============================ Serial Communication ============================ :TEP: 113 :Group: Core Working Group :Type: Documentary :Status: Draft :TinyOS-Version: 2.x :Author: Ben Greenstein and Philip Levis :Draft-Created: 11-Jul-2005 :Draft-Version: $Revision$ :Draft-Modified: $Date$ :Draft-Discuss: TinyOS Developer List .. Note:: This memo documents a part of TinyOS for the TinyOS Community, and requests discussion and suggestions for improvements. Distribution of this memo is unlimited. This memo is in full compliance with TEP 1. Abstract ==================================================================== This memo describes the structure and standard implementation of the TinyOS 2.x serial communication system for mote-to-PC data exchange. The system is broken into three levels (encoding, framing, and dispatch) to allow easy experimentation and replacement. It can also handle multiple packet formats: unlike 1.x, 2.x serial packets are not bound to the mote's radio packet format. Additionally, one of the supported packet formats is platform independent, so PC-side applications can communicate with arbitrary motes. 1. Introduction ==================================================================== Users need to read data out of a TinyOS network. The most common approach is to attach a mote to a PC or latop with a wired connection. While the interface on the PC side can vary from a serial cable to a USB device to IP, the mote generally talks to a serial port (UART). In TinyOS 1.x, the UART packet format is platform-specific, pushing a good deal of complexity into the protocol and PC-side tools in order to discover and properly handle platform diversity. TinyOS 2.0 introduces the notion of packet format dispatch, so a mote can support multiple UART packet formats simultaneously. This allows transparent bridging (e.g., an 802.15.4 base station) to exist in parallel with platform-independent communication, which allows simplifies the PC toolchain. This memo documents the protocols and structure of the TinyOS 2.x serial communication stack. 2. Serial Stack Structure ==================================================================== The TinyOS 2.x serial communication stack is broken up into four functional components. From bottom to top, they are o the raw UART, o the encoder/framer, o the protocol, o and the dispatcher. Structurally, they look like this: :: _____________________ | | | Dispatcher | Packet formatting. |_____________________| _____________________ | | | Protocol | Acknowledgements, CRC computation, |_____________________| windowing. _____________________ | | | Encoder/Framer | Translating raw bytes into frame |_____________________| delimiters, escape bytes. _____________________ | | | Raw UART | Platform code for reading/writing |_____________________| bytes over the serial connection. The bottom three provide a byte-level interface: only the Dispatcher provides a packet-level interface. The top three are all platform-independent: only the UART is platform-specific code. The lowest level of the stack is the raw UART. This HIL component provides functionality for configuring the UART (speed, stop bytes, etc.) as well as sending/receiving bytes. The Encoder/Framer sits above the raw UART. This component translates raw data bytes into packet bytes using a serial protocol's encoding. The Encoder/Framer assumes that a protocol's encoding has two kinds of bytes: delimiters and data bytes, and signals each in separate events to the component above. The Protocol component handles data and delimiter byte events. It is responsible for reading in and sending all protocol control packets. If the Protocol component starts receiving a data packet, it signals to the Dispatcher that a packet has started and signals the data bytes. When the data packet completes, the Protocol signals to the Dispatcher that the packet is complete and whether it passed the protocol-level CRC. The Dispatcher component handles data packet bytes and delimiters. It is responsible for reading data bytes into a message_t and signaling packet reception to components above it. The dispatcher can support multiple packet formats. Based on how message_t works (see TEP 111[tep111_]), this boils down to knowing where in a message_t a particular packet format begins (based on its header size). Section 3.4 describes how the default TinyOS 2.x implementation, ``SerialDispatcherC`` does this. 3. The 2.x Serial Stack Implementation ==================================================================== Section 2 describes the basic structure of the TinyOS 2.x serial stack structure. This section describes its actual implementation, including SerialActiveMessageC, which sits on top of the Dispatcher. All of the components except for UartC are part of the serial library that lives in ``tos/lib/serial``. 