security issues and challenges in wireless networks

Security Issues and Challenges in

Wireless Networks

Kishore Kothapalli

Bruhadeshwar Bezawada

Center for Security, Theory, and Algorithmic Research

(CSTAR)

International Institute of Information Technology

Hyderabad, INDIA

Introduction Wireless stations, or nodes, communicate over a wireless medium

Networks operating under infrastructure mode e.g., 802.11, 802.16,

Cellular networks

Networks operating with limited or no infrastructural support e.g., ad

hoc networks in AODV mode

Security threats are imminent due to the open nature of

communication

Two main issues: authentication and privacy

Other serious issues: denial-of-service

A categorization is required to understand the issues in each

situation.

Introduction – Wireless Technologies

Different technologies have been developed for different scenarios and requirements

WiFi is technology for Wireless LANs and short range mobile access networks

WiMAX is technology for last mile broadband connectivity

Wireless USB is technology for Internet connectivity on the go

Other technologies like Infrared (TV remotes etc), Bluetooth (soon to be obsolete) etc are short range

Extreme bandwidth but short range technologies are Gigabit wireless etc

Introduction

Fixed Infrastructure

Base stations that are typically not resource constrained.

Examples: sensor networks, and cellular networks.

Mobility of nodes but not of base stations.

Introduction

Ad hoc wireless networks

No infrastructural support.

Nodes also double up as routers.

Mobility of nodes.

Examples laptops/cellphones operating in ad hoc mode.

Image from www.microsoft.com

Introduction

Mixed mode

In between the two modes.

Some nodes exhibit ad hoc capability.

Introduction

To formalize study and solutions, need good models

for these networks.

Formal model to characterize the properties and

solutions

Models that are close to reality

Still allow for solution design and analysis.

Introduction

Solution properties

Light-weight

Have to use battery power wisely.

Other resources, such as storage, are also limited.

Local control

Many cases, only neighbours are known.

Any additional information gathering is expensive.

Introduction

Difficulty of modeling wireless networks as opposed to

wired networks:

Transmission

Interference

Resource constraints

Mobility

Physical carrier sensing

Outline

Introduction

Models of Wireless Networks

Various Layers and Current Solutions for each Layer

Security Issues and Threats at each Layer

Security Solutions

Open Problems


Unit disk graph model

Given a transmission radius R, nodes u,v are connected if d(u,v) ≤ R

u

R

v

u'


Unit disk graph model

Given a transmission radius R, nodes u,v are connected if d(u,v) ≤ R.

Too simple model – transmission range could be of arbitrary shape.

R

R

u

u

R

v

u'

Packet Radio Network (PRN) Can handle arbitrary shapes

Widely used Nodes u, v can communicate directly if they are

within each other's transmission range, rt.

u

v

w

v'


What is the problem?

Model for interference too simplistic

u

v

w

v'

w can still interfere at u PRN model fails to address certain interference

problems in practice

v

n – 2

s

t ≤ rt

≤ rt

≥ ri

≥ rt

What is the problem?

u

v

w

v'

Transmission Range, Interference Range Separate values for transmission

range, interference range. Interference range constant times

bigger than transmission range. Used in e.g., [Adler and

Scheideler '98], [Kuhn et. al., '04]


urt

vw

u'

ri

Transmission Range, Interference Range Separate values for transmission

range, interference range. Interference range constant times

bigger than transmission range. Used in e.g., [Adler and

Scheideler '98], [Kuhn et. al., '04]

What is the problem? Extension of unit disk model to

handle interference


urt

vw

u'

ri

Model Based on Cost Function

Gr = (V, Er), set of nodes V, Euclidean distance d(u, v) c is a cost function on nodes

symmetric: c(u,v) = c(v,u) [0,1), depends on the environment c(u,v) [(1 – )•d(u, v), (1 + ) •d(u, v)]

w

u

va

b

Edge (u,v) Er

if and only if c(u,v) ≤ r

Transmission and Interference Range

Transmission range rt(P), Interference range, r

i(P)

If c(v,w) ri(P), node v can cause interference at node w.

