IoTSSC - ZigBee, Wi-Fi ZigBee Low power low rate personal area - - PowerPoint PPT Presentation

IoTSSC - ZigBee, Wi-Fi

ZigBee

• Low power low rate personal area networking
• 10-20m range - why somewhat wider range?
• Zigbee operates in the 868MHz band in Europe

(915MHz in the US and Australia)

• Multiple channels in 2.4GHz band also allowed

worldwide

• Path loss is frequency dependent

𝑀 𝑒𝐶 = 20 log 𝑒 + 20 log 𝑔 + 92. 45 (d expressed in km, f expressed in GHz)

ZigBee applications

• Data rates are very low – 20kb/s in the 868MHz band,

250kb/s in the 2.4GHz band

• Typical applications: home/building automation,

industrial control, smart energy (Hive uses this).

• Star or peer-to-peer network topologies
• Two types of device:
• Full Function Device (FFD) - Can communicate with every type
• f device. Three modes: PAN Coordinator (Sends beacons

manages network-specific addresses), coordinator (acts as router), normal node.

• Reduced Function Device (RFD): Can only talk to a single FFD

ZigBee topologies

Credits: Christian Trödhandl

Protocol stack

Credits: A. Elahi and A. Gschwender

PHY and MAC layers

• The ZigBee PHY and MAC layers follow the IEEE

802.15.4 standard

• Transmission using Direct Sequence Spread

Spectrum (DSSS) – robust to noise.

• Multiple MAC layer modes
• CSMA/CA – wait random backoff before sensing the

channel if medium idle, transmit; otherwise repeat backoff; ACKs following data frames are optional

• Beacon mode – time divided into 16 equal slots;

coordinator send beacon first; attached devices can contend but need to align TX to slot; up to 7 guaranteed time slots (GTS) – for low latency apps

PHY and MAC layers

• CAP – Contention-based Access Period
• CFP – Contention Free Period

Data frame structure

Network layer (NWK)

In charge of

• Network start up
• Neighbour discovery, new device configuration
• Topology specific routing
• Security
• AES-128 encryption on links, key assumed known by all

parties (e.g. installed at manufacturing)

• Often this is not the case, a device not pre-configured

may be sent the key over the air (short window when this weakness can be exploited!)

Application Support Sublayer (APS)

• Maintain binding and groups tables
• Discovering devices and application services
• Provide communication endpoints for applications
• Initiating/responding to binding requests between

endpoints

• Filtering packets for non-registered end-points
• Fragmentation, reassembly and reliable data

transport

ZigBee in the IoT landscape

• Your phone most likely does not talk Zigbee
• A hub comprises a Zigbee Coordinator and some

Internet connectivity via a Wi-Fi/cellular/Ethernet interface

WLANs based on 802.11

• Wi-Fi (based on IEEE 802.11) by far the most popular WLAN

technology

• Two modes of operation: infrastructure and ad-hoc
• Infrastructure:
• De facto and the most wide spread
• Access Point (AP) man

anages access to wireless medium (synch, association, authentication, keys)

• However actual channel access is decentralised → each

station responsible for its own behaviour (within some protocol rules)

WLANs based on 802.11 (Wi-Fi)

• All communication is via AP
• Network can operate at
• 2.4GHz (802.11b/g) – 3 non-
• verlapping 20MHz channels;

max bitrate: 54Mb/s (OFDM)

• 5GHz (802.11a) - Depending on

country 12-28 channels; shorter range; Up to 108Mb/s (turbo: two 20MHz channels bonded)

802.11 channel access

• Carrier Sense Multiple Access with Collision avoidance (CSMA/CA)

and binary exponential backoff (i.e. CW doubled upon collisions)

• Access to medium regulated by an Arbitration Interframe Space

(AIFS), Contention Windows (min and max) and optionally number

• f transmission opportunities (packets in a burst).
• Parameters fixed by default (irrespective of network conditions) ->
• ften channel time wasted or too many collisions

802.11 MAC performance

• Example with 5 and 30 stations

(11Mb/s bitrate – 802.11b)

• Default CWmin = 16 sub-optimal
• The AP could potentially

compute the optimal settings if intended, and distribute these via management frames (beacons) every 100ms

How much throughput can a station obtain?

• Assume a network with n contenders
• Working with CWmin= 16, maximum number of attempts K
• Backlogged queues
• Denote τ the probability that a station transmits in a

randomly chosen slot time

• The conditional collision probability experienced by a frame
• Use a renewal-reward approach to compute τ, i.e. calculate

the expected number of slots and transmissions between the renewal the completion of packet transmissions.

