IoT Connectivity Part II
Syllabus
This module will cover the following
• Wireless Local Area Networks (WLAN)
• IEEE 802.11 based WLANs
• IEEE 802.11 enhancements
• Low-power Wide Area Networks (LPWAN)
• LoRaWAN
• Narrow-band IoT (NB-IoT)
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Wireless Local Area Networks (WLANs)
• Computer networking technology spanning limited areas (home, office buildings, campus), where devices communicate wirelessly
• The concept emerged in the 1970s, with the design of ALOHAnet, the first wireless packet data network interconnecting different islands in Hawaii
• Core idea of pure ALOHA:
• A station that has data to send, transmits immediately (no need to control who transmits) • If the message collided (i.e., was not acknowledged), retransmit “later”
• Problem: channel time wasted on collisions and on back-off
• Subsequent WLAN protocols refine this idea by using slotted channels and listening for the medium to be idle before a transmission attempt
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WLANs based on IEEE 802.11
• Wireless networks following the IEEE 802.11 specification are currently the de facto WLAN technology
• Different protocols in the IEEE 802.11 family bundled together under the Wi-Fi trademark
• Two modes of operation: infrastructure and ad-hoc
• Infrastructure is the default mode
• An Access Point (AP) may schedule the transmission of different stations during a contention free period
• Most commonly, channel access is decentralized and stations responsible for own behavior
• AP manages time synch (through beacons), association, and authentication
• Different nodes may play some roles of an AP in ad-hoc mode (e.g., periodic beaconing)
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Infrastructure IEEE 802.11 networks
•
Networks allowed to operate in the following ISM bands:
• 2.4GHz (802.11b/g) – three non-overlapping 20MHz channels; max bitrate: 54 Mb/s (OFDM)
• 5GHz (802.11a) – 12–28 channels (depending on the country); shorter range; bit rate: up to 108 Mb/s (when two 20MHz channels bonded)
Local and external communication always through the AP
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Medium access
• Channel time slotted (idle or busy slots)
• Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) – listen before TX
• Binary exponential back-off (BEB) – if unsuccessful, stations double the average time they wait before next transmission attempt
• Several contention parameters regulate access to the channel:
• Arbitration Interframe Space (AIFS), min and max Contention Windows (CW); number of packets
transmitted upon an attempt (Transmission Opportunity – TXOP)
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Throughput performance
• By default, the contention parameters are fixed, irrespective of the number of stations and traffic volume → channel time wasted or high collision rate, hence the suboptimal throughput
• Example: n = 5 and 30 stations transmitting 1,500-byte frames at 24Mb/s
• Default CWmin = 16 yields sub-optimal performance
• AP may be able to compute optimal configurations and distribute these periodically (every 100ms) via management frames (beacons)
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Hidden terminals
•
Stations whose transmissions cannot be detected by other stations, but which may collide with these at an intended receiver
Example: Station B transmits to C; Station D is unable to carrier sense the transmission of B (out of range), so may also attempt to transmit to C → frame collision.
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Exposed terminals
•
Stations that carrier sense the activity of others and defer transmission, although their frames would not collide at their respective destinations
Example: Station B is transmitting to A; Station C senses B’s activity and concludes it cannot transmit to D, hence defers → channel time potentially wasted.
