Lecture Notes on Wireless Networks, Lecture notes of Wireless Networking

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Wireless networks: lecture notes

R. Combes

January 29, 2016

Introduction

Foreword

This document is based on a course given to master’s students on the topic of “Wire- less Local Area and Ad Hoc Networks”. The course was given in Supelec (France) in January 2015. The target audience for this is engineering/math students with a basic understanding of the following topics

• elementary probability: conditioning, independence, convergence of random vari- ables.
• modelling of wireless channels: distance-dependent path-loss, shadowing, and fast-fading.
• physical layer techniques: channel coding, multiple access schemes (TDMA, CDMA, FDMA etc.)

8 CONTENTS

10 CHAPTER 1. WIRELESS NETWORKS: A PRIMER

• Security: the wireless medium is easily listened to by a malicious entity, and pas- sive eavesdropping (sniffing) is undetectable. Therefore data must be encrypted and authentication must be used to avoid theft of data or identity.
• Topology changes: nodes are mobile, so that their physical location is changing over time. One needs to keep track of nodes to ensure connectivity of the network. Also, in wireless networks with high mobility, connectivity might be intermittent (ON/OFF).

Standards for WLANS are only concerned with PHY and MAC layer, so that cross- layer optimization is usually not standardized, although some proprietary implemen- tations exist. In terms of quality of service, the PHY and MAC of standards such as 802.11 are purely best-effort, so that Quality of Service (QoS) never is taken into ac- count, beyond successful delivery of data.

1.2 Some 802.11 terminology

Although our goal is not to cover the 802.11 standard in detail, we provide some termi- nology used in this standard here. There are several network entities, called as follows:

• ”Stations”: entities exchanging data (laptops, computers, smart phones)
• ”Access points” (APs): gateways to a wired network
• ”Wireless medium”: the medium that carries data frames. It may be either Radio- Frequency or Infra-Red light
• ”Distribution system/backbone”: (only applies when there are several access points) the entity performing localization and routing. This entity is usually Ethernet- based (802.03). The connection between 2 APs might be wireless. In that case one talks about a wireless bridge.
• ”Basic service Set” (BSS): a group of stations and (possibly) access points

There are mainly three types of networks, with the following denomination:

1. Infrastructure BSS (ad-hoc): a set of stations linked directly without an access point. Those netowrks are generally short lived.
2. Infrastructure BSS (classic): a set of stations associated to a common access point. Here the access point relays all communication. Any station attempting to enter such a network must go through a procedure called association. Association is initiated by a station and granted by the access point.
3. Extended service set (ESS): BSS’s chained by a backbone network. An ESS is identified by a single SSID (Service Set IDentifier), which acts as the “name” of the network.

1.3. 802.11 PHY 11

The basic actions that a network can accomplish are called ”services”. Services granted by a network are the following:

• Distribution: move a frame from an AP to a station
• Integration: frame delivery to a non 802.11 network
• Association: register a station to an AP
• Reassociation: change the AP when Quality of Service (QoS) is poor
• Disassociation: termination of existing association
• Authentication: secure exchange of identity prior to sending data
• Deauthentification: termination of authentication
• Confidentiality: prevent eavesdropping through encryption
• MSDU delivery: delivery of data to the recipient
• Transmit power control: control of transmit power used by stations
• Dynamic frequency selection: detect and prevent interference (to other and from other systems e.g radar).

Nodes in a wireless network are, by definition mobile. When there are several access points available, a station should be associated to the access point which maximizes the received signal power. When a station exits the service area of an access point, a transition must occur to ensure that the station does not lose connectivity. There are three types of transitions:

• Absence of transition: a station stays in the service area of a given access point
• BSS transition: a station moves between 2 access points of the same ESS. This transition is in principle seamless and requires exchange of information between access points.
• ESS transition: a station moves between 2 access points of different ESSs. This transition is not seamless, and typically causes an interruption at the appication level. For instance if the station is using a VoIP (Voice over IP) application, this transition should cause a call drop. Seamless transition requires special upper layer protocols not included in the 802.11 standard.

1.3 802.11 PHY

1.3.1 Spectrum and transmit power

Two main physical media can be used:

• Radio Frequency (RF) in unregulated frequency bands (at 2.4 GHz or 5 GHz). The 2.4 GHz frequency band suffers from microwave oven interference, and does not propagate under rain and long distances, but propagates through walls.

1.4. 802.11 MAC 13

achievable data rate with arbitrarly low probability of error is an increasing function of Signal To Noise Ratio (SNR) at the receiver, by Shannon’s noisy channel coding the- orem. Hence, ideally, the receiver should measure the SNR and send that information back to the transmitter before she transmits a packet. In 802.11 there is no such feed- back, so that the transmitter must adjust the data rate based on the successes/failures of previous packets transmissions. A set of rates is defined by the BSS, and transmit- ters may change the data rate on a per-packet basis, including control packets. Recent standards use MIMO and OFDM, so that the number of available data rates is large. Designing a proper rate adaptation mechanism is critical for good performance.

1.4 802.11 MAC

1.4.1 Frame transmission

Modern computer networks are, for the most part, packet-switched networks, and 802. is no exception. Data is partitioned in small elementary units called “packets”, which are then transmitted by the networks in an independent manner. In 802.11 packet are called “frames” and the typical frame size is a few thousand bytes. Since the physical medium is unreliable due to fading, interference and noise, frame transmission follows the following rules:

• Each unicast frame must be acknowledged
• If no Acknowledgement (ACK) for a frame is received, the frame is considered lost, a retry counter is incremented (1 counter per frame) and the frame will be retransmitted at a later time.
• Transmitters are responsible for retransmission
• Each frame updates the Network Allocation Vector (NAV), see below.
• When a higher level protocol attempts to send a packet larger than a threshold , the packet is fragmentated into several frames which are sent separately. The threshold is called Maximal Transmission Unit (MTU).

