2013 Sixth International Workshop on Selected Topics in Mobile and Wireless Computing

On the Performance of ZigBee Pro and ZigBee IP in IEEE 802.15.4 Networks Mirko Franceschinis, Claudio Pastrone, Maurizio A. Spirito

Claudio Borean Research and Prototyping Telecom Italia Turin, Italy [email protected]

Pervasive Technologies (PerT) Area Istituto Superiore Mario Boella (ISMB) Turin, Italy [email protected]

HTTP. The convenience in using CoAP instead of HTTP is twofold: on the one hand the protocol is simpler, with less overhead; in addition, HTTP is built on top of the more complex TCP while CoAP relies on the lighter UDP as Transport (TRN) layer protocol.

Abstract— The wide spread of smart embedded devices and the trend towards their inclusion in a unique Internet of Things are making urgent their full integration within the TCP/IP protocol stack. This paper considers ZigBee, a short-range radio communication standard for embedded devices, and compares two alternative protocol stacks, namely ZigBee Pro and ZigBee IP, developed for the integration of ZigBee in the so called Internet of Things. The performance evaluation is founded on experimental results achieved using two test-beds built to this purpose and is based on service reliability and latency as metrics.

Coherently with the progressive inclusion of heterogeneous radio communication technologies and mobile smart objects in the future Internet architecture, the main goal of this paper is a performance comparison between ZigBee Pro and ZigBee IP, two alternative approaches designed for integrating ZigBee [5] in the IoT. They differ for how the integration is obtained and where it is positioned. ZigBee Pro leverages a gateway to interconnect a traditional ZigBee network to the TCP/IP domain; there is no awareness of Internet protocols in the ZigBee network. ZigBee IP moves some protocols of the TCP/IP stack directly in the ZigBee network: HTTP over TCP and (optionally) CoAP over UDP. ZigBee IP is an architecture key element being the only IEEE 802.15.4-based stack on top of which the Smart Energy Profile 2.0 (SEP 2.0), an Application (APP) layer under development, can be used.

Keywords— IEEE 802.15.4, ZigBee, 6LoWPAN, HTTP, CoAP, Internet of Things

I.

INTRODUCTION AND MOTIVATIONS

Recently, the traditional Internet has been undergoing a substantial evolution which is often referred to as the Internet of Things (IoT). The innovative trend that has been emerging is the key factor to enable a seamless integration of smart objects and communication technologies, which can be assimilated to different languages spoken by a population of heterogeneous objects. The IoT revolution is expected to lead to a unified framework for the management of a unique big communication network, the Internet of the future. The growing interest of different standardization organizations in defining full IP-based solutions for the integration of embedded devices in the IoT is also clearly emerging as a key driver for enabling a number of applications, with Smart Grids as one of the most promising application field for the IoT.

II.

A. IEEE 802.15.4 Standard IEEE 802.15.4 has been pushed by the need for addressing industrial, scientific and medical (ISM) applications by means of low-cost and low-power wireless devices. The standard includes the full specification of PHY and MAC layers for low rate wireless personal area networks (LR-WPAN). Routing protocols are outside the scope of IEEE 802.15.4, even if three different network topologies can be built on top of it: star, peerto-peer and cluster-tree.

The introduction of IPv6 has dramatically expanded the set of available IP addresses, today based on IPv4 format and next to the full depletion, permitting to almost indefinitely extend the number of Internet hosts. The definition of 6LoWPAN [1], an intermediate layer allowing the underlying Physical (PHY) and Medium Access Control (MAC) layers of the IEEE 802.15.4 standard [2] to be easily integrated in the IP world through IPv6, lets any object endowed with a communication interface make part of the network community. Conceptually similar to 6LoWPAN, other integration layers are currently developed to bring short range wireless technologies under the IP domain [3]. More recently, considering that embedded devices are seriously constrained in terms of resources they are endowed with (memory, energy, processing power), CoAP [4] has been proposed as a light alternative to the well-known

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STATE OF THE ART OF TECHNOLOGIES

