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<May 2007> doc.: IEEE 802.15-15-07-0724-00-wng0
Project: IEEE P802.15 Working Group for Wireless Personal Area Networks
(WPANs)
Submission Title: [ISA SP100.11a Hybrid DLL Proposal]
Date Submitted: [14 May 2007]
Source: [Jay Werb] Company [Sensicast]
Address [Massachusetts, USA]
Voice:[Add telephone number], FAX: [Add FAX number], E-Mail:[jwerb07@sensicast.com]
Re: [Information for IEEE 802.15 as per WNG]
Abstract: [DLL proposal for ISA SP100.11a containing TDMA and CSMA properties]
Purpose: [Information as to how the ISA SP100.11a WG anticipates using IEEE 802.15.4]
Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for
discussion and is not binding on the contributing individual(s) or organization(s). The material in this
document is subject to change in form and content after further study. The contributor(s) reserve(s)
the right to add, amend or withdraw material contained herein.
Release: The contributor acknowledges and accepts that this contribution becomes the property of
IEEE and may be made publicly available by P802.15.
Submission Slide 1 <Jay Werb>, <Sensicast>
<May 2007> doc.: IEEE 802.15-15-07-0724-00-wng0
Introduction
Submission Slide 2 <Jay Werb>, <Sensicast>
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Flexibility and Options
• The ISA-SP100.11a DLL is designed to be flexible
– Trade-offs are made by system designers
– They know more about their applications than we do
– Default settings will control the scope of validation & testing…
– … but systems won’t be limited to those defaults
– Default settings may be overridden by System Manager
• DLL supports a hierarchy of 5 device classes
– Backbone Routers (continuously powered)
– Dedicated Field Routers (continuously powered)
– Instruments with routing capability (battery powered)
– Instruments without routing capability (smaller batteries)
– Very low cost sensors (tiny batteries, CSMA only)
Submission Slide 3 <Jay Werb>, <Sensicast>
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Overview of TDMA/CSMA Hybrid
Features added to a baseline TDMA structure
– CSMA periods
• Fast network discovery using control channels
• Immediate next-hop propagation of unscheduled commands and alarms
• Support low end CSMA-only field devices
– Slot length is configurable to allow for
• Longer guard times (longer slots)
• Tighter implementations (shorter slots)
• Duocast (serial acknowledgement from two devices)
• CSMA at start of slot (prioritized access to shared slots)
– Flexible options for hybrid centralized/decentralized
operation
• System Manager can delegate slot allocation to Routers
• System Manager can allow Routers to create their own superframes & routes
Submission Slide 4 <Jay Werb>, <Sensicast>
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Stack Structure
Submission Slide 5 <Jay Werb>, <Sensicast>
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Field Device Stack Diagram
“Current” Version
Submission Slide 6 <Jay Werb>, <Sensicast>
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Field Device Stack Diagram
Recommended Changes
Submission Slide 7 <Jay Werb>, <Sensicast>
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What’s in the Lower Data Link Layer (LDLL)?
• Lower Data Link Layer (LDLL)
– A continuous sequence of channel hopping slots
• One slot per hop for “TDMA”
• Slower hopping sequence (~10-25 slots per hop) for “CSMA”
• May be combined for “Hybrid TDMA/CSMA”
– The LDLL is IEEE 802.15.4 as we wish it were
• Fewer “features”
• More flexibility
• IEEE 802.15.4 MAC not discussed here
– Treated as a pass-through to the PHY
• Designed to work with existing standard 15.4 MACs
– In silicon or otherwise “unchangeable”
– LDLL security similar to IEEE 802.15.4, but not the same
• Slot number used for replay protection
• Not covered here
– Just about “everything else” is different … and handled by
the UDLL
Submission Slide 8 <Jay Werb>, <Sensicast>
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Role of LDLL & UDLL Examples
LDLL UDLL & DLL Manager
Topology Immediate neighborhood “Mesh” (DLL subnet)
Time Synch Device level DLL subnet level
Time synch mechanics Manage time propagation messages
Building block Channel hopping slots Superframes
(Cyclic collections of slots)
Discovery Sends and receives packets Discovers and evaluates neighbors
Slot management Follow a hop sequence Manage thousands of slots cyclically
Diversity Handled in by UDLL Frequency, time, spatial diversity
Route Management Measure the quality of Accumulate & report statistics
specific links and manage the network topology
based on link quality information
Security Similar to 802.15.4 role Interface with Security Manager
Submission Slide 9 <Jay Werb>, <Sensicast>
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Lower
DLL
Submission Slide 10 <Jay Werb>, <Sensicast>
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DLL Structure
Submission Slide 11 <Jay Werb>, <Sensicast>
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TDMA, CSMA, and Hybrid
Submission Slide 12 <Jay Werb>, <Sensicast>
<May 2007> doc.: IEEE 802.15-15-07-0724-00-wng0
Types of Superframes
• TDMA only
– Channel-hopping slots of equal length
• CSMA only
– Collect ~10-25 slots on same channel