3.1 Raw UART: UartC -------------------------------------------------------------------- The UART HIL[TEP2_] is ``UartC``, which provides a byte-level interface to the underlying serial communication. It provides the ``SerialByteComm`` interface: :: interface SerialByteComm { async command error_t put(uint8_t data); async event void get(uint8_t data); async event void putDone(); } It also provides interfaces for configuring the serial port. *NOTE: These are not codified yet, and so working out the UART HIL seems like a good idea.* 3.2 Encoder/Framer: HdlcTranslateC -------------------------------------------------------------------- HdlcTranslateC is the serial encoder/framer. It uses the ``SerialByteComm`` interface and provides the ``SerialFrameComm`` interface: :: interface SerialFrameComm { async command error_t putDelimiter(); async command error_t putData(uint8_t data); async command void resetSend(); async command void resetReceive(); async event void delimiterReceived(); async event void dataReceived(uint8_t data); async event void putDone(); } As its name suggests, it uses the same encoding as the HDLC[HDLC_] protocol. ``0x7e`` is reserved as a frame delimiter byte, and ``0x7d`` is reserved as an escape byte. HdlcTranslateC maintains ten bits of state. The receive and send paths each have one bit to store whether they are using an escape byte, and the transmit path has a byte for when it sends an escaped byte. When HdlcTranslateC receives a delimiter byte, it signals delimiterReceived(). When HdlcTranslateC receives an escape byte, it sets the receiveEscape flag to true. When it receives any other byte, it tests to see if the receiveEscape flag is set; if so, it XORs the data byte with ``0x20`` and clears the flag. It signals dataReceived() with the byte. The most common use of escape byte is to transmit data bytes corresponding to the delimiter byte or escape byte. For example, ``0x7e`` becomes ``0x7d 0x5e``. HdlcTranslateC performs similar actions on the transmit side. When told to transmit the delimiter or escape byte as a data byte, it sets the transmitEscape flag to true, stores the data byte XOR ``0x20``, and sends an escape byte. When the escape byte is sent, it sends the stored data byte. 3.3 Protocol: SerialP -------------------------------------------------------------------- The SerialP component implements the serial protocol using PPP/HDLC- like framing (See RFC 1662[RFC1662_]). Type dispatch and buffer management are left to higher layers in the serial stack. The protocol is currently stop-and-wait in the host-to-mote direction and best effort in the mote-to-host direction. The first performance upgrade of this module will be to implement sliding window reliability in both directions. SerialP provides two byte-level interfaces to the upper layer for sending and receiving packets, respectively called SendBytePacket and ReceiveBytePacket. On the sending side, SerialP is responsible for encapsulation of upper layer packets. An upper layer component such as SerialDispatcherC initiates the sending of a packet by calling startSend, passing the first byte to send. SerialP collects subsequent bytes by signalling nextByte. Within the nextByte handler or between calls to nextByte, the upper layer should indicate the end-of-packet by calling completeSend. If completeSend is called from within a nextByte handler, SerialP will ignore the return of the call to nextByte. :: interface SendBytePacket { async command error_t startSend(uint8_t first_byte); async command error_t completeSend(); async event uint8_t nextByte(); async event void sendCompleted(error_t error); } SerialP maintains a small window of bytes that have been received by the upper layer and not yet sent to the UART. Depending on the timing requirements of the underlying UART, the size of this window can be changed. SerialP uses repeated calls to nextByte to keep this window filled. SerialP uses SerialFrameComm to