Legacy QoS Mechanisms

The following paragraphs describe legacy mechanisms.

FDD/TDD/OFDM

WiMAX incorporates a number of time-proven mechanisms to ensure good QoS. Most notable are TDD, FDD, FEC, FFT, and OFDM. The WiMAX standard provides flexibility in spectrum usage by supporting both FDD and TDD. Thus, it can operate in both FDD/OFDM and TDD/OFDM modes. It supports two types of FDD: continuous FDD and burst FDD.

In continuous FDD, the upstream and downstream channels are located on separate frequencies, and all CPE stations can transmit and receive simultaneously. The downstream channel is always on, and all stations are always listening to it. Traffic is sent on this channel in a broadcast manner using TDM. The upstream channel is shared using TDMA, and the BS is responsible for allocating bandwidth to the stations.

In burst FDD, the upstream and downstream channels are located on separate frequencies. In contrast to continuous FDD, not all stations can transmit and receive simultaneously. Those that can transmit and receive simultaneously are referred to as full-duplex capable stations while those that cannot are referred to as half-duplex capable stations.

A TDD frame has a fixed duration and contains one downstream subframe and one upstream subframe. The two subframes are separated by a guard time called transition gap (TG), and the bandwidth that is allocated to each subframe is adaptive. The TDD subframe is illustrated in Figure 1.

(Source: IEEE)

Figure 1: TDD subframe

Within a TDD downlink subframe, transmissions coming from the BS are organized into different modulation and FEC groups. The subframe header, called the FCH, consists of a preamble field, a PHY control field, and a MAC control field. The PHY control field is used for physical information, such as the slot boundaries, destined for all stations. It contains a map that defines where the physical slots for the different modulation/FEC groups begin.

The groups are listed in ascending modulation order, with QPSK first, followed by 16-QAM and then 64-QAM. Each CPE station receives the entire DL frame, decodes the subframe, and looks for MAC headers indicating data for the station. The DL data is always FEC coded. Payload data is encrypted, but message headers are unencrypted. The MAC control is used for MAC messages destined for multiple stations.

This variation uses burst single-carrier modulation with adaptive burst profiling in which transmission parameters, including the modulation and coding schemes, may be adjusted individually to each SS on a frame-by-frame basis. Channel bandwidths of 20 or 25 MHz (typical United States allocation) or 28 MHz (typical European allocation) are specified. Randomization is performed for spectral shaping and to ensure bit transitions for clock recovery.

Forward Error Correction (FEC)

WiMAX utilizes FEC, a technique that doesn't require the transmitter to retransmit any information that a receiver uses for correcting errors incurred in transmission over a communication channel. The transmitter usually uses a common algorithm and embeds sufficient redundant information in the data block to allow the receiver to correct. Without FEC, error correction would require the retransmission of whole blocks or frames of data, resulting in added latency and a subsequent decline in QoS.

Need QoS? Throw more bandwidth at it! Throughput and latency are two essentials for network performance. Taken together, these elements define the "speed" of a network. Whereas throughput is the quantity of data that can pass from source to destination in a specific time, round-trip latency is the time it takes for a single data transaction to occur (the time between requesting data and receiving it). Latency can also be thought of as the time it takes from data send-off on one end to data retrieval on the other (from one user to the other). Therefore, the better throughput (bandwidth) management, the better the QoS

Service Flow - WiMAX

Service Flow

Minimizing customer intervention and truck roll is very important for WiMAX deployments. The Provisioned Service Flow Table, Service Class Table, and Classifier Rule Table are configured to support self-installation and auto-configuration. When customers subscribe to the service, they tell the service provider the service flow information including the number of UL/DL connections with the data rates and QoS parameters, along with the types of applications (for example, Internet, voice, or video) the customer intends to run. The service provider preprovisions the services by entering the service flow information into the service flow database. When the SS enters the BS by completing the network entry and authentication procedure, the BS downloads the service flow information from the service flow database. Figure 1 provides an example of how the service flow information is populated. It indicate that two SSs, identified by MAC address 0x123ab54 and 0x45fead1, have been preprovisioned. Each SS has two service flows, identified by sfIndex, with the associated QoS parameters that are identified by qosIndex 1 and 2, respectively. qosIndex points to a QoS entry in the wmanIfBsServiceClassTable that contains three levels of QoS: Gold, Silver, and Bronze. sfIndex points to the entry in the wmanBsClassifierRuleTable and indicates which rules shall be used to classify packets on the given service flow.

(Source: Intel)

Figure 1: Service flow provisioning

When the SS with MAC address 0x123ab54 registers into the BS, the BS creates an entry in the wmanIfBaseRegisteredTable. Based on the MAC address, the BS will be able to find the service flow information that has been preprovisioned. The BS will use a dynamic service activate (DSA) message to create service flows for sfIndex 100001 and 100002, with the preprovisioned service flow information. This can be seen in Figure 1. It creates two entries in wmanIfCmnCpsServiceFlowTable. The service flows will then be available for the customer to send data traffic

How WiMAX Works

Like most data communications, WiMAX relies on a process consisting of a session setup and authentication. The RLC manages and monitors the quality of the service flow. With WiMAX, this process is a series of exchanges (DLs and ULs) between the BS and SS. A complex process determines what FDD and TDD settings will be used for the service flow, FEC, sets encryption, bandwidth requests, burst profiles, and so on. The process starts with channel acquisition by the newly installed SS.

