Receive Side of System Simple Receiver Design Simple Receiver

Receive Side of System Simple Receiver Design Simple Receiver
Receive Side of System
Simple Receiver Design
• Reliably discriminate flag byte from noise
• Synchronize receiver clock to the incoming
• Store incoming data until CRC can be
determined and compared
• CRC Match -> pass data to higher level
• CRC Mismatch -> discard data
• Sample input channel 4x per bit period
• Continuously shift data into storage
registers (1 register per bit-period sample)
• After each new bit is shifted in, XOR
register with 0x7E to test for flag
• After successfully identifying a flag byte,
use the timing data to determine phase of
bit stream clock
Simple Receiver Design contd...
Decoding 0x7E Flag
• Continue to shift data into storage registers,
sampling near center of bit period
• Test for stop flag after each bit is shifted in
• Contemporaneously calculate CRC
• When stop flag is identified, compare CRC
calculated at receiver to that sent from
transmitter – forward or discard data
• Continue to monitor channel (flag can serve as
both start and stop flags, so new data may be
coming immediately)
HighHigh-level Data Link Control
(HDLC) Protocol
• OSI Layer 2 – data link layer
• Generic packet form
• Ubiquitous protocol – though a bit old, it’s a
Ex: A Modern Phy/MAC
Phy/MAC Protocol
• 802.15.4 – recently (2003) ratified
• 900 MHz and 2.4 GHz
• 2.4 GHz transceivers available for < $4 in
unit quantity! Includes FW protocol stack
• Complex – not suitable for 55:036 projects
;-) (27 page app. note to blink an LED)
IEEE 802.15.4 Basics
• 802.15.4 is a simple packet data protocol for lightweight
wireless networks
– Channel Access is via Carrier Sense Multiple Access with collision
avoidance and optional time slotting
– Message acknowledgement and an optional beacon structure
– Multi-level security
– Three bands, 27 channels specified
Designing with 802.15.4 and
• 2.4 GHz: 16 channels, 250 kbps
• 868.3 MHz : 1 channel, 20 kbps
• 902-928 MHz: 10 channels, 40 kbps
Industrial Wireless Applications Summit
San Diego, California
9 March 2004
Jon Adams
Director, Radio Technology and Strategy
Motorola Wireless and Mobile Systems Group
Tempe, Arizona
[email protected]
+1 480.413.3439 Office
– Works well for
• Long battery life, selectable latency for controllers, sensors, remote
monitoring and portable electronics
ZigBee Qualification Group
ZigBee Alliance
– Configured for maximum battery life, has the potential to last as
long as the shelf life of most batteries
Copyright 2004 The ZigBee Alliance, Inc.
Copyright 2004 The ZigBee Alliance, Inc.
PHY Performance
IEEE 802.15.4 MAC Overview
Employs 64-bit IEEE & 16-bit short addresses
– Ultimate network size can reach 264 nodes (more than we’ll probably
– Using local addressing, simple networks of more than 65,000 (2^16) nodes
can be configured, with reduced address overhead
802.15.4 has excellent
performance in low
SNR environments
Three devices specified
– Network Coordinator
– Full Function Device (FFD)
– Reduced Function Device (RFD)
Simple frame structure
Reliable delivery of data
AES-128 security
CSMA-CA channel access
Optional superframe structure with beacons
Optional GTS mechanism
Copyright 2004 The ZigBee Alliance, Inc.
Copyright 2004 The ZigBee Alliance, Inc.
Data Frame format
Acknowledgement Frame Format
One of two most basic and important structures in 15.4
Provides up to 104 byte data payload capacity
Data sequence numbering to ensure that packets are tracked
Robust structure improves reception in difficult conditions
Frame Check Sequence (FCS) validates error-free data
Copyright 2004 The ZigBee Alliance, Inc.
• The other most important structure for 15.4
• Provides active feedback from receiver to sender that
packet was received without error
• Short packet that takes advantage of standardsspecified “quiet time” immediately after data packet
Copyright 2004 The ZigBee Alliance, Inc.
MAC Command Frame format
Beacon Frame format
• Mechanism for remote control/configuration of
client nodes
• Allows a centralized network manager to
configure individual clients no matter how
large the network
Copyright 2004 The ZigBee Alliance, Inc.
Beacons add a new level of functionality to a network
Client devices can wake up only when a beacon is to be broadcast,
listen for their address, and if not heard, return to sleep
Beacons are important for mesh and cluster tree networks to keep all
of the nodes synchronized without requiring nodes to consume
precious battery energy listening for long periods of time
Copyright 2004 The ZigBee Alliance, Inc.
ZigBee is Mesh Networking
Protocol Limitations
ZigBee Coordinator (FFD)
ZigBee Router (FFD)
ZigBee End Device (RFD or FFD)
Mesh Link
Star Link
Yes – channel access constraints!
Yes – collision avoidance strategy!
Yes – addressing!
Yes – frequency agility for multipath
• Yes – error correction mechanism!
• Yes – encryption!
• No – data compression!
Copyright 2004 The ZigBee Alliance, Inc.
Chipcon CC2420
• QLP-48 SMT package (7mm x 7mm)
• 2.4 GHz Direct Sequence Spread
Spectrum (DSSS) transceiver
• 10 dBm RF input power
• 250 kbps data rate
• 3.6V (max) power supply
• Bundled 802.15.4 protocol stack
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