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Request to Send/Clear to Send mechanism
• RTS/CTS control frames introduced to mitigate these problems
• Additional overhead introduced that may reduce performance in the absence of hidden/exposed terminals
• Particularly useful when multiple packets are sent upon the same channel access (burst or frame aggregation)
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IEEE 802.11 frame structure
• Scope of addresses changes depending on the operation mode (infrastructure/ad-hoc) and direction of packet (to/from AP)
• Address 1: Destination/BSSID
• Address 2: Source/BSSID
• Address 3:
Destination/Source
• Address 4: Used only in WDS mode (bridge)
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Statistical differentiation of traffic in IEEE 802.11
Random access protocol → strict latency guarantees difficulty to guarantee
• Latest specification mandates different queues and different contention parameters for different traffic types
• Four “Access Categories” (AC):
• Best Effort (BE), Background (BK), Video (VI),
Voice (VO)
• Internal collision resolution mechanism
• Multicast traffic (content after beacon –
CAB) and beacons take priority
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IEEE 802.11n enhancements
• Wider bandwidth through channel bonding (up to 40MHz)
• Spatial multiplexing through Multiple Input Multiple Output (MIMO)
• Up to four spatial streams (concurrent transmissions between pair of devices)
• More spectrally-efficient modulation and coding schemes, hence higher data rates, e.g., 8 bits per symbol quadrature amplitude modulation (64-QAM) – up to 65Mb/s per single stream, or 72.2Mb/s if shorter guard between packets
• Lower MAC overhead through frame aggregation
• Selective retransmissions
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IEEE 802.11n frame aggregation
• MSDU – MAC service data unit
• MPDU – MAC protocol data unit
• MSDUs in orange corrupted by channel errors
• Content retransmitted marked below
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IEEE 802.11ac enhancements
• Deployed in the 5GHz frequency band
• Channels with up to 160MHz bandwidth
• Even more spectrally-efficient modulation and coding schemes supported, e.g., 256- QAM (433Mb/s over a single 80MHz channel), but better channel conditions needed
• Up to eight spatial streams
• Downlink multi-user MIMO (MU-MIMO) – transmit a combination of packets intended for different clients, based on estimate channel state; each client decodes only the message intended for it (Gb/s total throughput)
• Whether such high data rates are needed in IoT is questionable; one application that can benefit may be virtual/augmented reality
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Low-power Wide Area Networks (LPWAN)
•
Mostly long range wireless communications suitable for battery powered devices
Particularly suitable for low rate IoT applications
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Low-power Wide Area Networks (LPWAN)
Multitude of factors influence the choice of LPWAN for an application
1. Coverage-throughput trade-off: low frequency narrow bandwidth vs. higher frequency and larger bandwidth
2. Deployment cost and interference: licensed vs. unlicensed spectrum
3. Complexity: centralized vs. decentralized access
4. Compatibility with existing wireless/cellular systems
5. Support for mobility (e.g., automotive IoT)
6. Latency: guaranteed scheduled air time vs. contention-based multiplexing
7. Security
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Long Range Wide Area Networking (LoRaWAN)
• Deployed in unlicensed frequency bands: 433MHz, 868MHz (Europe), 915MHz (Australia and North America), 923MHz (Asia)
• Access subject to duty-cycle and TX power regulations, e.g., 0.1% to 10% per duty-cycle depending on the channel used
• Transmission range over 10km in rural areas
• Access to eight different frequencies at a time allowed
• Protocol stack works on top of a proprietary chirp spread spectrum PHY layer (LoRa) – rates between 0.3 and 5.5kb/s
• Two additional “high speed” frequency shift keying (FSK) modulated channels (support 11 and 50Kb/s data rates)
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LoRaWAN architecture
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LoRa PHY layer
Chirp spread spectrum modulation
Key idea
• Moving a radio frequency (RF) tone through time linearly
• Different symbols (0/1 bit sequences) break signal in different places in terms of time and frequency – chirping
• Spread Factors (SF) 7 to 12
• Signal spread across the entire band used
• Robust to multipath fading and Doppler shift (due to movement)
• Possible to demodulate signals 20dB below noise floor
Illustration
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LoRa bit rate
• Symbol rate (SF bits per symbol):
𝑅! = 𝐵𝑊
2″#
• Bit rate:
• Data whitening, interleaving, Forward Error Correction (FEC) then applied
• With a FEC code with coding rate CR = k/n, for every k bits of information, transmit n bits (i.e., adding redundancy)
𝑅$ = 𝑆𝐹
𝐵𝑊 2″#
• Effective bit rate
𝑅%&& =𝐶𝑅⋅𝑆𝐹𝐵𝑊 2″#
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LoRa bit rate vs. spread factor BW = 125KHz, default code rate CR = 4/5
Higher rates
• Wider channels (250 and 500kHz) permitted in some geographic regions (with certain SF)
• Example: BW = 500KHz, SF = 7,
𝑅!”” = # 7 $%% = 21.875Kb/s
SF
Chirps/symbol
Bitrate
7
128
5.469kb/s
8
256
3.125kb/s
9
512
1.758kb/s
10
1024
977b/s
11
2048
537b/s
12
4096
293b/s
$ &!