If c(v,w) rt(P) then v is guaranteed to receive the message from

w provided no other node v' with c(v, v') ≤ri(P) also transmits at the

same time.

w

rt(P)v'

ri(P)

u

v c(v,w) rt(P)

c(v, v') ri(P)

Carrier Sensing

Virtual carrier sensing using RTS/CTS. Physical Carrier Sensing

Provided by Clear Channel Assessment (CCA) circuit.

Monitor the medium as a function of Received Signal Strength Indicator (RSSI)

Energy Detection (ED) bit set to 1 if RSSI exceeds a certain threshold

Has a register to set the threshold in dB

Physical Carrier Sensing

Carrier sense transmission (CST) range, rst(T, P)

Carrier sense interference (CSI) range, rsi(T, P)

Beyond the CSI range, sensing is not possible.

Both the ranges grow monotonically in T and P.

w

vr

st(T,P)v'

v''

rsi(T,P) c(w,v) rst(T, P)

c(w, v') rsi(T, P)

c(w, v'') rsi(T, P)

Outline

Introduction


Various Layers and Current Solutions at each layer


Security Solutions

Open Problems

Various Layers of Interest – Physical Layer

Physical Layer

802.11 standard supports several data rates between

11 Mbps and 54 Mbps

802.16 support multiple data rates from 2Mbps to 300

Mbps

Several modulation schemes in use and support

different conditions and data rates AM, FM, PSK, BPSK, QPSK, FDM, OFDM, OFDMA, ...

Physical Layer – WiFi

Stands for Wireless Fidelity Range of Technologies

Technology that uses IEEE 802.11 protocol standards

802.11b operates at 2.4 Ghz using DSSS Has three non-overlapping channels with 11mbps max

802.11g operates at 2.4 Ghz resp, with 20 Mhz, OFDM Achieves 54 Mbps and inter-operable to 802.11b

802.11a operates at 5GHz using OFDM About 4-8 (depending on country) non-overlapping

channels Bandwidth achieved is 54 Mbps

Various Layers of Interest – MAC Layer

MAC Layer

Medium access control is an important requirement.

Collision detection (CSMA/CD) not possible unlike

wired networks.

Hence using Collision avoidance (CSMA/CA)

Functions of MAC

Scanning, Authentication, Association, WEP, RTS/CTS,

Power Save options, Fragmentation


802.11 MAC

Use Physical Carrier Sensing to sense for a free medium.

Explicit ACKs to indicate reception of packet.

Results in the problem of hidden node.

Use Virtual Carrier Sensing using RTS/CTS.

DATADATA


Virtual Carrier Sensing cannot solve the exposed node problem.

A and D cannot succeed simultaneously.

DATADATA

AB

CD

Other MAC Techniques

Cell phone networks

Node to base station and vice-versa.

Fixed frequency for communication pair (FDD).

Separate frequencies for each pair.

Different technologies Analog/CDMA/GSM support

different number of simultaneous communications per

band.

802.16 has a Receive/Grant model which is basically TDD (Time-Division Duplexing)

More efficient than FDD.

MAC Layer

More recent solutions address issues such as,

especially with respect to ad hoc networks

self-stabilization

Dynamism

Efficiency

Fairness

Various Layers – Network Layer Route packets in the network.

Routing in infrastructure based networks is similar to

IP routing

All the base stations have a wired IP interface which is

used by the routers/switches to forward data

Issues like handoffs are handled through techniques

like Mobile IP or Cellular Handoffs or Soft-handoffs as

done in Mobile WiMAX

Now, for network without infrastructure the problem is

difficult as the routes are transient

Various Layers – Network Layer

Ad hoc networks

No easy solutions but different proposals exist.

Two kinds: proactive and reactive

Proactive: Maintain lot of state, proactive updates.

Example: DSDV, DSR

Reactive: Minimal state, react to changes.

