Dale’s Web Log

While the Federal Communications Commission moves ahead with planning for the upcoming 700MHz spectrum auction, the White Space Coalition has submitted a second prototype white space wireless broadband device to the FCC for testing. White space devices could use the so-called white space in the current analog television spectrum (2MHz to 698MHz) to deliver wireless broadband service. Former FCC chief engineer Edmond Thomas (and current technology policy advisor for the law firm of Harris, Wiltshire & Grannis, which is representing the Coalition) told Ars that he believes white space broadband could deliver download speeds of up to 80Mbps, which would make it extremely competitive with fiber-to-the-premises solutions like Verizon’s FiOS networks.

The newest white space prototype is manufactured by Philips Electronics of North America and consists of a TV tuner, a digital processing board, and a PC which provides the UI, control, and signal processing. It’s proof-of-concept hardware intended to demonstrate that it’s possible to sense the presence of TV signals and transmit wireless IP data in a way that does not interfere with TV. According to an FCC filing seen by Ars Technica, the new prototype is capable of picking up analog and digital television signals as well as wireless microphone signals (which operate in the same part of the spectrum). It works similarly to the Microsoft-manufactured spectrum sensing device submitted earlier this year. Microsoft also submitted a transmission device to the FCC for testing which will be used to show that white space broadband transmissions won’t interfere with TV signals.

There are a few screenshots in the FCC submission. It’s quite simple: the user selects a type of signal to scan for, and the application shows the results. If the sensing module picks up television transmission on a particular channel, then that part of the spectrum will not be used for white spaces broadband in that particular area.

The goal of the White Space Coalition is simple: take advantage of unused television spectrum to provide wireless broadband. Although analog television transmissions will cease in February 2009, digital TV signals will continue to use the spectrum between 54MHz and 698MHz. That is a highly desirable chunk of spectrum because the signals can easily pass through walls and other solid objects, giving them a much greater reach than WiFi or even WiMAX, both of which operate in higher frequency bands.

Television broadcasters have vigorously opposed the usage of the white spaces, citing fears that wireless broadband will interfere with TV signals. The current round of FCC testing is designed to ensure that the prototype white space broadband devices don’t cause any interference problems at all. “Like the personal/portable prototype devices previously submitted by Microsoft on the Coalition’s behalf, the Philips prototype is designed to demonstrate that operating parameters set forth by the Coalition… will provide incumbent licensees in the television bands with the interference protection to which they are entitled,” reads the FCC filing.

The White Space Coalition is comprised of Dell, EarthLink, Google, HP, Intel, Microsoft, and Philips Electronics. The FCC should conclude its testing of the white space broadband prototypes in July and the first rules governing the use of the spectrum by wireless broadband devices should be released in October 2007. Once that happens, the IEEE will likely begin the work of standardizing the tech. If all goes as planned, white space broadband service could begin in the US as soon as February 2009.

Cisco Systems Inc. is now looking at the prospects of several of its networking divisions introducing WiMax products, according to industry sources, reversing a sometimes combative stance on the emerging wireless technology from the company.

“The wireless, cable, and Linksys groups are all looking at WiMax,” a source tells Unstrung. “These have different motivations and different products.”

Cisco will face the eternal conundrum that it always faces when entering a new wireless market, the source adds: “The question will be, do these internal groups do the work… or who will they buy?”

Another source, however, says at least some of the development is internal and the company is already working on WiMax. This could result in additional WiMax capabilities for its municipal networking offerings.

In the past, WiMAX Forum member Cisco has pooh-poohed the technology’s chances of success as a wide-area wireless access technology. Most notably, when CTO Charlie Giancarlo said the business case for WiMax was “not compelling” in November 2004. The firm has softened its stance a little since then but still has a white paper on its site explaining why it won’t build WiMax base stations.

The company’s official stance on WiMax is still fairly muted. “Cisco always looks at different wireless technologies,” allows Ben Gibson, director of mobility solutions marketing at the firm, but he adds: “WiMax is certainly not nearly as far along in the market as wireless LAN.”

Cisco, however, would by no means be the only major networking company to change its position on WiMax as the market evolves. Just recently, Qualcomm Inc. bought into mobile WiMax, while Ericsson AB decided to get out of the market and concentrate on cellular 4G updates.

Certainly there is more support for WiMax from mobile operators and other service providers now than there was in 2004 or 2005 — when Cisco first got sniffy on WiMax. In the U.S. alone, Clearwire LLC and Sprint Nextel Corp. are working on multi-billion dollar WiMax rollouts through 2008 and beyond.

— Dan Jones, Site Editor, Unstrung

Sprint wants to shrink WiMax base stations even further so that they can be used to enhance data transfer speed and capacity in the home.

Sprint has already revealed some of its plans to ensure decent WiMax coverage inside larger buildings with picocell WiMax radios for campuses, offices, shopping malls, and conference centers with picocells. The Reston, Va., operator also has an RFP out for what it calls “Low Cost Internet Base Stations” but most others in the industry call home base stations or “femtocells.” (

A home base station, or femtocell, is a low-cost, low-power 3G cellular radio system that users can put in their dwellings to boost bandwidth and coverage and enable new applications such as fixed/mobile convergence (FMC) in the home. Such mini-base stations have become more interesting to vendors as operators have started to take the devices more seriously, both as an alternative to WiFi hotspots or as a complementary technology.

