Author: Pablo Gutiérrez Peón

Deterministic Networking Course

May 2014 Institute of Computer Engineering Vienna University of Technology

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From wired to wireless

Wireless technologies

IEEE 802.15.4 (WPAN)

IEEE 802.11 (WLAN)

IEEE 802.11-based approaches

Summary

The most noticeable differences Wired Wireless

Transmission medium Wire: Twisted pair, coaxial cable, fiber…

Air/vacuum/water: Broadcast radio, terrestrial microwave, satellite…

Type of media Guided: Physical path along which signals are propagated.

Unguided: Employ antenna for transmitting through air, vacuum, or water.

Quality of a data transmission depends on… Medium is decisive.

Bandwidth is decisive. Directionality is also important. Time-varying and asymmetric propagation properties.

Data transfer rate (Usually) Higher than wireless (more reliable). (Usually) Lower than wired (less reliable).

Interferences (competing signals in overlapping frequency bands)

Emanations from nearby cables or unguided transmissions. Of particular concern.

Number of receivers Point-to-point. Point-to-multipoint. Half-duplex/Full-duplex.

(Usually) Half-duplex. Lack of full connectivity (devices might be hidden from each other)

Topologies (Usually) Static Dynamic

Security Needs direct physical access. Anyone can hear the transmission. 3/34

Constraints

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Data transfer rate (Usually) Lower than wired.

Number of receivers (Usually) Half-duplex.

Not only less data can be transmitted…

…but also signal changes threat determinism (real-time)

Interferences Stations can move Other objects can move

+

+ +

The main challenge for real-time

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Interferences

Stations can move

Other objects can move

Assuming that normally, stations are static and other objects have a limited effect on the signal strength when moving…

The main problem are Quite common scenario: Every station which wants to send data starts to transmit

data. But only one station can send data each time. That starts a contention.

To guarantee determinism: All stations that want to exchange information must be

under the same coordination mechanism. Other wireless devices which can cause interferences must

be kept out of the operation area.

Classified by range

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WPAN <10m

~1Mbps

WLAN <100m

~11-54Mbps

WMAN <5km

70Mbps

WWAN <15km

Wireless Wide Area Network LTE

Wireless Metropolitan Area Network IEEE 802.16 WiMAX

Wireless Local Area Network IEEE 802.1 WiGig IEEE 802.11 WiFi

Wireless Personal Area Network IEEE 802.15.1 WPAN/Bluetooth IEEE 802.15.3 High-Rate WPAN / Ultra Wide Band IEEE 802.15.4 Low-Rate WPAN / ZigBee, WirelessHART, 6LoWPAN, MBStar.

Data Link

Layer

<Upper layers>

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Layered protocols

MAC (Medium Access Control layer)

LLC (Logical Link Control layer)

OSI

mod

el

Standardizes PHY and MAC

Alliances/Foundations to promote the standard,

develop new features and ensure compliant devices.

PHY (Physical layer)

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IEEE 802.15.4 (WPAN) - Standard for WPANs. - Used to convey information over relatively short distances. - Involving little or no infrastructure. - Featuring small, power-efficient (to extend battery life), inexpensive solutions. - Low data rate wireless connectivity (up to 250 kbps. Scalable down to the

needs of the sensor and automation). - Simple and flexible protocol.

IEEE 802.11 (WLAN) - Standard to provide wireless connectivity for fixed, portable, and moving

stations within a local area (WLANs). - To operate within independent and infrastructure networks as well as mobility

(transition) within those networks. - Devices can communicate directly with another device outside the

independent or infrastructure network.

IEEE 802.15.4 & 802.11 general features

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Components & topology Full-Function Device (FFD) - Can serve as a personal area network coordinator (at least one coordinator to

form a WPAN). Reduced-Function Device (RFD) - Cannot serve as a coordinator. - Intended for very simple applications (e.g. light switch).

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MAC layer - Deterministic features Beacon - For network discovery. - To support synchronization (only on beacon-enabled PANs). - To describe the GTSs assignment. Two different periods between two beacon frames - Contention Access Period (CAP).

Communication using slotted CSMA/CA. - Contention-Free Period (CFP).

Communication using Guaranteed Time Slots (GTSs).

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MAC layer - Guaranteed time slots GTSs within the Contention Free Period

- Up to 7 GTSs on each CFP. - Each GTS can occupy more than 1 slot.

No interactions with the Contention Access Period

- A sufficient portion of the CAP remains for contention-based of other network devices or new devices wishing to join the network.