send a delimiter between frames, a serial-level type field, the bytes of the packet, and a two-byte frame CRC. For mote-to-host gap detection and link reliability, a sequence number may also be sent (not currently activated). After sending an entire frame and receiving the last putDone event from below, SerialP signals sendCompleted to indicate the success or failure of a requested transmission. Packet reception is also managed by SerialP and the interface provided to the upper layer is ReceiveBytePacket: :: interface ReceiveBytePacket { async event error_t startPacket(); async event void byteReceived(uint8_t b); async event void endPacket(error_t result); } Upon receiving an interframe delimiter and a new frame's header, SerialP signals the upper layer indicating that a packet is arriving. For each byte received, SerialP signals byteReceived. (Note: SerialP signals on byte k-2 when byte k arrives, because the implementation precludes it from knowing when it has encountered the 2-byte CRC in the frame footer until after it has received it. Lagging behind by two bytes makes it possible to hide all frame details from the upper layer.) Once SerialP receives the complete frame it signals endPacket with a value of SUCCESS. If instead it loses sync during reception it signals endPacket with FAIL. SerialP acknowledges frames it receives. Acknowledgements have a higher priority than data transmissions and consequently, data frames may be slightly delayed. However, acknowledgement information is stored in a queue separate from the data buffer, so a data packet to be transmitted may begin spooling into SerialP while SerialP is actively sending an acknowledgement. 3.4 Dispatcher: SerialDispatcherC -------------------------------------------------------------------- SerialDispatcherC handles the data packets that the Protocol component receives. It uses the SendBytePacket and ReceiveBytePacket interfaces, and provides parameterized Send and Receive interfaces. The parameter in the Send and Receive interfaces (``uart_id_t``) determines the packet format contained in the message_t. SerialDispatcherC places a one-byte header, the packet format identifier, on the packets sent and received through SerialP. SerialDispatcherC uses a parameterized SerialPacketInfo interface to be able to handle various packet formats: :: interface SerialPacketInfo { async command uint8_t offset(); async command uint8_t dataLinkLength(message_t* msg, uint8_t upperLen); async command uint8_t upperLength(message_t* msg, uint8_t dataLinkLen); } When SerialDispatcherC receives the first data byte of a packet from SerialP, it stores it as the packet type and calls SerialPacketInfo.offset() to determine where in a message_t that packet format begins. It then spools data bytes in, filling them into its message_t buffer. Similarly, on the send side, it first sends the type byte and spools out data bytes starting from the index denoted by the call to offset(). SerialDispatcherC uses the two length commands, dataLinkLength and upperLength, to translate between the two notions of packet length: above, length refers to the payload excluding header, while below it refers to the payload plus header. A component that provides communication over the serial port with uart_id_t *U* MUST wire a component implementing SerialPacketInfo to SerialDispatcherC with uart_id_t *U*. The file ``Serial.h`` contains reserved uart_id_t's for supported packet formats. Currently, only platform independent active messages (``TOS_SERIAL_ACTIVE_MESSAGE_ID``, described in Section 3.5), 802.15.4 active messages (``TOS_SERIAL_802_15_4_ID``), mica2 CC1000 packets (``TOS_SERIAL_CC1000_ID``) and the error code ``TOS_SERIAL_UNKNOWN_ID`` are reserved. New packet formats MUST NOT reuse any reserved identifiers. 