Sprint has already made it clear that it anticipates that in-building coverage will be an important aspect of its WiMax rollout. “Femtocells are on our radar,” says a spokesman for the operator.

The spokesperson, however, wouldn’t be drawn on any specifics about particular RFPs. “As a matter of policy we don’t comment on RFPs,” he says.

Sprint is said to be looking for several hundred thousand of these mini-base stations along with smaller — but still significant — numbers of picocell-scale equipment for its WiMax deployment, which is at a testing phase right now and due to go live in many major cities in the U.S. in 2008. CDMA and EV-DO support could also be part of the specifications for these appliances.

The problem for vendors wishing to compete will be pulling together these disparate networking technologies. Indoor wireless specialist RadioFrame Networks Inc. has been upfront in its plans to bring a WiMax home base station to market. The company has already worked with Nextel on indoor systems and must be considered a strong contender for any new contracts.

AirWalk Communications Inc. , Airvana Inc. , and Samsung Corp. are all in the running since the three vendors are established in the CDMA business. Samsung also has the WiMax chops and experience, especially now that it has been involved in early deployments in South Korea.

Silicon could be key in enabling a multi-radio mini-base station. Qualcomm Inc. has already started to express an interest in developing chips for this type of application. Major players such as Texas Instruments Inc., as well as smaller companies like PicoChip Designs Ltd. , are already working on dedicated femtocell chipsets. Meanwhile, RadioFrame is extending its own OmniFrame silicon to support WiMax.

Insider analyst Brown, author of the recent “3G Home Base Stations: Femto Cells & FMC for the Masses” report, says that no matter what happens with Sprint’s WiMax-related work, femtocells are one of the new hot technologies in the world of wireless.

“A major operator will roll out a femtocell deployment this year,” says Brown. He predicts that either a CDMA operator in the U.S. or a European operator that has a serious GSM footprint but few 3G networks will be first.

WiMAX (802.16) Service Specific Convergence Sub-layer (CS)

The Service Specific Convergence Sub-layer is at the top of the 802.16 protocol stack. It is responsible for performing the following functions:

• Accepting higher layer PDU’s from the higher layer service
• Classifying and processing the higher layer PDU’s
• Delivering the PDU’s received from the higher layer to the appropriate MAC SAP
• Receiving CS PDU’s from the peer and delivering them to the higher layer

At the present time, there are only two convergence sub-layer definitions for 802.16; the ATM CS and the Packet CS. Thus, only these types of traffic can be transported over the 802.16 network. The standard does leave open the possibility of specifying additional CS’s in the future.

WiMAX (802.16) Packet Convergence Sub-layer (CS)

The first Convergence Sub-layer (CS) I will discuss is the Packet CS. While the packet CS is generic in that it supports any number of packet protocols including IP, PPP, and Ethernet, the most common implementation will likely be for IP networks. In addition to the generic functions above, the packet CS also has the “optional” responsibility for suppressing and rebuilding the packet header information to save bandwidth over the wireless link.

So, how exactly is a protocol data unit (PDU) mapped into the wireless network? Well, essentially the entire packet contents are wrapped into a MAC “Service Delivery Unit” (SDU) – i.e. the packet is encapsulated with the MAC SDU header information (see figure below). Essentially, this is an additional field added to the packet identifying the Packet Header Suppression Identifier (PHSI) if used.

Once the packet has been encapsulated into an SDU, the SDU must be classified and associated with a connection identifier (CID) for transmission to the appropriate peer node. Each CID is known as a “service flow”. The process of classification involves matching each packet against a set of protocol specific matching criteria (e.g. ip address, priority, QoS, etc.) to determine the appropriate CID, or service flow, for that packet. If a packet fails to match any of the classifiers defined for the system, that packet is discarded.

Payload Header Suppression

As we all know, there is a lot of repetitive data in the packet headers on an IP network (Source destination addresses, port numbers, version numbers, etc.). There have even been a fair number of efforts to implement compression schemes based on that repetitiveness (Van Jacobsen compression being the most popular). The folks on the 802.16 committee decided it would be a good idea to build a header compression/suppression scheme into the specification. This way, they would ensure that all WiMAX devices that supported Header Suppression would be interoperable.

The 802.16 Header Suppression technique works by having the MAC SDU of the sending node suppress the header information while the receiving node restores it. Simple enough right? Well not so fast, the sending and receiving nodes need to have a method of sharing connection information such that the suppressed header information is known/recoverable by both nodes. That’s where the Payload header Suppression Index (PHSI) and the Payload Header Suppression Field (PHSF) come into play.

As I described above, the transmitting node uses a classification process to assign packets into specific service flows. For systems that support Payload Header Suppression (PHS), the classifiers will include a PHS rule. On the receiving side, the node uses the CID and PHSI to restore the suppressed header.

