Introduction

UWB MAC sub layer corresponds to the MAC sub layer of the Standard ISO/OSI-IEEE 802 as shown in Figure 1.1


Figure 1.1 The Seven Layers of the OSI Model

Let me introduce the two important layers of the OSI model.

Data Link Layer

The data link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the physical layer. MAC sub layer lies in the Data Link layer. The MAC layer concerns itself with the access control method and determines how use of the physical transmission is controlled.

Physical layer

The physical layer defines all the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. This includes the layout of pins, voltages, cable specifications, Hubs, repeaters, network adapters, Host Bus Adapters (HBAs used in Storage Area Networks) and more.

MAC features within Wireless USB devices

Wireless USB devices must be well-behaved MAC Layer devices. Its major role in wireless USB device is to Transmit or Receive protocol related packets sent by the Host via analog circuitry within PHY. All protocol related packets must follow the MAC layer frame format.

UWB MAC variations in different applications

Hosts identify the class of a device from the Beacon Behavior field of the Wireless USB Device Capabilities – UWB descriptor. Wireless USB device defines three categories of devices providing different degrees of awareness of MAC Layer mechanisms:

Self Beaconing: These devices are fully aware of MAC Layer protocol and do all related beaconing. In the self-beaconing MAC, coordination of devices within radio range is achieved by exchange of beacon frames.

Beacons provides the basic timing for the network & carry reservation & scheduling information for accessing the medium. When a device is enabled, it scans one or more channels for beacons & selects a channel. If no beacons are detected in the selected channel, the device creates its Beacon Period by sending a beacon. A Self Beaconing device must support several Wireless USB device requests in order to allow the host to manage DRP reservations.

MAC Layer channel time is organized into super-frames. 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 256us). The MASs at the beginning of a super-frame are allocated for use by the Beacon Period.

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.

Direct Beaconing: These devices are unaware of MAC Layer protocol and rely on the host for direction to properly beacon and detect neighbor devices. These devices only implements the Wireless USB channel must support Wireless USB define capabilities that allow a host to learn about neighbors of the device that are out of range from the host, but in range of the device (called hidden neighbors), and to have the device transmit a beacon defined by the host.

Non Beaconing: These devices have reduced transmit power and receiver sensitivity so that they don’t interfere with, and are not interfered with, by neighbor devices that the host cannot detect. These devices don’t need to Beacon.

Limitations of Non-Beaconing Devices

  • Non-Beaconing Devices have reduced transmitting power & receiver sensitivity It means these devices should be very close to the host within the cluster (within 10 meters).
  • Non-Beaconing Devices Protocol layer takes many responsibilities of the UWB MAC such as: acknowledgement policies, retransmission of the frames.

Wireless USB device (Non-Beaconing)

Wireless USB MAC of this IP supports non Beaconing characteristics. Devices that are unaware of the MAC layer channel & have restricted transmit & receive capabilities such that they don’t need to beacon are called “Non-Beaconing Devices”.

Wireless USB device can be attached with a ECMA-368 Standard compatible PHY & is having ECMA-369 Standard MAC-PHY Interface. Devices may have to scan several PHY channels before finding the appropriate host. Depending on user preferences and device capabilities, a device may choose to automatically connect to a host, or wait for a user to instruct the device to make a connection (possibly by pushing a button). When a Wireless USB Host becomes active, it must choose a PHY channel (Band Group) in which to operate the Wireless USB channel.

TX MAC / RX MAC are implemented most of transmit and receive MAC functions according to the High Rate Ultra Wideband PHY and MAC Standard ECMA-368. These modules synchronize with the clock provided by PHY.

The ECMA-368 standard specifies UWB operation in the frequency range of 3.10 to 10.60 GHz, for unlicensed operation. In this range are 14 bands, each 528 MHz wide. The lower (in frequency) 12 bands are grouped into four groups of three bands each, and the upper two of the 14 bands are grouped into a fifth group. The band allocation is summarized in Figure 1.2.

Figure 1.2 Band Group Allocations

Features of Wireless USB MAC:

  • Supports speeds up to 480 Mbps at MAC-PHY interface
  • Supports Control, Bulk and Interrupt transfers
  • Supports Data Bursting
  • ECMA 368 standard compatible MAC PHY Interface.
  • Supports standard ECMA-368 compatible interface between MAC & PHY.
  • CRC check on the packet Payload.
  • Supports Header Error detection
  • Supports ZERO payload length detection

Features of ECMA 368 Standard compatible PHY:

  • The function of the PHY device is to transfer bits over the air interface.
  • Standard support for the error correction for all bits in the logical packet (PHY/MAC Header + frame body).
  • CRC checks the MAC & PHY header.
  • Reason for putting PHY on a separate chip – The PHY incorporates a significant amount of analog hardware, while the MAC is typically an all digital component.
  • Frame transmission in both single frame and burst mode.
  • Frame reception for both single frame and burst mode transmission
  • PLCP header error indication for both PHY and MAC header structures

MAC sublayer performs the following functionalities

  • Frame transmission in both single frame and burst mode.
  • Frame reception for both single frame and burst mode transmission.
  • Performs the Frame Check Sequence (FCS) over the frames transmitted/ received.
  • Utilizes the ECMA-368 MAC PHY interface to communicate with Physical layer.
  • Wireless USB MAC operates in the protocol time slots provided by the Host & do IN & OUT transactions based on the opportunities provided by the Device Free running Timer.
  • Since Wireless USB MAC is a non-beaconing, while OUT transactions, it does the reception of the two type of protocol packets, Control frames & Data frames, while IN transactions, it does the transmission of only Data frames.

Implementation of the MAC in hardware consists of the following general blocks:

Transmitter
Its main role is to support IN transaction & it behaves as a transmission link between Protocol & Host with support of analog circuitry in third party PHY chip.

Receiver
Its main role is to support OUT transaction & its behaves as a reception link between Protocol & Host with support of analog circuitry in third party PHY chip.

CRC Generator
Its main role is to generate 32-bit CRC upon the transmitted frame & compares the generated 32-bit CRC with the 4-byte FCS within the received frame.

MAC-PHY interface
The MAC-PHY interface is used to continue proper communication & synchronization between Data Link layer & Physical layer. The ECMA-368 MAC-PHY interface is shown in Figure 1.3

Figure 1.3 ECMA 369 Standard MAC PHY Interface

Summary

MAC is a sub block of the wireless USB device & its major role includes the transmission of Protocol packets at the allocated time slots within DTCTA or DNTSCTA block provided by the Host & reception of the Protocol packets at the allocated time slots within DRCTA block or next MMC Time slot provided by the Host. It also provides an ECMA 369-standard compatible MAC-PHY Interface to attach with a third party PHY chip.

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