– Each collection treated as a single CSMA
period
– Slow channel hopping
• Hybrid TDMA/CSMA
– TDMA slots followed by CSMA period
• This TDMA/CSMA ordering is not written in stone
See Case Study (next slide)
– TDMA and CSMA have independent channel
hopping patterns
• TDMA follows an underlying sequence of one hop per slot
• CSMA follows a slower sequence
Submission Slide 13 <Jay Werb>, <Sensicast>
<May 2007> doc.: IEEE 802.15-15-07-0724-00-wng0
A Case Study
WNSIA-Style Operational Superframe
Submission Slide 14 <Jay Werb>, <Sensicast>
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Channel Hopping Sequences
Submission Slide 15 <Jay Werb>, <Sensicast>
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Channel Hopping Sequence
Implementing a WiFi Coexistence
Strategy
Slot-to-slot channel choices can be made to avoid
consecutive channel usage within a single Wi-Fi channel
Submission Slide 16 <Jay Werb>, <Sensicast>
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<May 2007>Example: “Hop sequence 5”
doc.: IEEE 802.15- 15-07-0724-00-wng0
Interlaced with 14 other identical but offset
sequences
Slots within a set of coordinated hop sequences
Submission Slide 17 <Jay Werb>, <Sensicast>
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Hop pattern
Optimized for retry in CSMA channel
Avoid the channels “around” the CSMA channel for
better frequency diversity during CSMA retries
Submission Slide 18 <Jay Werb>, <Sensicast>
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Upper DLL
TDMA Baseline
Submission Slide 19 <Jay Werb>, <Sensicast>
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Typical TDMA as viewed by a single
router
~2 slots/second used
Submission Slide 20 <Jay Werb>, <Sensicast>
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Example:
8 Nodes sharing a TDMA superframe
Submission Slide 21 <Jay Werb>, <Sensicast>
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Upper DLL
CSMA Baseline
Submission Slide 22 <Jay Werb>, <Sensicast>
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CSMA
• CSMA is fully supported by SP100.11a
• Typically combined with TDMA (next section)
• Typically hosted by “continuously powered” routers
– If you have power available, you might as well provide some level of
services when you are not committed to other communications
• Battery-powered routers may briefly enable CSMA on startup
– Facilitate network formation
• CSMA provides key advantages
• Fast neighbor discovery
• Immediate transmission of unscheduled commands and alarms
• Support low-end field devices (tiny batteries, CSMA only)
• Similar to 802.15.4 CSMA-CA with enhancements
• Channel hopping
• Retry involves channel and link diversity
Submission Slide 23 <Jay Werb>, <Sensicast>
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Example of CSMA Operation
Submission Slide 24 <Jay Werb>, <Sensicast>
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8 Routers
With interlaced CSMA Hopping
Sequences
Submission Slide 25 <Jay Werb>, <Sensicast>
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Upper DLL
TDMA/CSMA Hybrid
Submission Slide 26 <Jay Werb>, <Sensicast>
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TDMA/CSMA Hybrid
• TDMA/CSMA Hybrid is a fully-supported mode of operation
– Scheduled messages in TDMA slots
– Retries and unscheduled messages in CSMA periods
– CSMA periods used for subnet discovery
– Intended for continuously powered routers
• Since they have the power, they might as well run their receivers whenever possible
• Battery powered routers may briefly enable CSMA during network formation
• TDMA slots provide
– Predictable performance
– Managed bandwidth
– Reduced contention for CSMA bandwidth
• CSMA periods enable
– Fast network discovery using control channels (next slide)
– Immediate transmission of unscheduled commands and alarms
– Support for low-end CSMA-only field devices
Submission Slide 27 <Jay Werb>, <Sensicast>
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TDMA/CSMA Hybrid
with 3 CSMA Control Channels
Submission Slide 28 <Jay Werb>, <Sensicast>
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DLL Manager
Decentralized Operation
Submission Slide 29 <Jay Werb>, <Sensicast>
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Centralized and decentralized Slot allocation
• Flexible range of centralized/decentralized options
Centralized: Network Brain allocates and manages all slots in the network
Decentralized: Network Brain authorizes Routers to create and manage superframes
Hybrid: Network Brain may delegate slot allocation to capable Routers
Resource-limited routers need not support decentralized options
• Delegated slot allocation: The basics
– Superframes created by Network Brain
– Blocks of slots within superframes are delegated to routers
– Router autonomously allocates delegated slots to its neighbors
– Mechanics of delegation is standardized
– Router’s use of slots is opportunity for “value add”
Submission Slide 30 <Jay Werb>, <Sensicast>
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Example of delegated slot allocation 8 routers
sharing a superframe
Different routers have different slots allocated to them
which they manage on their own
Submission Slide 31 <Jay Werb>, <Sensicast>
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