Included within the PHS function are two options known as Payload Header Suppression Valid (PHSV) and Payload Header Suppression Mask (PHSM). When the PHSV option is enabled, the transmitting node must verify the payload header before suppressing it. With PHSM, the nodes negotiate which header bytes to suppress and which to transmit. For example, the static header fields (IP address, version, etc.) can easily be suppressed, while the more dynamic fields (ACK number, length, etc.) can continue to be transmitted.

Payload Header Suppression Protocol

• A packet to be transmitted over the 802.16 network is received on the wired interface of the transmitting node and forwarded to the Convergence Sublayer (CS).
• The CS classifies the packet and provides as an output an uplink service flow, a CID, and a PHS rule. The PHS rule identifies the PHSF, PHSI, PHSM, PHSS, and PHSV to be used for this packet.
• If the PHSV value is set, or is not provided as a classification output, the subscriber terminal will compare the bytes in the packet header with the bytes in the PHSF that are slated to be suppressed. If they match, the subscriber will suppress the bytes in the PHSF with the exception of those bytes masked by the PHSM. If they do not match, the transmitting node sets the PHSI to 0 and the header is not suppressed.
• The transmitting node appends the PHSI to the packet and transmits it to the receiving node.
• At the receiving node, the packet received over the air interface is forwarded to the service flow associated with the received CID.
• The convergence sublayer on the receiving node uses the CID and the PHSI to recover the PHSF, PHSM, and PHSS.
• The packet is then reconstructed with the original packet header fields and forwarded normally.
• If PHSV was enabled on the transmitter, then the PHSF bytes recovered by the receiver are guaranteed to match what was transmitted.
• If PHSV was not enabled on the transmitter, then there is no guarantee that the PHSF bytes recovered by the receiver will match what was transmitted.

The figure below is an excerpt from the 802.16 specification and depicts the decision logic involved in PHS.



I hope this is useful. I know it’s helping me learn the protocol. Next I will start into the specific packet CS’s covered by the specification (Ethernet, PPP, and IP).

This is the first in a multi-part series of posts I am working on. I am attempting to familiarize myself with the 802.16 protocol standards and the WiMAX forum efforts. I find that the best way to learn stuff is to write it down. I hope this helps others out there, and by all means, if you see and error, please comment and I will correct it.

Introduction

The IEEE 802.16 working group is the standards body developing the specifications for the next generation of broadband wireless networks – both fixed and mobile. The WiMAX (Worldwide Interoperability for Microwave Access) forum is an interoperability initiative that will certify that broadband wireless radios manufactured by various vendors comply with the IEEE 802.16 specification and – more importantly – are interoperable from vendor to vendor through testing.

WiMAX Range and Throughput

The throughput and range of a WiMAX link is very dependent upon a number of factors including; transmit power, antenna gain, directionality, modulation scheme, forward error correction codes, the terrain, density, height of tree cover, presence of hills and valleys, bodies of water, etc. (nothing new here, these are all the typical issues that need to be dealt with in wireless communications). Despite this, it is not uncommon to see statements in the media describing WiMAX multipoint coverage as being capable of extending coverage up to 30 miles from a base station or tower location. In certain (very specific) cases this is true. However, typical operating ranges fall into the 8-10 mile range for line of site (LOS) installations and 4 – 5 miles for non or near-line of sight (NLOS) installations.

Realized throughput is dependent upon many of the same factors that address the achievable link distance. WiMAX supports a number of different modulation schemes and coding rates. Also, it is important to remember though that WiMAX is a shared network service, meaning that this bandwidth will be shared amongst all of the users within a given WiMAX cell – much like cable modem users share their Internet bandwidth with other subscribers in the neighborhood.

802.16 Reference Model

802.16 is essentially a wireless implementation of Layers 1 and 2 of the OSI model (Physical Layer and Link Layer). The Physical layer is referred to as the PHY. The link Layer is referred to as the MAC (Media Access Control) layer. The MAC is comprised of 3 sub-layers; the Service Specific Convergence Sub-layer (CS), the Common Part Sub-layer (CPS), and the Security Sub-layer (SS).

The Service Specific Convergence Sub-layer provides the interface between the WiMAX network and the higher level protocols. Data received from the higher layer protocols is mapped into the appropriate WiMAX identifiers for transmission over the network (for example, Ethernet Addresses are converted to WiMAX station identifiers). Currently there are only two CS interfaces defined; one for ATM traffic, and one for packet (IP) traffic.

The MAC CPS performs the bulk of the MAC layer processing. It is responsible for managing system access, bandwidth allocation, and connection management.

The MAC Security Sub-layer provides authentication, secure key exchange, and payload encryption.

The WiMAX Physical Layer (PHY) actually consists of multiple sub-specifications, each one dependent upon the application and frequency spectrum to be utilized. There are three basic frequency bands to be concerned with:

• 10 GHz to 66 GHz Licensed Frequencies
• Licensed Frequencies below 11 GHz
• Unlicensed frequencies below 11 GHz (primarily 5 to 6 GHz)

Coming next….a look at the Convergence Sublayer of the MAC.