Three frequency bands are available: 868 MHz (Europe), 915 MHz USA) and 2.4 GHz (worldwide). A direct sequence spread spectrum (DSSS) modulation allows the PHY layer to assure robustness against interferers whose transmission power is higher than IEEE 802.15.4 devices. Bit rates respectively of 20, 40 and 250 kbps characterize the mentioned bands. The standard classifies the devices of a LR-WPAN in two categories, full-function devices (FFDs) and reduced-function devices (RFDs), assigning only to FFDs advanced capabilities like forwarding packets in multi-hop communications and

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providing node’s synchronization. RFDs communicate with FFD only. An IEEE 802.15.4 network always includes a unique PAN Coordinator, a FFD responsible of initializing and managing the network. Other components with specific roles are Coordinators (only FFDs) and Devices (FFDs or RFDs).

ad hoc routing protocol. At APL: a collection of application profiles related to specific scenarios like Building Automation (BA), Smart Energy (SE) and Home Automation (HA). The last standard revision in 2012 defines a feature set named ZigBee Pro, whose overall architecture is depicted in Fig. 2. It adds new features concerning routing (multi-casting, many-to-one routing) and advanced security mechanisms (symmetric-key key exchange). ZigBee Coordinator, Routers and End-device are the types of devices defined, symmetrically reflecting by roles, functions and capabilities the ones introduced in IEEE 802.15.4. Even the network topologies (star, mesh and cluster tree) partially remind those of IEEE 802.15.4, although the concept of multi-hop routing is peculiar to ZigBee, whose routers act to extend the network coverage.

The channel access scheme for transmitting MAC PDUs depends on the working mode: beacon / non-beacon enabled. If beacons are not conceived, unslotted CSMA-CA is adopted and the PAN coordinator permanently listens to the channel to be able to receive data from RFDs. With beacon enabled mode the PAN coordinator periodically sends the network nodes a beacon containing synchronization information. A superframe, delimited by two consecutive beacons, includes a mandatory activity period and an optional inactivity period. In the former, a contention access period managed by slotted CSMA-CA is followed by an optional contention free period governed by a TDMA-like approach with slots pre-assignment in order to reduce latencies. So far, the standard has passed through three consecutive versions [2]. We consider the one ratified in 2006, on which 6LoWPAN is defined. As depicted in Fig. 1, the general MAC frame format fills 23 bytes for the header, covering different fields. The additional cyclic redundancy check (CRC) bytes and those used for encryption/decryption purposes through AES-CCM code with 32, 64 or 128 bits at the Link layer make the MAC payload to be comprised of a minimum of 81 bytes.

Fig. 2. ZigBee Pro network architecture.

C. ZigBee IP The basic ZigBee IP principle is to push the use of TCP/IP protocol stack in ZigBee-based solutions. An exclusively IPbased system is defined abstracting the protocols at PHY and MAC layers. Fig. 3 displays ZigBee IP architecture, reporting the protocols used at each layer. PHY and Link layers are based on IEEE 802.15.4 in the 2.4 GHz band. NWK layer includes IPv6, ICMP and RPL [6] protocols. RPL is associated with 6LowPAN, the adaptation layer allowing the exchange of IPv6 packets over IEEE 802.15.4 based network. TRN layer can adopt either TCP (mandatory, to support HTTP) or UDP (optional, to support CoAP).

Fig. 1. IEEE 802.15.4 MAC Frame format. Numbers indicate the field size, measured in bytes.

B. ZigBee Pro ZigBee covers low-power wireless networking built on top of IEEE 802.15.4-2003, inheriting the PHY and MAC layers definition. The Network (NWK) layer specification and the framework for the application layer (APL) are instead original. The APL includes the application support (APS) sub-layer, the application framework (AF) and the ZigBee Device objects (ZDO). The APS is an interface between the NWK layer and the AF, the building block which contains application objects where the application logic is implemented.

1) IPv6 and 6LowPAN IPv6 is intended to replace IPv4, whose weakness is the forthcoming exhaustion of the available addresses, due to the increasing number of new devices connected to the Internet. Mapping network addresses on 128 bits instead of 32, an IP address can be assigned to any device in the Internet. In this sense IPv6 is an IoT enabling factor, every smart device being univocally identified within the big network. The differences between IPv4 and IPv6 are not limited to the addressing space (even, they are not interoperable). We are just interested in the significant increases of packet header size from IPv4 (20 bytes, 8 for addressing) to IPv6 (40 bytes, 32 for addressing).