- Each device must ensure its transaction is within the limits of the GTS and CFP.

Request of GTSs

- The PAN coordinator allocates GTSs. Stores the information to manage them (starting slot, length, direction and device address).

- GTSs are only available for communications between the PAN coordinator and a device. - Each device may request one transmit GTS and/or one receive GTS. - How to request?

1. A request command shall be sent to the PAN coordinator. 2. On receipt, PAN coordinator shall check if there is available capacity. 3. An ACK to the request command is sent to the device. 4. The device tracks beacons. If no GTS descriptor for the device appears in the

beacon, the GTS was not allocated.

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MAC layer – Data transfer CSMA/CA mechanism

- Non-beacon-enabled PANs use unslotted CSMA/CA. - Beacon-enabled PANs use slotted CSMA/CA aligned with the start of the beacon transmission.

Channel state

- If channel idle: device transmits data. - If channel busy: device waits for a random period before trying to access the channel. ACKs are sent immediately.

Data transfer to a coordinator (beacon-enabled PANs)

1. Device listens for the network beacon. 2. Device synchronizes to the network beacon. 3. Device transmits its data frame to the coordinator. 4. Coordinator sends ACK frame.

Data transfer from a coordinator (beacon-enabled PANs)

1. Coordinator indicates it has a message pending in the beacon. 2. Device receives beacon and transmits a command message requesting the data. 3. Coordinator sends an ACK to indicate the successful reception of the data request. 4. Data is sent by the coordinator. 5. Device sends back an ACK, if requested.

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Components & topology STA (STAtion)

It is the addressable unit (the origin or/and destination of a message).

AP (Access Point)

Provides access to the DS via the wireless medium for associated STAs.

DS (Distribution System)

Interconnects AP.

BSS (Basic Service Set)

Area in which the service is provided.

ESS (Extended Service Set)

Network made of BSSs connected by the DS.

Topology - Infrastructure network. All communications over AP. - Ad-hoc network. Direct communication between STAs. - Mesh network. STAs exchange messages with

neighbours and using the multi-hop capability, messages can be transferred between STAs that are not in direct communication.

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MAC layer - Responsibilities Reliable data delivery a) Two-frame exchange. Acknowledgement of frames. b) Four-frame exchange.

1. Source sends Request To Send (RTS). RTS alerts all stations that are within the reception range of the source that an exchange is under way. These stations refrain from transmission in order to avoid collision.

2. Destination responds with Clear To Send (CTS). Same to RTS, but to alert stations around destination.

3. Source transmits data. 4. Destination responds with ACK.

Access control Distributed Coordination Function (DCF). <Slides #15 & #16> Security -

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MAC layer - Access control (I) Original 802.11 MAC

- Distributed Coordination Function (DCF). - Point Coordination Function (PCF).

802.11e MAC (Quality of Service – QoS amendment)

Hybrid Coordination Function (HCF): - Enhanced Distributed Channel Access (EDCA). - HCF Coordinated Channel Access (HCCA).

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MAC layer - Access control (II) Distributed Coordination Function (DCF)

- CSMA/CA.

Point Coordination Function (PCF)

- AP is the Point Coordinator (PC). Contention and contention-free periods. - PCF can be used to coordinate packet transmission, but it does not rely on timing information, and

thus cannot provide real-time guarantee. - Also, when a station gets access to the channel, it may occupy it for a non-deterministic time

interval.

Enhanced Distributed Channel Access (EDCA) – 802.11e

Based on prioritization. CSMA/CA + Inter Frame Space (IFS) + Backoff mechanism. - Transmission Opportunity (TXOP). STA has the right to transmit as many frames as it can during

its TXOP. - Access Category (AC). Data classified into different priority levels. A station includes up to 4 MAC

queues. AC plays with different sets of parameters (IFS and backoff) to be used for contending the channel.

HCF Coordinated Channel Access (HCCA) – 802.11e

- Channel access handled by the Hybrid Coordinator (HC). - Based on a polling mechanism (stations are polled). HC gives to the stations the right to use a

TXOP.

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MAC layer - Deterministic features (I) 802.11e - Time divided in superframes

- Contention Period (CP). EDCA used. However, HC can initiate Controlled Access Phases (CAP) at any time, whenever a transmission of real-time critical data is necessary.

- Contention Free Period (CFP). HCCA used.

Admission control and scheduling algorithm

Specific ones are not currently recommended by IEEE 802.11 standard. Both components can be designed with respect to the specific application.