Channel Acquisition

The MAC protocol includes an initialization procedure designed to eliminate the need for manual configuration. In other words, the subscriber takes the SS out of the box, plugs in power and Ethernet, and connects almost immediately to the network. The following paragraphs describe how that is possible without laborious user setup or service provider truck roll.

Upon installation, the SS begins scanning its frequency list to find an operating channel. It may be preconfigured by the service provider to register with a specified BS. This feature is useful in dense deployments where the SS might hear a secondary BS due to spurious signals or when the SS picks up a sidelobe of a nearby BS antenna. Moreover, this feature will help service providers avoid expensive installations and subsequent truck rolls.

After selecting a channel or channel pair, the SS synchronizes to the DL transmission from the BS by detecting the periodic frame preambles. Once the PHY is synchronized, the SS will look for the periodically broadcasted DCD and UCD messages that enable the SS to determine the modulation and FEC schemes used on the BS's carrier.

Initial Ranging and Negotiation of SS Capabilities

Once the parameters for initial ranging transmissions are established, the SS will scan the UL-MAP messages present in every frame for ranging information. The SS uses a backoff algorithm to determine which initial ranging slot it will use to send a ranging request (RNG-REQ) message. The SS will then send its burst using the minimum power setting and will repeat with increasingly higher transmission power until it receives a ranging response.

Based on the arrival time of the initial RNG-REQ and the measured power of the signal, the BS adjusts the timing advance and power to the SS with the ranging response (RNG-RSP). The response provides the SS with the basic and primary management CIDs. Once the timing advance of the SS transmissions has been correctly determined, the ranging procedure for fine-tuning the power is done via a series of invited transmissions.

WiMAX transmissions are made using the most robust burst profile. To save bandwidth, the SS next reports its PHY capabilities, including which modulation and coding schemes it supports and whether, in an FDD system, it is half-duplex or full-duplex. The BS, in its response, can deny the use of any capability reported by the SS. See Figure 1 for an illustration of this process.

Figure 1: Channel acquisition process between an SS and BS

It should be noted here how complex this setup procedure is. The purpose thus far is to ensure a high quality connection between the SS and the BS.

SS Authentication and Registration

Wi-Fi has been dogged with a reputation for lax security. Perhaps the best "horror story" deals with a computer retailer who installed a wireless LAN. A customer purchased a Wi-Fi equipped laptop and, anxious to enjoy it, powered it up in the parking lot of the retailer. The new laptop owner was immediately able to tap into the retailer's Wi-Fi network and was able to capture some customer credit card information. Fortunately, the new laptop owner was a journalist, not a con artist. The story, much to the chagrin of the national retailer and the Wi-Fi industry, made the national news. The Wi-Fi industry has had to work hard to shake the reputation of having loose security measures. A similar story will not easily, if ever, occur with WiMAX.

Each SS contains both a manufacturer-issued factory-installed X.509 digital certificate and the certificate of the manufacturer. The SS in the Authorization Request and Authentication Information messages sends these certificates, which set up the link between the 48-bit MAC address of the SS and its public RSA key, to the BS. The network is able to verify the identity of the SS by checking the certificates and can subsequently check the level of authorization of the SS. If the SS is authorized to join the network, the BS will respond to its request with an authorization reply containing an authorization key (AK) encrypted with the SS's public key and used to secure further transactions.

Upon successful authorization, the SS will register with the network. This will establish the secondary management connection of the SS and determine capabilities related to connection setup and MAC operation. The version of IP used on the secondary management connection is also determined during registration.

IP Connectivity

After registration, the SS attains an IP address via DHCP and establishes the time of day via the Internet Time Protocol. The DHCP server also provides the address of the TFTP server from which the SS can request a configuration file. This file provides a standard interface for providing vendor-specific configuration information. Se Figure 2 for an illustration of this process.

Figure 2: SS authentication and registration

Connection Setup

Now comes the connection setup, where data (the content) actually flows. WiMAX uses the concept of service flows to define one-way transport of packets on either the DL or the UL. Service flows are characterized by a set of QoS parameters, such as those for latency and jitter. To most efficiently utilize network resources, such as bandwidth and memory, WiMAX adopts a two-phase activation model in which resources assigned to a particular admitted service flow may not be actually committed until the service flow is activated. Each admitted or active service flow is mapped to a MAC connection with a unique CID. In general, service flows in WiMAX are preprovisioned, and the BS initiates the setup of the service flows during SS initialization.

In addition, the BS or the SS can dynamically establish service flows. The SS typically initiates service flows only if there is a dynamically signaled connection, such as a switched virtual connection (SVC) from an ATM network. The establishment of service flows is performed via a three-way handshaking protocol in which the request for service flow establishment is responded to and the response acknowledged.

In addition to supporting dynamic service establishment, WiMAX supports dynamic service changes in which service flow parameters are renegotiated. These service flow changes follow a three-way handshaking protocol similar to the one dynamic service flow establishment uses.