DH

Figure 4-2. Physical Wireless USB Connection Topology

Figure 4-2 illustrates an example physical topology enabled by Wireless USB. The host has a radio range of about 10 meters. Devices within the host’s range can establish a secure relationship with the host and become part of the host’s Wireless USB Cluster. All communication flows between the host and devices are point-to- point which means the physical topology of Wireless USB is a 1:1 match with the defined logical communications topology familiar to USB architecture. Likewise the client software-to-function relationship remains unchanged (see Section 5.2 in the Universal Serial Bus Specification, Revision 2.0).

Wireless USB also defines a specific class of device called the Wire Adapter (see Chapter 8) that bridges between a Wireless USB bus and a USB 2.0 bus. The effect on the communications topology is essentially a cascading of USB busses.

4.3Wireless USB Communication Flows

Wireless USB retains the familiar concepts and mechanisms and support for Endpoints, Pipes, and Transfer types, please refer to the USB Specification, Revision 2.0 for details. This section describes additions required to support Wireless USB. Subsequent sections cover co-existence, host and device requirements and an overview of the methods employed to manage data communications over a Wireless USB Channel.

A channel is a transmission path between nodes. The wireless physical layer (i.e. PHY) formats radio transmissions in a frequency range, via encoding and other techniques into a channel (or set of channels) through which basic bit streams are transmitted and received. A data link layer (on top of the PHY layer) encodes/decodes bit streams into/out of data packets, furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer includes the MAC (Media Access Control) and LLC (Logical Link Control) for managing information over the physical channel. Application layers utilize the low-level channel services provided by the MAC and PHY.

Wireless USB utilizes the MAC Layer and PHY, which define several access methods for accessing the MAC Layer channel, including Beacons (for discovery and some distributed control) and Distributed Reservation Protocol (DRP - for TDMA type data communications). MAC Layer channel time is organized into superframes as illustrated in Figure 4-3. Super-frames begin with a Beacon Period (BP) and are 65 milliseconds in duration. Super-frames are logically segmented into 256 Media Access Slots (MAS – each 256μs). The MASs at the beginning of a super-frame are allocated for use by the Beacon Period.

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Figure 4-3. Basic Layout of Channel Time organization for a MAC Layer

Wireless USB defines a Wireless USB Channel which is encapsulated within a set of MAC Layer super frames via a set of MAC Layer MAS reservations (DRPs). The Wireless USB Channel is a continuous sequence of linked application-specific control packets, called MMCs (Micro-scheduled Management Commands), which are transmitted by the host within MAC Layer reservations (see Figure 4-4). MMCs contain host identifying information, I/O control structures and a time reference to the next MMC in the sequence (i.e. a link). These links provide a continuous thread of control which can be simply followed by devices that join the Wireless USB Cluster. This encapsulated channel provides the structure that serves as the transmission path for data communications between a host and devices in a Wireless USB Cluster.

Figure 4-4. General Model of a Wireless USB Channel

The Wireless USB Channel is an efficient and extensible medium access protocol that provides low latency and fine grained bandwidth control. The Wireless USB Channel allows a host to rapidly and efficiently change the amount of channel time allocated to individual function endpoints. The Wireless USB Channel is used for both cluster broadcast and point-to-point data communications.

When establishing a Wireless USB Channel, the host allocates a Broadcast Cluster ID and uses it as the Destination DevAddr in describing the MAC Layer channel reservation (DRP IEs) and in the MMCs. These are the control mechanism for a host to manage the Wireless USB Channel. MMCs are used to broadcast command and I/O control information to all devices belonging to the Wireless USB cluster. MMCs are also used to communicate channel time allocations for point-to-point data communications, which are specifically between the host and individual function endpoints in the cluster. The addressing context for MMCs includes the host DevAddr and a Broadcast Cluster ID. The Broadcast Cluster ID for the Wireless USB channel is assigned by the host via a process that is appropriate to ensure the value is tightly coupled with the Wireless USB application and is unique for the lifetime of the Wireless USB Channel. The lifetime of a Wireless USB channel spans the time from when a host first starts the channel (i.e. after a boot or re-boot) to the next time the host needs to boot or reboot. The intent is that a host retains Wireless USB channel parameters across power states, PHY channel changes, etc. Refer to Section 4.13 for details about how a device identifies a host to connect to. The addressing context for point-to-point data communications includes the host DevAddr, the device DevAddr and a Stream Index value. A single stream index value is allocated by the host for each Wireless USB Channel.

A Wireless USB Channel consists of a continuous sequence of MMC transmissions from the host. The linked stream of MMCs is used primarily to dynamically schedule channel time for data communications between host applications and Wireless USB Endpoints. An MMC specifies the sequence of micro-scheduled channel time allocations (MS-CTAs) up to the next MMC within a reservation instance or to the end of a reservation instance. It may be followed by another MMC without the existence of MS-CTAs between the two MMCs. In this case, the MMC is only used to convey command and control information. The channel time between two

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MMCs may also be idle time, where no MS-CTAs are scheduled. The general layout of the MMC is defined below with detailed information elements for the MMC defined in Sections 5.2 and 7.5.