ZigBee presents additional features with respect to IEEE 802.15.4. At NWK layer: ciphering using AES 128-bit; association, configuration, authentication, joining and leaving procedures; neighbor and route discovery; AODV as reactive This paper has been partially supported by the European FP7 project BUTLER, under contract no: 287901. In addition, Telecom Italia wishes to thank KIC EIT ICT Labs, SecSES Secure Energy Systems, that partly supported this research activity as well.

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bytes. If packet fragmentation and reassembling occurs, one additional byte of overhead is required. To conclude the 6LoWPAN overview, the RPL algorithm [6] was designed to deal with IPv6 routing in IEEE 802.15.4 networks. 2) TCP and UDP TCP and UDP are the alternative TRN layer protocols available in the traditional TCP/IP stack: TCP is connectionoriented and offers end-to-end reliability guarantees, UDP provides a connection-less service with no guarantee at all. The respective header sizes (20-24 bytes for TCP, only 8 for UDP) reflect this. The impact on the packet size left free for upper layers protocols is shown in Fig. 5.

Fig. 3. ZigBee IP network architecture.

IPv6 over Low power Wireless Personal Area Networks (6LoWPAN) [1] is an adaptation layer making IEEE 802.15.4 networks coexist with IPv6 protocol stack. To transmit IPv6 packets over IEEE 802.15.4 links, encapsulation and header compression mechanisms are defined. The MTU of an IPv6 packet is 1280 bytes long, not compatible with the maximum MAC frame size defined by the IEEE 802.15.4 standard: 127 bytes, 25 used for frame overhead and 102 for payload. Header for security purposes at Link layer is responsible for consuming up to 21 additional bytes, thus limiting to 81 bytes the available space for an IPv6 packet (see Fig. 1). Being the IPv6 header 40 bytes long, only the remaining 41 bytes could be reserved to the upper layers, as shown in Fig. 4.

Fig. 5. The packet format using TCP (above) or UDP (below). Adaptation layer 6LoWPAN is adopted.

3) CoAP and HTTP The Constrained Application Protocol (CoAP) [4] reminds HTTP but is intended for constrained IoT nodes. The increasing use of smart and mobile devices make SE and BA domains of potential interest for CoAP and other protocols similarly conceived. Bit rate in the order of tens or hundreds of kbps and high error rates radio channel negatively influence communications that often have to rely on multi-hop strategies to obviate the typically short communication range. CoAP is based on a representational state transfer (REST) architecture in which resources are server-controlled abstractions made available by an application process and identified by universal resource identifiers (URIs) [7]. It provides a request/response interaction model between application endpoints and includes key web concepts such as URIs and content-types, making easily HTTP translations for integration with the web. CoAP manages resource discovery on both client and server sides of the protocol.

Fig. 4. Packet fields and available payload size for TRN and upper layers, in case of IPv6 only (above) or with the 6LoWPAN adaptation layer (below).

CoAP works quite similarly to HTTP. A method code is used by a client to send a request for an action on a resource, identified by a URI, available on a server. The server answers the request with a response code. These interactions are managed asynchronously, relying on UDP but exploiting the support for optional reliability with exponential back-off and transaction IDs between end-points transport. CoAP adopts a

Even if UDP is used, only 33 bytes – not enough – are available for application data. Header compression based on HC1-encoding allows 6LoWPAN to compress the 40 bytes of the IPv6 header using just 2 to 7 bytes. The beneficial impact is summarized in Fig. 4 with the two alternatives – with/ without 6LoWPAN – in terms of packets format and used/available

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two-layer approach: a messaging layer to deal with UDP and the asynchronous nature of the interactions; at the same time, the request/response interactions are supervised using method and response codes. CoAP is designed as a single protocol, with messaging and request/response features of the header.

The application associated with the identified scenario has a unique client side, hosted on the PC-test, and multiple server sides, one for each IEEE 802.15.4 node. Requests are sent by the client selectively to one server, which replies back with a response. The specific application depends on the stack, in particular the APP layer protocol: HTTP or CoAP for ZigBee IP, the application profile for ZigBee Pro. Internally, the IEEE 802.15.4 network exploits multi-hop routing capabilities to deliver client requests to the selected server node: the routing protocol is AODV for ZigBee Pro and RPL for ZigBee IP.

