Internet of Things Amr El Mougy Alaa Gohar

ZigBee Older and better established than Bluetooth (more trials) Key advantage: mesh networking capabilities Protocol stack is heavier than BLE Higher energy consumption than BLE Lower rates than BLE: 250 kb/s (2.4GHz), 40 kb/s (915MHz), 20 Kb/s (868 MHz) Longer range than Bluetooth (up to 300m) Key applications: smart grids, monitoring and surveillance, healthcare, M2M

ZigBee Stack PHY supports operation in multiple ISM frequencies (over 26 channels), including 2.4GHz. Rates up to 250 Kb/s MAC is very versatile. Supports star and mesh topologies, contention-based and contention-free communications, beacon and non-beacon modes, acknowledged and unacknowledged communications NWK layer supports starting a network, joining/leaving a network, addressing (using 64-bit MAC), synchronization, security, routing Application profiles include home automation and energy monitoring

IP-Based ZigBee Benefits of supporting IP-based ZigBee networks The pervasive nature of IP networks allows use of existing infrastructure. IP-based technologies already exist, are well-known, and proven to be working. Open and freely available specifications vs. closed proprietary solutions. Tools for diagnostics, management, and commissioning of IP networks already exist. IP-based devices can be connected readily to other IP-based networks, without the need for intermediate entities like translation gateways or proxies. Challenges IPv6 header size is 40 octets. UDP header is 8 octets, MAC header in 802.15.4 can be up to 46 octets (if security is enabled)  may leave only 33 octets for application layer data (ZigBee packets can be 127 bytes max) IPv6 requires that links support frames of 1280 octets because it does not allow fragmentation  fragmentation cannot occur at the network layer, has to occur at the link layer

IPv4: we have a problem

IPv6

IPv4 vs. IPv6 Addresses

Solution: 6LowPAN The 6LowPAN protocol is an adaptation layer allowing to transport IPv6 packets over 802.15.4 links Uses 802.15.4 in unslotted CSMA/CA mode (strongly suggests beacons for link-layer device discovery) Based on IEEE standard 802.15.4-2003 Fragmentation / reassembly of IPv6 packets Compression of IPv6 and UDP/ICMP headers Mesh routing support (supports star and mesh topologies) Low processing / storage costs

6LowPAN Stands for IPv6 over Low Power Personal Area Networks It is a framework for supporting IPv6 over 802.15.4 links 802.15.4 packets are encapsulated in an IPv6 datagram Provides routing, compression, and fragmentation of IPv6 packets ZigBee 6LowPAN-hc supports auto-configuration of IP addresses and compression of IPv6 headers ZigBee 6LowPAN-nd supports neighbor discovery in the presence of sleepy nodes Considers neighbor lifetime Max 127 octets Preamble 802.15.4 MAC Header IPv6 Header UDP Payload FCS 23 40 8 54 2

ZigBee IP Stack A collection of standards and specifications defined for interoperability and streamlining IEEE 802.15.4-2006 MAC/PHY IETF 6lowpan-hc adaptation layer and IETF 6lowpan-nd neighbour discovery IPv6 network layer RH4 routing header Hop-by-hop header RPL option TCP/UDP transport layer IETF ROLL RPL routing PANA/EAP/EAP-TTLSv0/TLS security Public key (ECC and RSA) and PSK cipher suites mDNS/DNS-SD service discovery support

ZigBee IP Stack NWK layer supports different routing algorithms and route repair NWK layer supports neighbor discovery Routing is energy-efficient (ROLL = routing over low power and lossy networks) TCP and UDP are supported for various IoT applications A variety of security options are available Methods for hosting a DNS server on every device for enabling service discovery

Node Types Full Function Device (FFD): Can communicate with every type of device. A FFD can operate in three different modes: PAN Coordinator: Sends beacon frames, provides routing information, manages short, network-specific addresses Coordinator: Acts as router Normal device Reduced Function Device (RFD): Can only talk to a single FFD

Network Topologies Peer-to-Peer Clustered Network Star Topology Each FFD can communicate with any other device within its range. A RFD may only communicate with a single FFD at a given time. Network Topologies Clustered Network Used for establishing larger networks. Each cluster has a single cluster head that is responsible for coordination within the cluster. The PAN coordinator manages routing and synchronization functions for the entire network. In very large networks, multiple PAN coordinators may be present Star Topology The PAN coordinator chooses a unique (within its radio range) PAN id. Nodes that perform association can only talk with this PAN id

Medium Access Control (MAC) Layer MAC operates in Beacon or Beacon-less modes Beacon mode is managed by the FFD, which will periodically broadcast beacon messages announcing the beginning of a frame In beacon-less mode, nodes communicate using Carrier-Sense Multiple Access (CSMA) – more on this later MAC also supports acknowledgments Association/disassociation with PAN coordinator Nodes are identifiable by a 64-bit MAC address PAN coordinator maintains a list of addresses for the network

Carrier Sense Multiple Access Any node receiving two or more frames at the same time will not be able to distinguish them  collision To reduce the chance of collisions, nodes listen before they talk and wait for a random period called the backoff If the medium is busy, the node waits for a random duration and repeats the process Acknowledgments are optional. If employed, they are sent without CSMA Hidden Terminal Problem Exposed Terminal Problem

Beacon Mode A round (superframe) is divided into 16 equally sized slots. Coordinator regularly sends beacon frames in the first slot. The beacon frames are used to synchronize the attached devices, identifies the PAN, and describes the superframe structure. Any device that wishes to send data uses the CSMA/CA mechanism, but aligns the sent frames to the slots. The PAN coordinator may assign guaranteed time slots (GTS)to devices for low-latency or fixed data bandwidth. Up to 7 GTS can be allocated in this way at the end of the superframe.

Frame Types The IEEE standard defines four different frame types: A beacon frame: Sent by the coordinator to announce the network and contains the superframe structure. A data frame: Used for data transfer An acknowledgment frame: To confirm the successful reception of a frame. A MAC command frame: For handling MAC peer entity control transfers.

Summary of MAC Protocols BLE advertisements: basic ALOHA (or less) Connected BLE: TDMA ZigBee non-beacon: CSMA ZigBee beacon: mixture of slotted ALOHA and TDMA Each MAC protocol has advantages and disadvantages ALOHA: lowest maximum throughput, simplest protocol. Suitable for low traffic TDMA: Maximum throughput if load is high. Requires central controller. Maybe wasteful is reserved slots are not used CSMA: Compromise between throughput and complexity. Throughput decreases as load increases Slotted ALOHA: somewhere in between ALOHA and CSMA