𝑞 = 1 − 1 − 𝜐 𝑜−1

How much throughput can a station obtain?

• The expected number of attempts to transmit a packet is
• The expected number of slots during back-off

where bk = 2kCWmin/2 is the mean length of back-off stage k

• The transmission attempt rate can be expressed as

𝐹(𝑆) = 1 + 𝑞 + 𝑞2 + ⋯ + 𝑞𝐿 𝐹(𝑌) = 𝑐0 + 𝑞𝑐1 + 𝑞2𝑐2 + ⋯ + 𝑞𝐿𝑐𝐿 𝜐 = 𝐹(𝑆) 𝐹(𝑌)

How much throughput can a station obtain?

The throughput a station obtains is basically the average amount of data transmitted over the average slot duration A slot can be idle, or occupied with a successful transmission or a collision. The duration of an idle slot is fixed by the standard (e.g. 9μs for 802.11a/g). If packets are of the same length L and transmitted with the same PHY rate C, the duration of a slot containing a successful transmission is the same as the duration of a slot that contains a collision, i.e.

𝑈𝑐𝑣𝑡𝑧 = 𝑈𝑄𝑀𝐷𝑄 + 𝑀 𝐷 + 𝑇𝐽𝐺𝑇 + 𝑈

𝐵𝐷𝐿 + 𝐵𝐽𝐺𝑇

𝑇 = 𝐹[𝐸𝑏𝑢𝑏] 𝐹[𝑇𝑚𝑝𝑢]

How much throughput can a station obtain?

Hence the average duration of a slot is The throughput obtained by a station is thus

𝑇 = 𝜐 1 − 𝑞 𝑀 1 − 𝜐 𝑜𝑈𝑗𝑒𝑚𝑓 + 1 − 1 − 𝜐 𝑜 𝑈𝑐𝑣𝑡𝑧 𝐹 𝑇𝑚𝑝𝑢 = 𝑄𝑗𝑒𝑚𝑓𝑈𝑗𝑒𝑚𝑓 + 1 − 𝑄𝑗𝑒𝑚𝑓 𝑈𝑐𝑣𝑡𝑧 = 1 − 𝜐 𝑜𝑈𝑗𝑒𝑚𝑓 + 1 − 1 − 𝜐 𝑜 𝑈𝑐𝑣𝑡𝑧

Hidden terminal problem

A B C D Radio Range A is transmitting to B. If C senses medium it will not hear A (out of range) and so it could transmit and destroy the frame sent from A to B. Not being able to detect (carrier-sense) a potential competitor is called the “hidden terminal problem”

Exposed terminal problem

A B C D Radio Range B is transmitting to A. If C senses the channel, it will hear the activity and falsely conclude it cannot transmit to D. This is called the “exposed terminal problem” As you may conclude, it is all about knowing what is going on at your receiver. What the sender “sees” may not be what the receiver will see.

Request to Send/Clear to Send

RTS/CTS control frames help mitigate this problem but may introduce additional overhead However, also useful to avoid collision when bursting

802.11 frame structure

Statistical traffic differentiation

Current hardware supports multiple queues, one for each “Access Category” (AC):

• Best Effort (BE), Background

(BK), Video (VI), Voice (VO)

• Additional Content after

beacon queue (CABQ) typically used for multicast traffic.

• Beacon queue (which does not

really work like a queue: always stores ONE frame that gets updated)

802.11n enhancements

• Channel bonding (up to 40MHz)
• Multiple-Input Multiple Output (MIMO)

– up to 4 spatial streams

• Superior modulation and coding schemes (with 64-

QAM - up to 65Mb/s per single stream, or 72.2Mb/s if shorter guard between packets)

• Frame aggregation (lower overhead)
• Selective retransmissions

802.11n frame aggregation

Source: Cisco

802.11ac enhancements

• Channel bonding up to 160MHz (2x 80MHz streams)
• Highest modulation and coding schemes supported is

256-QAM (433Mbs/s on single 80MHz channel)

• Up to 8 spatial streams
• 802.11ac introduces multi-user MIMO (Gb/s rates) –

the idea is to send a combination of packets to multiple clients at the same time – spatial multiplexing (good channel sounding required)

• Question: is so much throughput needed for IoT

applications?

Questions?