HTTP uses TCP as TRN layer protocol, while CoAP uses the lighter UDP. HTTP header is human readable: it directly appears in text format and its size can considerably vary. It is frequently larger than 60 bytes thus requiring 6LoWPAN packet fragmentation. ‘Status code’ and ‘content type’ are the only mandatory fields: a short header is about 40 bytes, which makes possible to send no more than around 20 bytes without incurring in packet fragmentation. CoAP has a shorter header: 4 or 5 bytes, such that fragmentation is avoided.

The common reference network scenario, so far conceptual, has been put into effect and mapped into two different testbeds, respectively correspondent to the ZigBee Pro and the ZigBee IP configurations. The unique ZigBee IP test-bed can be fruitfully utilized for both the protocol choices, HTTP over TCP and CoAP over UDP.

The function of HTTP in ZigBee IP is to let the exchange of XML-based messages. In order to decrease the size of the exchanged messages, reducing XML verbosity and the cost of parsing, EXI (Efficient XML Interexchange), a binary XML format, is a solution. Beside HTTP, service discovery protocols are exploited at APP layer, enabling address resolution and service discovery without the need for a central server. III.

1) ZigBee Pro Configuration Fig. 7 shows the test-bed configured for ZigBee Pro, which includes the following hardware and software components. x

A PC-Test with Windows XP SP3 hosts the clientside of TestHarness application [8], implemented in C#; the PC-Test runs the tests and collects measurements too

x

5 ZigBee nodes – HA profile

FRAMEWORK FOR EXPERIMENTAL COMPARISON

A. Experimental Framework: Architecture and Scenario Three stacks, in which the protocols/standards are reported by increasing layer, are considered for comparison purposes: 1.

IEEE 802.15.4 – ZigBee Pro - ZigBee Gateway

2.

IEEE 802.15.4 - 6LoWPAN - IPv6 - TCP – HTTP

3.

IEEE 802.15.4 - 6LoWPAN - IPv6 - UDP - CoAP

o

1 node, connected to PC-test, as ZigBee node attached to the ZigBee Gateway

o

4 ZigBee nodes as HA smart plugs, one of them hosts the application server-side

The first architecture concerns ZigBee Pro which exploits a ZigBee Gateway to connect IEEE 802.15.4 with TCP/IP stack. The second and the third items refer to ZigBee IP, simply differing for the highest protocol layers: HTTP over TCP alternatively to CoAP over UDP. A reference network scenario has been identified to run meaningful tests. Schematically reported in Fig. 6, it involves the following main architectural pieces: x

4 nodes (labeled Node 1, …, Node 4) belonging to an IEEE 802.15.4 network

x

1 PC-test running the application that launches, coordinates and generally executes the tests

x

an interface – the GW node – to interconnect the PC-test with the IEEE 802.15.4 network

Fig. 7. ZigBee Pro test-bed.

2) ZigBee IP Configuration The test-bed enabling both the ZigBee IP configurations is schematically depicted in Fig. 8.

Fig. 6. Common network scenario.

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x

4 Zolertia Z1 devices create an IEEE 802.15.4 network and leverage Contiki OS [9] v2.x, which provides an implementation of 6LoWPAN

x

One node selected as the recipient of the APP request during a test works as a HTTP/CoAP server, depending on the ZigBee IP version

x

A PC-test with Ubuntu OS v11.10 hosts a HTTP/ CoAP client, implemented in Java language

x

1 additional Zolertia Z1 node connected to the PCtest exposes border router functionalities to let the client application on the PC-test interact with the server application in the IEEE 802.15.4 network

x

IV.

An application available in Contiki OS, combined with the Zolertia Z1 node acting as border router, creates a virtual interface between the IEEE 802.15.4 network and the IPv6 host on the PC-test

EXPERIMENTAL TESTS

A test campaign characterized by the following statements, settings and actions has been carried out.