Example: AODV

Other Important Layers

Transport layer

This is important layer especially since the wireless

medium suffers from high bit-error rate and collisions.

To offset this wireless technologies rely less on TCP’s

reliability mechanism

This is mostly handled at physical layer through

techniques like FEC and other error correcting codes

Application Layer

Notion of an application layer protocol

Email/Web/Games/SMS/MMS

Outline

Introduction




Security Solutions

Open Problems

Threats in Present Solutions – MAC Layer

Denial of Service

Can hog the medium by sending noise continuously.

Can be done without draining the power of the

adversary.

Depends on physical carrier sensing threshold.

zA

Threats in Present Solutions – MAC Layer

802.11 standard uses Access Control Lists for

admission control.

If MAC address not in the list, then the node is denied

access.

But easy to spoof MAC addresses.

00:1A:A0:FD:FF:2E

00:0C:76:7F:DF:49

00:13:D3:07:2F:A8

00:2F:B8:77:EA:B5

Threats in Present Solutions – Network Layer

Ad hoc networks

Network layer

Denial-of-service attacks

Broadcast nature of communication

Packet dropping

Route discovery failure in ad hoc network

Packet rerouting


Denial-of-service

Easy to mount in wireless network protocols.

One strategically adversary can generally disable a dense part of the network.

zA

Nodes Disrupting Routes

SourceSource

Destination

Can simply engage in conversation and drain battery

power of other nodes – power exhaustion attack

Send lot of RREQ messages but never use the routes.

zA

RREQ(a)

RREQ(b)

RREQ(c)

….



Broadcast nature of communication

Each message can be received by all nodes in the transmission range

Packet sniffing is a lot easier than in wired networks.

Poses a data privacy issue

s

tA


Route discovery in ad hoc networks AODV discovers route by RREQ/RREP. Few adversarial nodes can fail route discovery. Difficult to detect route discovery failures. Also vulnerable to RREP replays.

RREQ

RREQ


Packet dropping

Wired networks can monitor packet drops reasonably

Such mechanisms are resource intensive for wireless networks

AODV has timeouts but no theoretical solutions Difficult to distinguish packet drops, say RREQs, from

non-existence of route itself

Nodes some times behave selfishly to preserve resources


Packet rerouting – also known as data plane attacks.

Attacker reveals paths but does not forward data along

these paths.

Control plane measures do not suffice.

Difficult to trace in wired networks also [Gouda, 2007].

s

t

Application Layer

Easy to infect mobile devices.

Rerouting content through the base station poses

privacy issues.

Bluetooth networks and ad hoc networks do not have a

base station facility.

Contrast with wired networks with firewalls, filters,

sandboxes.


Outline

Introduction




Security Solutions

Open Problems

Security Solutions

Requirements

Need solutions that do not add any perceivable burden

Cryptography can help

Public key solutions

Public key operations about 1000 times slow compared to symmetric key

operations.

Cost of SHA-1 = 2 microseconds

Cost of RSA signature verification = order of millisec

Symmetric key solutions for privacy and authentication

Issue: How to distribute and manage keys?

Security Solutions for 802.11 Networks

Previous WEP (Wired Equivalent Privacy) based on RC4 is prone to attacks

Privacy is not guaranteed as the key streams could be easily recovered

Weaknesses in RC4 are well documented

Authentication is weak as well due to weak encryption technique

Challenge-response using pre-shared keys is prone to attacks if encryption is weak

Previous WEP Solution using RC4

RC4 is a Vernam Cipher meaning primary operations are XOR with pseudo-random bytes

Per-packet encryption key is 24-bit IV concatenated to a pre-shared key

Integrity Check Vector (ICV) is CRC-32 over plain-text (used as Message Authentication Code)

Data and ICV are encrypted using per-packet encryption key

Problem

RC4 is weak (as the IV is reused) and can allow an attacker to get the key stream used

The ICV can enable one to check the validity of the key stream recovered

802.11 Hdr Data

802.11 Hdr DataIV ICV

Encapsulate Decapsulate

WEP Authentication Model

WEP Authentication Based on RC4

Authentication key is distributed out-of-band

Access Point generates a randomly generated challenge

Station encrypts challenge using pre-shared secret

Problem: Challenge-responses of valid users can be recorded and key stream can be recovered due to RC4 working

Attacker can use the keys to encrypt any future challenges

Challenge (Nonce)

Response (Nonce RC4 encrypted under shared key)

Wireless Node

APAP

Shared secret distributed out of band

Decrypted nonce OK?