The MS-CTAs within a reservation instance can only be used by the devices that are members of the associated Wireless USB Cluster. The direction of transmission and the use of each MS-CTA is fully declared in each MMC instance. An MMC can declare an MS-CTA during any channel time following the MMC. Section 5.2 provides detailed requirements for using MMCs to accomplish the Wireless USB protocol via the Wireless USB channel. Figure 4-5 illustrates an example MMC sequence within an instance of a MAC Layer channel reservation for Wireless USB.

Params DRP IE

Figure 4-5. Example Wireless USB Channel Control Sequence within a MAC Layer Reservation

An MMC contains the information elements necessary to identify the Wireless USB Channel, declare any MSCTAs, or other information elements that are used for command and control. The general structure of an MMC packet is defined in Section 7.5 and the exact structure of the information elements contained in an MMC is defined in Sections 7.5 and 5.2.1. The MMC is a broadcast control packet that is for receipt only by devices that are members of the Wireless USB cluster. The host must use the Broadcast Cluster ID value in the DestAddr field of an MMC packet’s MAC header. This technique identifies this packet transmission as a broadcast targeting all devices in a Wireless USB cluster, and avoids potential confusion at Non-Wireless USB devices in listening range of the host. The MMC data payload must be encapsulated within a MAC Layer secure packet; however its data payload is transmitted in plain text, thus using the security encapsulation for authentication purposes only.

A host is required to implement the MAC Layer protocol, establish and maintain (one or more) Wireless USB Channels by allocating sequences of MAC Layer MAS reservations (i.e. DRPs). A device may implement the full MAC Layer protocol; however it is nominally only required to implement the Wireless USB protocol which operates within the Wireless USB Channel.

This section reviews a small subset of MAC features that are relevant to Wireless USB. It is not intended to provide a working knowledge of the MAC Layer and also uses many terms defined in the MAC Layer standard, please refer to reference [3] for full details. Section 4.3.8 describes the requirements on a host for establishing and maintaining a Wireless USB Channel, both for data communications within the Wireless USB Cluster and interactions with other MAC Layer devices. Section 4.3.7 describes the minimum requirements on a device to accomplish data communication over the Wireless USB Channel. This specification includes all information required to implement Wireless USB Devices. For the host, it contains only the information required to implement a Wireless USB Host; it does not include all information required for a compliant MAC Layer implementation.

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4.3.1Wireless USB Channel Time

Identical to USB 2.0, a Wireless USB Host maintains a free running timer that effectively defines USB channel time. USB channel time can be used by isochronous devices and is used by Directed Beaconing devices and is also utilized by several other features of Wireless USB. Accuracy of the USB channel time must be 20ppm (matching PHY requirements).

Current USB channel time is communicated by the host in MMCs. Each MMC contains a 24-bit timestamp value that indicates when (in USB channel time) the MMC was transmitted. The timestamp consists of two parts, a 1/8th millisecond value and a microsecond value. The microsecond counter is 7 bits, and counts from 0 to 124, and then wraps back to zero. The 1/8th millisecond value is 17 bits and wraps to zero after reaching a value of all 1’s. The 1/8th millisecond value increments when the microsecond value wraps from 124 to 0. The timestamp indicates the time when the beginning of the MMC packet was transmitted.

Figure 4-6. MMC Time Stamp

There are no requirements on a specific start value when the host first starts a USB Channel. However, once the timestamps are started they must continue to be provided based on the free-running timer as long as the host is producing MMCs. USB channel time must not be affected by changes in the host’s beacon start time or any other event during normal operation.

4.3.2MMC Transmission Accuracy

The host must ensure that MMC transmissions begin on microsecond boundaries with +/-40 nanosecond accuracy and that the timestamp value in the MMC accurately reflects the USB channel time. For example – it would be incorrect for a host to produce an MMC with the current time stamp and then wait 2 microseconds to send the MMC.

Figure 4-7. MMC Time Stamp Accuracy

Figure 4-7 shows a timescale for the free running timer in a host. The host intends to send an MMC when its free running timer reaches microsecond X. The host has a small window of 40 nanoseconds before to 40 nanoseconds after microsecond X on its free running timer to start transmission of the MMC. If the MMC transmission starts earlier or later, the host has not met the accuracy requirement for the time stamp in MMC.

4.3.3USB Time across Device Wire Adapters

For a device wire adapter USB channel time must be consistent (same rate and same value) across its upstream bus and its downstream bus. A device wire adapter is required to synchronize its downstream SOF packets with the USB channel time on its upstream wireless connection. The DWA generated FrameNumber values in SOFs must match the 1/8th millisecond values in MMC timestamps on the upstream wireless bus.