In Fig. 8 the cloud represents the IEEE 802.15.4 network whose Zolertia Z1 nodes resort to multi-hop to communicate with each other. The fifth Zolertia Z1 mote, as a border router, is a network interface allowing the interaction under the same IPv6 domain between IEEE 802.15.4 nodes and PC-test. In the figure Node 3 is assumed to run the application server side.

x

GET requests are respectively associated with ZigBee Pro through the TestHarness application and with ZigBee IP through HTTP/CoAP

x

To avoid packet fragmentation using ZigBee IP, CoAP/HTTP requests have a limited APP payload size: 3-byte resource size assures no fragmentation at all, 20 bytes guarantee CoAP only

x

For each test session, the interval between two consecutive requests is equal to 2 s, the test ends when 2000 GET requests are successful

x

The GET request target, selected at the beginning of the test session, is kept valid until its conclusion

x

Although AODV and RPL are the ZigBee Pro and ZigBee IP multi-hop routing protocols in IEEE 802.15.4 networks, in this scenario the path to and from the network interface is forced so that node i is in direct connection with nodes i-1 and i+1 only (if existing), thus configuring a linear topology as reported in Fig. 8. Two viable solutions have been identified to practically obtain this: manual setting of routing tables locally at each node when HTTP/CoAP is considered, a deployment forcing the network topology as far as ZigBee Pro is used.

Fig. 8. ZigBee IP test-bed.

Additional software information is not included in Fig. 8. The Java module realizing HTTP/CoAP client functionalities makes use of the software libraries Jetty (for HTTP) and Californium (for CoAP). The alternative JCoAP library was tested too, but finally discarded. The command format at the client side specifies the resource URI to be queried, the type of request (e.g., GET), the time interval between two consecutive requests. The cumulative number of requests, the overall percentage of requests obtaining a successful response and the mean time elapsed between request transmission and response reception are computed and updated. The implementation jointly provided with Contiki distribution has been used for HTTP while Erbium [10] library has been adopted for CoAP.

A. Performance Evaluation TABLE I. displays the percentage of GET requests that have received no response by a specific target node. Losses are negligible, performances are good in any scenario (the reported number of bytes – 3 or 20 – refers to the resource size).

The invoked resource is responsible for packet size. In principle it can be chosen as desired. However, fragmentation occurs if an XML file of about 100 bytes is requested. CoAP does not suffer from fragmentation if the resource content size is limited to about 10 bytes. HTTP needs a payload of at most a few bytes to avoid fragmentation.

TABLE II. shows the average delay (in ms) of successful GET requests for different target nodes, selected APP protocol and (for CoAP) payload size. It can be observed that: x

ZigBee Pro latencies are about halved w.r.t. ZigBee IP, either with CoAP or HTTP, but the order of magnitude is hundreds of ms for both

x

CoAP performs slightly better than HTTP, as shown by delays related to requests with same size

x

The impact of the payload size is negligible, provided that packet fragmentation does not occur

B. Performance Metrics 1) Unsatisfied Request Rate The probability that a request might not be satisfied is calculated as the ratio between the number of requests that do not get back a response and the total number of requests generated by the client. The unsatisfied request rate is a pure number representing a percentage of unfavorable events.

TABLE I.

2) Service Latency On a per-request basis, the delay is defined as the period elapsed between the instant when the client side of the APP protocol assigns a GET request to the TRN protocol and the instant when the related response is delivered (successful request) at the same layer. Delay distributions and moments of different order (e.g., the mean value) can be derived by aggregating delay samples according to some criteria.

Application HTTP (3 bytes) CoAP (3 bytes) CoAP (20 bytes) ZigBee Pro

UNSATISFIED REQUEST RATE [%] Number of Routing Hops 1-hop

2-hop

3-hop

4-hop

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

0.25 0.25 0.20 0.00

0.60 0.60 0.65 0.00

A linear regression on the experimental samples reported in TABLE III. using both the functions y = m1 * x + q1 and y = m2

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* x (forcing q2 = 0) in the unknown m1, m2 and q1, where x is the number of hops. We can conclude that the contribution to the final latency due to each additional hop is approximately constant. The estimated values of m1, m2 and q1, approximated to the closest integer (1 ms accuracy) are reported in TABLE III. along with the related Mean Squared Error (MSE) in ms2. TABLE II. Application HTTP (3 bytes) CoAP (3 bytes) CoAP (20 bytes) ZigBee Pro TABLE III. Application HTTP (3 bytes) CoAP (3 bytes) CoAP (20 bytes) ZigBee Pro

MEAN DELAY [MS] Fig. 9. Delay probability distribution function for HTTP.

