considered to be a “clean” band. If interference happens on the band, the performance will decrease.

It is possible to limit bandwidth in each WLAN (Rate Limiting) defining these limits as Uplink and Downlink. This allows small improvements in certain scenarios where interference is a problem due to saturated radio band. It is not a solution but it may rather be considered as a temporary fine tuning.

Quality of Service

The rapid expansion of wireless networks is making it possible for new services and applications with digital equipments (PDAs, PCs, STBx, etc) to access information anywhere and at any time. The number of wireless terminals with 3P applications is growing rapidly, and therefore, it is important that the quality model in wireless networks be consistent with requirements.

As next generation for IPTV services will include a price tag for premium services, each application will deal with the proper level agreement. The service contract will fix the Quality of Service (QoS) expected by the application and the price the user is willing to pay for the service.

Four QoS modules are available in this technology: CoS, ToS, Layer4 and heuristic. The devices have the option to respect incoming QoS marking, transmitting packets through the Ruckus to the receiver with no change. This is useful if packets get the CPE with QoS preclassification.

Next section will describe different Wifi QoS technologies

QoS Preclasification inside Wifi

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.

Heuristic QoS

This QoS classification method allows systems to inspect the size of the packet crossing the equipment and they would be redirect to the proper queue to process the packet. This method is based in packet inspection.

It is recommended to use CoS or ToS preclassification (packets are already marked properly when get the Ruckus) in an IPTV. It is also recommended to use a specific home network just for video transmission. Both recommendations will allow that all the traffic marked as video by the service provider is retransmitted with the highest priority within the home network.

In many cases, the most common practice would be prioritizing video traffic once it reaches the home network by applying advanced classification based on protocol type, origin/destination port or even packet size (heuristic). This is a not reliable way of prioritizing since there is a lot of IPTV traffic which is not UDP or which does not use the typical ports for TV reception. For example, communication traffic between the STB and the middleware for control/management, which may be standard web traffic (TCP).

Security

As main security features, Ruckus devices implement:

• Traffic transmission using WPA2.
• Two different networks and SSIDs, one for data and one for the IPTV service.

Management

Management protocols allowed are:

• Device access: HTTP, HTTPS, Telnet, SSH.
• O&M: TR-069, SNMP, Automatic & Manual Firmware Upgrade and Syslog.

An interesting implemented feature is the option to manage the any Adapter device from the Access Point with no need to access directly into the adapters.

A desirable feature would be an independent network for management where management traffic is actually is prioritized, which involves guaranteeing a high priority small bandwidth since this traffic amounts small traffic transmission.

Another desirable feature is the option for an automatic configuration of QoS parameters in every device in the home network once it is changed, for example, on the access point.

Advanced Features

The device is able to provide information about the WiFi band where the devices are transmitting. This feature, combined with an event management system will allow the O&M module to collect all the necessary data in order to know if the physical level is optimal for video transmission.

Roadmap

The most expected feature will be the fully standardization of 802.11n. This will permit to know if WiFi may become a mature enough technology to support massive IPTV deployments. The standard is currently approved in draft 4.0, but a full standardization is expected in end of 2009.