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As an example, consider Figure 4-8 where the top part of the figure illustrates the upstream and downstream buses on a DWA. Bus A is a Wireless USB bus connecting the Host PC to a Device Wire Adapter (DWA). Bus B is a USB 2.0 hi-speed connection between the DWA and a hi-speed device. The bottom part of the figure shows a sequence of time when the USB channel time generated by the host is crossing a millisecond boundary. The vertical dotted lines in the figure show 1/8th millisecond points of USB channel time. The DWA is required to make the transmission times of the SOFs on the downstream bus be consistent with the upstream channel time as illustrated in the figure.

The FrameNumber value of the SOFs on the downstream wired bus must match bits 13:3 of the 1/8th millisecond value in the timestamp of any MMCs transmitted during a 1/8th millisecond period. Similarly, bits 2:0 of the 1/8th millisecond value in the MMC timestamp must reflect the SOF instance (microframe) on the downstream bus.

Figure 4-8. USB time across hierarchical buses

Note, HWAs are not required to synchronize their downstream USB channel time with their upstream USB 2.0 channel time (i.e. SOFs). HWAs must meet all Wireless USB channel time accuracy requirements.

4.3.4Device and Application Co-existence

The Wireless USB Channel is mapped onto reserved MAC Layer channel time, so the host is required to satisfy the MAC Layer protocol beaconing requirement. This means hosts must manage two-hop topology in order to respect declared reservations from other devices that carry reservation declarations (DRP IEs) in their beacon data structures. DRP IEs are used to reserve MAC Layer channel time for the Wireless USB Channel and the host must utilize MAC Layer beacons to propagate the Wireless USB reservations to neighbors of the host and individual devices (see Section 4.3.8). Note that protection of Wireless USB reservations requires that under most circumstances a host and devices from the Wireless USB Cluster transmit beacons during the Beacon Period. Figure 4-9 illustrates an example.

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WUSB

Host

Figure 4-9. Example Two-Hop Topology

The host (WUSB Host) and device (WUSB Dev) communicate via the Wireless USB Channel. The device has neighbors Dev A and Dev B which are not neighbors of the host (i.e. are not in range of the host, so therefore cannot successfully receive the host’s Beacon). Therefore, in order to protect the Wireless USB Channel reservation, the device needs to transmit a Beacon that includes a DRP IE similar to that of the host. Dev A will observe the device’s beacon and become aware of the Wireless USB Channel reservation and thus avoid transmitting during the advertised MAC Layer channel time.

A Wireless USB device may choose to be fully MAC Layer aware. These device implementations must meet the MAC Layer standards for participating in the MAC Layer channel, including managing beacon period synchronization and processing neighbor beacons. As noted above devices can choose to implement only support for communicating via the Wireless USB Channel, which means they have no knowledge of the encapsulating MAC Layer protocol. If all devices in the Wireless USB cluster have the same neighbors as the host, then only the host needs to transmit a beacon (with a DRP IE(s)) to reserve the MAC Layer channel time for the Wireless USB channel. However, when devices have neighbors that are different (than what the host observes), they need to transmit a beacon which can be observed by its neighbors that are not neighbors of the host. Wireless USB accomplishes this by providing mechanisms that allow a host to perform proxy processing on behalf of the non-MAC Layer protocol devices.

Wireless USB defines protocol mechanisms to allow a host to direct devices to capture and propagate MAC Layer channel protocol information without having knowledge of the MAC Layer channel protocol (see section 4.3.7.2 describing Directed Beaconing devices). These mechanisms are used by a host to command Wireless USB (only) devices to capture and/or transmit raw packets at specific times, relative to the Wireless USB Channel time. Using these mechanisms, a host can command devices to capture frames during the beacon period and forward them to the host for processing. The host can accumulate this information and then determine whether the device has neighbors that are out of range of the host. When a host determines that a device needs to transmit a MAC Layer beacon, the host constructs the device beacon, forwards it to the device via a standard Wireless USB request on the Default Control Endpoint, then directs the device to transmit the stored beacon packet at a Wireless USB Channel time that occurs co-incident with an open beacon slot time. The requirements on the host for managing Directed Beaconing devices are detailed in Section 4.3.8.

Over time, the host will encounter situations when it has too much or too little MAC Layer channel time reserved for the current Wireless USB Channel communications load. There are two basic forms of overbudgeting which need to be handled: Short-term and Long-term. The definition of a Short-term over-budget situation is where the host has no scheduled transfers for the Wireless USB Channel. When this occurs, the host may release the remainder of the current MAC Layer reservation by instructing all affected devices in the cluster to transmit a UDR (Unused DRP Response) control packet. MAC Layer compliant devices that hear the UDR can then use the remainder of the MAC Layer channel reservation time. A short-term over-budget condition does not cause any modifications to the MAC Layer channel reservations for the Wireless USB Channel, as declared in the beacon data structures transmitted by devices from the Wireless USB Cluster. A Long-term over-budget scenario can result in a modification to the MAC Layer channel reservation time as declared in cluster’s beacons. For example, the host may detect that it has much more MAC Layer channel time required than the aggregate communication load requires. It can therefore release a portion of its reservation by reducing the number of reserved MASs declared in its DRP IEs for the Wireless USB Channel.

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