Number of Routing Hops 1-hop

2-hop

3-hop

4-hop

275 228 227 108

550 456 454 226

841 683 686 345

1134 919 919 485

ORDINARY LEAST SQUARES FOR LINEAR REGRESSION Linear Regression Coefficients m1

q1

MSE

m2

q2

MSE

287 230 231 125

-17 -4 -6 -22

22.70 5.25 2.45 35.25

281 229 229 118

0 0 0 0

70.87 7.29 7.49 112.30

Fig. 10. Delay probability distribution function for CoAP.

Fig. 9, Fig. 10 and Fig. 11 show the delay probability distribution functions (pdf) respectively for HTTP, CoAP (both with 3-byte resource size) and ZigBee Pro. They provide an alternative way to show that latencies under ZigBee Pro are shorter than ZigBee IP in both its versions. V.

Fig. 11. Delay probability distribution function for ZigBee Pro.

CONCLUSIONS

This work is motivated by the large number of embedded devices endowed with an IPv6 address expected to make part of a unique big network: adapting the complex protocols of TCP/IP stack to smart resource-constrained devices is a must.

REFERENCES [1]

IETF, “IPv6 over Low power WPAN (6lowpan)”, [Online], available: http://datatracker.ietf.org/wg/6lowpan/ [2] “IEEE Std. 802.15.4-2003/2006/2011, IEEE Standard forWireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LRWPANs)”, October 2003, September 2006, June, 2011. [3] J. Nieminen, B. Patil, T. Savolainen, M. Isomaki, Z. Shelby, and C. Gomez, “Transmission of IPv6 Packets over Bluetooth Low Energy,” IETF, 2012. [4] Z. Shelby, K. Hartke, C. Bormann, and B. Frank, “Constrained Application Protocol (CoAP)”, [Online], July 16, 2012, available: http://datatracker.ietf.org/doc/draft-ietf-core-coap [5] “ZigBee Alliance website”, [Online], available: http://www.zigbee.org/ [6] Hui, J. and JP. Vasseur, “The Routing Protocol for Low-Power and Lossy Networks (RPL) Option for Carrying RPL Information in DataPlane Datagrams”, RFC 6553, March 2012. [7] W. Colitti, K. Steenhaut, and N. D. Caro, “Integrating Wireless Sensor Networks with the Web,” in Extending the Internet to Low power and Lossy Networks (IP+SN 2011), Chicago, Illinois, USA, 2001. [8] Presented at the Fourth International Conference on Sensing Technology (ICST), Lecce, Italy, June 3-5, 2010 [9] Contiki web page, http://www.contiki-os.org/ [10] http://people.inf.ethz.ch/mkovatsc/erbium.php [11] C. Pastrone, C. Borean, “GAL: A Gateway Abstraction Layer for fast application development in ZigBee Networks”, in 2nd European ZigBee Developers Conference, Munich, Germany, June 2008. [12] Energy@home Whitepaper, http://www.energyhome.it/Documents/Others/110518Energy@Home_Whitepaper

A performance comparison of ZigBee IP and ZigBee Pro, two alternative approaches for the integration of ZigBee with the TCP/IP stack, has been carried out. The analysis, conceived on the percentage of unsatisfied requests and on the responses latencies, proves that: (a) both the approaches guarantee negligible request losses and (b) ZigBee Pro outperforms ZigBee IP, which can however achieve better performance if CoAP is used instead of HTTP. Nonetheless, it is worth stressing that, even if some adaptation is needed with the actual Internet, ZigBee IP brings TCP/IP directly within the network, while ZigBee PRO can be exposed to TCP/IP environment only through a ZigBee Gateway [11]. Further considerations concern service latencies. First, the impact of the payload size is negligible only if fragmentation does not occur; otherwise, delays can jump from hundreds of milliseconds to several seconds. Instead, the number of hops is always responsible for growing delays. Second, latencies can be remarkable, especially in ZigBee IP networks, in presence of multiple hops: the observed mean delay is 485ms for ZigBee PRO, 919ms/1134ms for ZigBee IP with CoAP/HTTP, when routing relies on four hops. Specific Energy@Home [12] use cases, strictly requiring limited service latencies, could suffer from too many hops: alternative communication technologies like Wi-Fi could be identified as more adequate.

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