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MAC layer - Deterministic features (II) The standard itself emphasizes the limitations of the wireless medium

When providing QoS services, the MAC endeavors to provide QoS “service guarantees” within the limitations of the medium properties. In other words, particularly in unlicensed spectrum, true guarantees are often not possible. However, gradations of service are always possible; and in sufficiently controlled environments, QoS guarantees are possible.

The polling service based on admitted TS provides a “guaranteed channel access” from the scheduler in order to have its QoS requirements met. This is an achievable goal when the WM operates free of external interference (such as operation within the channel by other technologies and co-channel overlapping BSS interference). The nature of wireless communications may preclude absolute guarantees to satisfy QoS requirements. However, in a controlled environment (e.g., no interference), the behavior of the scheduler can be observed and verified to be compliant to meet the service schedule.

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802.11 limitations HCCA overhead - HCCA is the best alternative that 802.11e offers for deterministic

communication. - However, its polling mechanism causes additional overhead. When

transmitting frames with a small payload, it results in a huge communication overhead.

- TDMA is more promising. 2 examples of TDMA-based MACs in the following slides.

802.11e not implemented yet

- There are no COTS (Commercial Of-The-Shelf) devices implemented yet.

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RT-WiFi – Introduction Summary - TDMA-based approach using 802.11.

- It supplements the existing MAC mechanisms. Can coexist with standard 802.11 stations. Transparent to other layers.

- Provides: - Deterministic timing guarantee. - High sampling rate.

- Based on: - Centralized channel and time management (Network Manager) to

access the channel according to a strict time schedule.

Network Manager

- It is running on top of the RT-WiFi Acess Point (AP). - Configures RT-WiFi network based on the requirements specified by a

designer. - Dynamically allocates communication resources and configures data link

layer of RT-WiFi AP and stations.

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RT-WiFi – MAC design (I) Architecture

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RT-WiFi – MAC design (II) Traffic scheduling Link scheduler (@ Network Manager) coordinates channel access among stations. Definitions: - Link. A link defines the communication behavior within a time slot. 4 link

types: transmit, receive, broadcast and shared. - Superframe. Sequence of consecutive time slots (TDMA). Defines

communication pattern with neighbors and will repeat itself infinitely. - Device profile. Each node keeps its device profile. When a device joins the

network, the link scheduler gets its device profile. Then it constructs the link scheduler and sends the superframe and link configuration information back to the station using the beacon frame.

RT-WiFi – MAC design (III) Beacon frame

- Every beacon frame contains the scheduling information for the nodes. - Stations synchronize to the master clock in the RT-WiFi AP periodically through

beacon frames.

Clock synchronization

- The Timer (@ Network Manager) maintains global synchronization among all RT-WiFi nodes and triggers timing events.

- Based on the Timing Synchronization Function (TSF) in IEEE 802.11. - It uses beacon frames to deliver timing information from the AP to the nodes.

Flexible channel access controller

- (@ Network Manager) Configures the hardware parameters based on what the designer specifies. It allows to chose parameters related to sampling rate, reliability and the co-existence with regular WiFi networks.

Association process

1) CSMA/CA to access the channel. 2) 802.11 authentication and association process. 3) After association, RT-WiFi waits for the next beacon frame, which contains TDMA

schedule information. 4) Then, the node synchronizes its local clock with TSF. 23/34

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RT-WiFi – Limitations Not fault tolerance!!!

Latency and packet loss ratio comparison in an interference-free environment: Latency and packet loss ratio comparison in an office environment:

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IsoMAC – Introduction Summary - TDMA-based approach using 802.11.

- It supplements the existing MAC mechanisms. Can coexist with standard 802.11 stations. Transparent to other layers.

- Provides: - A way to satisfy soft real-time flows.

- Based on: - Centralized channel management (flexWARE Controller) to access the

channel according to a strict time schedule. flexWARE Controller - Scheduler and resource manager for the whole system. - Schedules IsoMAC network based on the requirements specified by the

stations. - Dynamically allocates communication resources and configures data link

layer of IsoMAC AP and stations. Can reschedule the communication and distribute new schedule to all involved nodes.

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IsoMAC – MAC design (I) Architecture

flexWARE Controller (FC) - Scheduler and resource manager for the whole system. - Forwards real-time traffic between the backbone and FAPs. flexWARE Access Point (FAP) - Acts as a bridge between the wired and wireless medium. - Represents the master clock for the FN and comprises the MAC. flexWARE Node (FN) - Interconects automation devices through a wireless link with the FAP.