3.5 SerialActiveMessageC -------------------------------------------------------------------- SerialActiveMessageC is a platform-independent active message layer that operates on top of the serial communication stack. SerialActiveMessageC is a configuration that wires SerialActiveMessageP to SerialDispatcherC with uart_id_t TOS_SERIAL_ACTIVE_MESSAGE_ID and wires SerialPacketInfoActiveMessageP to SerialDispatcherC with uart_id_t TOS_SERIAL_ACTIVE_MESSAGE_ID: :: includes Serial;`` configuration SerialActiveMessageC { provides { interface Init; interface AMSend[am_id_t id]; interface Receive[am_id_t id]; interface Packet; interface AMPacket; } uses interface Leds; } implementation { components new SerialActiveMessageP() as AM, SerialDispatcherC; components SerialPacketInfoActiveMessageP as Info; Init = SerialDispatcherC; Leds = SerialDispatcherC; AMSend = AM; Receive = AM; Packet = AM; AMPacket = AM; AM.SubSend -> SerialDispatcherC.Send[TOS_SERIAL_ACTIVE_MESSAGE_ID]; AM.SubReceive -> SerialDispatcherC.Receive[TOS_SERIAL_ACTIVE_MESSAGE_ID]; SerialDispatcherC.SerialPacketInfo[TOS_SERIAL_ACTIVE_MESSAGE_ID] -> Info; } SerialActiveMessageP is a generic component so that it can be used to sit on top of any packet-level communication layer. It does not filter packets based on destination address or group. It assumes that if the packet was received over the serial port, it was destined to the node. This saves PC-side tools from having to discover or consider the ID and group of a mote. Platform-independent active messages do not have a CRC (they assumes the serial stack CRC is sufficient), and have the following header: :: typedef nx_struct SerialAMHeader { nx_am_addr_t addr; nx_uint8_t length; nx_am_group_t group; nx_am_id_t type; } SerialAMHeader; 3.6 Packet Format -------------------------------------------------------------------- A data packet in the TinyOS 2.x serial stack has the following format over the wire. Each protocol field is associated with a specific component: :: ____________________________________________ | | | | | | | | | | | | | | | | |_|_|_|_|_______________________________|__|_| F P S D Payload CR F F = Framing byte, denoting start of packet: HdlcTranslateC P = Protocol byte: SerialP S = Sequence number byte: SerialP D = Packet format dispatch byte: SerialDispatcherC Payload = Data payload (stored in SerialDispatcherC): SerialDispatcherC CR = Two-byte CRC over S to end of Payload: SerialP F = Framing byte denoting end of packet: HdlcTranslateC Payload is a contiguous packet that SerialDispatcherC reads in. Note that any data bytes (P - CR) equal to 0x7e or 0x7d will be escaped to 0x7d 0x5e or 0x7d 0x5d accordingly. For example, a platform independent AM packet of type 6, group 0x7d, and length 5 to destination 0xbeef with a payload of 1 2 3 4 5 would look like this: ``7e 40 09 00 be ef 05 7d 5d 06 01 02 03 04 05 7e`` Note that the group 0x7d is escaped to 0x7d 0x5d. The protocol field (P) is 0x40 (64), corresponding to ``SERIAL_PROTO_ACK`` (in Serial.h). 4. Access Abstractions ==================================================================== Two generic components: SerialAMSenderC and SerialAMReceiverC connect to SerialActiveMessageC to provide virtualized access to the serial stack. Each instantiation of SerialAMSenderC has its own queue of depth one. Therefore, it does not have to contend with other SerialAMSender instantiations for queue space. The underlying implementation schedulers the packets in these queues using some form of fair-share queueing. SerialAMReceiverC provides the virtualized abstraction for reception. These abstraction are very similar to TinyOS's radio abstractions, namely, AMSenderC and AMReceiverC. See Section 4 of TEP 116[TEP116_] for more information. Unlike the services in the TEP 116, the serial component virtualizations provide no snooping capabilities. 5. Author's Address ==================================================================== | Philip Levis | 358 Gates | Computer Science Laboratory | Stanford University | Stanford, CA 94305 | | phone - +1 650 725 9046 | email - pal@cs.stanford.edu | | | Ben Greenstein | Center for Embedded Networked Sensing | UCLA 3563 Boelter Hall | Los Angeles, CA 90095-1596 | | phone - +1 310 206 3925 | email - ben@cs.ucla.edu 6. Citations ==================================================================== .. [TEP2] TEP 2: Hardware Abstraction Architecture. http://cvs.sourceforge.net/viewcvs.py/tinyos/tinyos-1.x/beta/teps/txt/tep2.txt?view=markup .. [TEP111] TEP 111: message_t. http://cvs.sourceforge.net/viewcvs.py/tinyos/tinyos-1.x/beta/teps/txt/tep111.txt?view=markup .. [TEP116] TEP 116: Packet Protocols. http://cvs.sourceforge.net/viewcvs.py/tinyos/tinyos-1.x/beta/teps/txt/tep116.txt?view=markup .. [HDLC] International Organization For Standardization, ISO Standard 3309-1979, "Data communication - High-level data link control procedures - Frame structure", 1979. .. [RFC1662] PPP in HDLC-like Framing, Internet Engineering Task Force (IETF), 1994