When a frame is received, the receiver calculates a Cyclic Redundancy Check (CRC), and compares it to the value of the CRC contained in the header of the received frame. The CRC is a logical function of the received bits, ensuring that one may detect any error burst of length less than a fixed size. Typically, a CRC of length m can detect an error burst of length m or less. If the CRC is correct, an ACK frame is sent to the sender. Otherwise a NACK frame is sent and the frame is considered lost. A frame contains the following elements: control data, NAV, receiver and sender MAC adresses, sequence number, payload (the actual data) and CRC. Frames are not transmitted in a continuous manner, so that frames are not adjacent to avoid time synchronisation issues, and to allow sensing by other transmitters. The

14 CHAPTER 1. WIRELESS NETWORKS: A PRIMER

spacing used follows a set of rules, and different spacing is used based on the frames sent type:

• (S) Short SIFS : RTS/CTS (see below) and ACK/NACK. Those frames have high priority.
• (P) PCF PIFS: used by the PCF (see below), any node using PCF that wants to seize the medium must wait for it to be idle for a time equal to PIFS.
• (D) DCF DIFS: used by the DCF (see below), any node using DCF that wants to seize the medium must wait for it to be idle for a time equal to PIFS.
• (E) Extended EIFS: used when there has been an error in the previous frame transmission.

It should be noted that the acronym IFS stands for Inter Frame Spacing.

1.4.2 Resource allocation

Contrary to cellular networks, in wireless local area networks and ad-hoc networks, the channel is shared between all nodes, and there is no central entity that takes care of the resource allocation. Resource allocation consists in determining, at any given time, which node may use which network resource (i.e subcarriers, antenna etc.). Proper resource allocation is necessary to prevent the adverse effects of excessive interference. In our setting, when a node transmits, it is unaware of whether or not other nodes are transmitting. The decision to transmit must be taken in a distributed manner. In fact, the channel allocation in Ethernet (IEEE 802.3) follows the same principle. In both cases, this design principle is chosen because of low complexity and cost.

Listen before talking

The simplest idea to perform distributed resource allocation is to implement “listen before you talk’. Each node senses the wireless channel before sending data. The most basic form of sensing is physical sensing: each node measures the received power level, and compares it to a thresold. If the received power is above the threshold, the channel is deemed busy, otherwise the channel is deemed idle. It should be noted that although physical sensing is a passive operation (the sensing node does not transmit anything), it consumes power, so that it should only be used when virtual sensing is not feasible (see below). When node A can sense transmissions of node B, we say that node A hears node B.

The network allocation vector (NAV) and virtual sensing

In order to reduce the need for physical sensing, another mechanism called NAV is used. When a node transmits a frame, the frame includes a numerical value called NAV, which

16 CHAPTER 1. WIRELESS NETWORKS: A PRIMER

1.5 Modelling of wireless networks

1.5.1 Signal propagation and Interference

We quickly recall the standard models for the propagation of radio waves through a wireless channel. Consider a transmitter receiver pair located at distance r from each other. The transmitter transmits with power P , and the received power is modelled as P l(r)eσY^ Z with:

• Path-loss: l(r) = min(Ar−α, 1), with α ≥ 2 and A a constant.
• Shadowing: Y a standard Gaussian variable and σ the shadowing standard devia- tion.
• Rayleigh Fading: Z and exponentially distributed random variable with parameter

We recall that path-loss accounts for the distance-dependent loss due to propagation, shadowing accounts for absorption by obstacles such as walls, and fast-fading accounts for the fact that the signal typically travels through several uncorrelated paths where reflections on walls cause random phase changes. Interference is described by two types of models:

• Protocol model: based on the transmitted power and the path loss exponent, one defines a transmission range for each node. When a receiver is within the trans- mission range of two nodes transmitting simultaneously, any transmission is con- sidered unsuccessful. This model is a strong simplification of the physical reality, but allows to model a set of interfering nodes as a graph G = (V, E). The ver- tices V are the nodes, and two nodes may transmit simultaneously if and only if they are not linked by an edge. As a consequence, a set of nodes may transmit simultaneously iff they form an independent set of G.
• Signal-To-Noise-plus-Interference (SINR) model: when a node transmits to an- other node, transmission is successful if and only if the SINR at the receiver is above a target value. This model is also called the physical model. Signal re- ceived from other transmitting nodes is treated as noise.

1.5.2 Traffic models

As in cellular networks, the main modelling problem comes from the fact that the state of buffers, the location of transmitting nodes and the location of users are time-varying. We will consider several types of models, corresponding to different time-scales:

• PHY-level model: there are 2 nodes, a transmitter and a receiver.
• MAC model, full-buffer model: there is one receiving node and several trans- mitting nodes. Transmitting nodes always have a packet to transmit at any given time

1.6. REFERENCES 17

• MAC-level model, queuing model: there is one receiving node and several trans- mitting nodes. Transmitting nodes have a buffer where packets to be sent are stored. Packets arrive dynamically to the buffer of each node.
• Flow-level model: the number of nodes may vary across time. A node enters the network when it initiates a data flow (a series of packets to be transmitted). The node leaves the network when the last packet of her flow has been successfully delivered.

It should be noted that choosing the proper time scale is critical to ensure that the pro- posed models are both tractable and give a reasonable representation of the physical reality of the network.

1.6 References

The 802.11 standard is available at [15]. A more complete exposition of the standard and practical implementation of Wireless LANs is found in [11]. A comprehensive exposition of both wireless channel modelling can be found for instance in [30].

Part I

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