Security Solution for 802.11 Networks: 802.11i Model

Solution Requirements Mutual authentication Scalable key management for large networks Central authorization and accounting Support for extended authentication like smart cards Key Management Issues

Need to dynamically manage keys to avoid manual reconfiguration difficulties especially for large networks

Current Standard: 802.11i or WPA2

802.1X for Authentication Based on EAP (Extensible Authentication Protocol)

Port based authentication

Access denied if port authentication fails

CCMP (Counter Mode CBC-MAC Protocol) using AES for confidentiality, integrity and origin authentication

Dynamic Key Management

802.1X Authentication

802.1X Key Management

LEAP use dynamically generated WEP keys to secure authentication data

EAP-TLS –Station and Access Point use public-key certificates through a TLS tunnel

Session key can be exchanged

Mutual-authentication as both parties have digital certificates

EAP-TTLS and PEAP –Only server-side certificate is needed

Simplifies implementation where certificate management is difficult

EAP-GSS where the authenticator is required to be in contact with a KDC

Key Derivation in 802.11i

Key Derivation in 802.11i

At the end of EAPOL: Station and Server share a Master Key: MK (E.g., Using EAP-TLS)

Both the Station and the AP derive a new key, called the Pairwise Master Key (PMK), from the Master Key.

Radius Server moves PMK to AP

A 4−way handshake between the station and the AP to derive, bind, and verify a Pairwise Transient Key (PTK).

Key Confirmation Key (KCK), as the name implies, is used to prove the posession of the PMK

Key Encryption Key (KEK) is used to distributed the Group Transient Key (GTK) Temporal Key 1 & 2 (TK1/TK2) are used for encryption.

The KEK is used to send the Group Transient Key (GTK) from AP to the station

The GTK is a shared key among all stations connected to the same authenticator (AP), to secure multicast/broadcast traffic

802.16 Authentication

Security Solutions for 802.16 Networks

802.16 or popularly WiMAX use X.509 certificates for authentication

Subscriber Station authentication using X.509 certificate Establish security association (SAID) Authentication Key (AK) exchange AK is encrypted using public key of SS Authentication is completed when both SS and BS verify possession AK

AK is used to exchange the TEK (Traffic encryption key)

Base station generates TEK randomly and encrypts using KEK generated from AK

802.16 uses AES in CCM mode for privacy

Mutual authentication is possible through EAP-TLS etc (802.16e)

Security in Ad Hoc Mode

Ad hoc networks cannot use RADIUS type authentication

Problem: if RADIUS type authentication is used, every station will need to store every other station’s credentials

Moreover, authentication will have to be using EAP-TLS which is computationally intensive

Problem: mutual authentication is trouble some

Other Security Requirements

Cryptographic mechanisms for confidentiality

Key establishment for confidentiality

Public-key management to prevent replacement of keys

Symmetric key management to protect from compromise

Denial-of-service resistance in contention mechanisms at MAC layer

Security in Ad Hoc Networks

Security Mechanisms

Pro-active : Prevents an attacker from launching an attack say by using cryptographic mechanisms

Requirement is establishment of necessary cryptographic material

E.g., Routing Attacks

Reactive : Relies on detection and mitigation of attacks

Benign behaviour is defined and behaviour analysis is done to detect malicious behaviour

E.g., Packet Forwarding attacks

Key Management in Ad Hoc Networks- An

Overview

Key management – Manage a set of secure communication

channels so that

Use as few keys as possible

Avoid centralized infrastructure during sessions

Minimal cryptographic/message overhead

Ensure “reasonable” security

Two scenarios

Broadcast security

Peer-to-peer security

Security Solutions – Broadcast Security

Base station and a set of nodes.