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IsoMAC – MAC design (II) Traffic scheduling (1/2)

Resource request frame: - Used to specify the bandwidth for a traffic flow. - Includes detailed traffic specification (latency, jitter, update time, payload

size). - Can be sent at any time. The allocation is made dynamically. 1) A station (FN) sends a resource request frame to the coordinator (FC). 2) Admission control of the coordinator uses the traffic specification to

decide whether the new node can be accepted or not. - Depending on the already admitted traffic flows and the available

resources.

IsoMAC – MAC design (III) Traffic scheduling (2/2)

Communication cycle: - Scheduled phase (SP). Real-time traffic.

RT traffic transmitted within assigned timeslots during the scheduled phase. Further divided in timeslots for downlink (DL) and uplink (UL).

- Contention phase (CP). Best-effort and management (M) (i.e. messages for clock sync).

Ordinary DCF and EDCA used. At least one data frame can be transmitted.

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IsoMAC – MAC design (IV) Beacon frame - Every beacon frame contains the scheduling information for the nodes. - To avoid a device to start a transmission which can delay beacon, the

Contention Phase has a restricted phase at its ending. Clock synchronization - Uses IEEE 1588 Precision Time Protocol (PTP) and software time-stamping. - The master clock is at the FAP. Error recovery - Uses 802.11 ACKs. - ACKs for downlink frames are postponed till the uplink sub-phase. - The coordinator (FC) uses its knowledge of the schedule to determine if an

error has happened. - Scheduled phase is further subdivided and a recovery phase is added where

frame retransmissions can take place.

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IsoMAC – MAC design (V) Association process

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IsoMAC – Evaluation HCCA vs IsoMAC - Testbench: Time at which a node (FN) receives data updates from 3 traffic flows. - Results: Histograms show that IsoMAC is more accurate than HCCA.

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Determinism on a wireless medium

Main problem: Interferences - Every station which wants to send data starts to transmit data. - But only one station can send data each time. To guarantee determinism… - All stations must be under the same coordination mechanism. - Other wireless devices which can cause interferences must be kept out of the

operation area.

802.15.4

- Determinism using Guaranteed Time Slots (GTS). TDMA mechanism. - Beacon used to send scheduling. Between 2 beacon frames:

- Contention Free Period (CFP). GTS used. - Contention Access Period (CP). CSMA/CA used.

802.11

Two MAC protocols: - EDCA. Based on traffic prioritization. - HCCA. Based on a polling mechanism to give stations Transmission Opportunities

(TXOPs). Between 2 beacon frames: - Contention Free Period (CFP). HCCA. - Contention Period (CP). EDCA.

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802.11 limitations

HCCA polling mechanism - Causes communication overhead (specially when transmitting small payloads). There are no COTS implemented. Therefore, 802.11-based solutions are proposed (RT-WiFi and IsoMAC).

RT-WiFi

- TDMA. - Beacon frame is used to send the schedule (which time slot they can use) to the

stations and synchronize their clocks. - It is not fault tolerance.

IsoMAC

- TDMA. - Beacon frame is used to send the schedule (which time slot they can use) to the

stations. - Clock synchronization using IEEE 1588 Precision Time Protocol. - Time between two beacon frames is divided in:

- Scheduled Phase (SP): Real-time traffic transmitted within assigned timeslots. First downlink and then uplink traffic. It also has a recovery phase.

- Contention Phase (CP): Best-effort and management traffic.

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[1] William Stallings. Data and Computer Communications. Pearson Education, Inc. Upper Saddle River, New Jersey, USA, 8th edition, 2007. [2] IEEE Standards Association. IEEE Std 802.11-2012. Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. 3 Park Avenue, New York, USA, 29 March 2012. [3] IEEE Standards Association. IEEE Std 802.15.4-2011. Part 15.4: Low-Rate Wireless Personal Area Networks (LR-WPANs). 3 Park Avenue, New York, USA, 5 September 2011. [4] Yi-Hung Wei, Quan Leng, Song Han, Aloysius K. Mok, Wenlong Zhang, Masayoshi Tomizuka. RT-WiFi: Real-Time High-Speed Communication Protocol for Wireless Cyber-Physical Control Applications. 2013 IEEE 34th Real-Time Systems Symposium, pp. 140-149. [5] Henning Trsek, Jürgen Jasperneite. An isochronous medium access for real-time wireless communications in industrial automation systems - A use case for wireless clock synchronization. inIT - Institut Industrial IT, Ostwestfalen-Lippe University of Applied Sciences, 32657 Lemgo, Germany. 2011.