• 50Kb/s achievable with FSK modulation instead of CSS
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LoRaWAN protocol stack
• Lightweight and secure stack layered on top of PHY
• Access to the medium regulated by gateway; slotted channel, different operating regimes configurable
• Payload encrypted at end node; only application can decrypt (message hidden to gateway)
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LoRaWAN device types
Three classes of the devices defined in LoRaWAN specification
• Different channel’s access behaviors and reliability offered with each class
• Class A: Slotted channel, ALOHA-style access; each uplink transmission followed by two
short downlink receive windows; any device must support Class A operation
• Class B: Additional receive windows available to end nodes at scheduled times → downlink with deterministic latency
• Class C: Continuously open receive widow, except when transmitting → network server can initiate transmission any time (low latency, but continuous power required)
• No acknowledgement of receipt, unless explicitly requested by sender
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LoRaWAN packet structure
• CRC available only for uplink messages
• MAC header indicates protocol version and message type (management, uplink/downlink, ACK required)
• Application payload between 51 and 222 bytes, depending on data rate.
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LoRaWAN security
Two layers of security
Network Security Key (nwkSkey) – secures link between end node and gateway
MIC calculated over the “network” part of the message – works as a signature
Application Security Key (appSkey) – end-to-end confidentiality between end device and application → network operator cannot inspect data; only service provider can
Symmetric encryption (AES-128) secures both link and end-to-end communication
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Device activation
Over The Air Activation (OTAA)
• End device follows a join procedure
• Device can attach to any LoRaWAN network
• Network keys updated on a per session basis
• Roaming possible
• Network server must respond to join requests each time a device (re)starts that adds overhead
Activation by Personalization (ABP)
• End device pre-registered to the network
• DevAddr and keys stored in end device (must be
secured) and network server
• End node tied to a particular network
• Simpler from application server point of view
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Narrow-band IoT (NB-IoT)
Extension to cellular networks to support IoT applications
• Standardized by 3GPP (Release 13)
• Particular focus on reliability (licensed spectrum deterministic transmissions)
• Applications: smart cities (fault
diagnostics and control), eHealth
• Deployment types:
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NB-IoT characteristics
Bandwidth: 180kHz
Data rates: 25kb/s (downlink) and 64kb/s (uplink, multi-tone)
Latency: < 10ms (transmissions aligned with LTE frames)
Reliability: Hybrid ARQ scheme
Energy efficiency: base station controls transmit power and sleep modes via signaling
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Cellular IoT (CIoT) architecture
Apart from a new radio interface, service capability exposure function (SCEF) added
Red path: control plane Blue path: user plane
• UE – User Equipment
• RAN – Radio Access
Network
• SGW – Serving Gateway
• PGW – Packet gateway
• MME – Mobility Management Entity
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NB-IoT downlink
• Each NB-IoT subframe spans one physical resource block (PRB)
• One PRB spans 12 subcarriers in the frequency domain and one time slot (1ms)
• Frame duration: 10ms
• Data transmitting in Narrow-band Physical Downlink Shared Channels (NPDSCH)
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NB-IoT uplink
Up to 48 subcarriers can be configured
• New single tone signal with frequency hopping used on Narrow-band Physical Random Access Channels (NPRACH)
• UE randomly select one subcarrier to
transmit preamble
• Hops one subcarrier between symbols 1–2 and 3–4 and 6 subcarriers between 2–3
• Then pseudo-random hopping
• Base station responds instructing which
uplink resources to be utilized for TX
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