Base station sends updates to all the nodes using broadcast.

N = number of satellite nodes

Authentication and privacy is required

Trivial Solution

Each node shares a key with the base station. Storage is O(N) for sender and does not scale well Authentication is expensive especially if messages need to be

broadcast

K6

K8

K1

K7

K4

K2

K5K3

K1, K2, K3, K4, K5, K6, K7, K8

Broadcast Security

Maintain a set O(log N)

Each satellite node gets a subset of log n keys of S. Privacy: use XOR of keys to communicate with the user

Authentication: sender adds MAC using all its keys

Each node verifies signatures that can be generated using its subset of keys

K1, K2, K3, K4, K5

MACK1(M) MACK2(M) MACK5(M)MACK4(M)MACK3(M)Message

K1, K3, K5

K1, K2, K4

K1, K3, K4

K2, K5, K4

K1, K2, K5

K1, K2, K3

K1, K5, K4

K2, K5, K3

Broadcast Security

Collusion is an issue

A larger pool of keys can be selected

For N users O(log N) keys can give good results

Scales well as the sender only needs to give a new subset of keys to a new user

K1, K3, K5

K1, K2, K4

K1, K3, K4

K2, K5, K4

K1, K2, K5

K1, K2, K3

K1, K5, K4

K2, K5, K3

K1, K2, K3

K4, K5, K6,

K7, K8

Security Solutions

Privacy in a Peer-to-peer situation

Public-key cryptography can be of use but expensive

Key distribution is a major hurdle given that communicating parties are

not known in advance

Anyone can communicate with any one

Trivial Solution: one unique key per pair of users work

Expensive

Not scalable if new user gets added

Revocation is little more tricky

Scalable approach : key pre-distribution

Point-to-Point Security

Point-to-Point security

Need a key for every pair of nodes in an n node network.

Trivial solution requires storing n – 1 keys at every node.

Not scalable on the space usage.

A B

CD

KAB

KAD

KAC

KBC

KCD

KBD

KCDC-D

KBDB-D

KBCB-C

KADA-D

KACA-C

KABA-B


Random Key Pre-distribution

A B

CD

Pool of KeysK1, K2, K3, K4, K5, K6,K7, K8, K9, K10, K11,K12, K13, K14, K15

K1, K2, K5, K6 K3, K9, K5, K11

K12, K11, K13, K15K1, K15, K9, K13

K5

K11

K1+K15+K13

K1

E

F

G

K1, K5, K9, K13G

K3, K5, K7, K9, K15F

K10, K4, K5, K8, K7E

K1, K15, K9, K13D

K12, K11, K13, K15C

K3, K9, K5, K11B

K1, K2, K5, K6A


Issues in Random Key Pre-Distribution

May need Intermediaries for key establishment

Storage is High Experimental: 250 keys out of 10,000 keys may be necessary

An active adversary is dangerous

Collusion effect is unknown due to the randomness of key distribution

Might require privacy mechanisms to hide key sharing patterns

Revocation issues exist

Probabilistic arguments for size of key storage and connectivity possible

Practice proves otherwise, especially for sparse graphs

Some Solutions –Key Establishment

Multi-path Key Establishment

A B

CD

Pool of KeysK1, K2, K3, K4, K5, K6,K7, K8, K9, K10, K11,K12, K13, K14, K15

K1, K2, K5, K6 K3, K9, K5, K11

K12, K11, K13, K15K1, K15, K9, K13

K5

K11

K1+K15+K13

K1

E

F

G

K1, K5, K9, K13G

K3, K5, K7, K9, K15F

K10, K4, K5, K8, K7E

K1, K15, K9, K13D

K12, K11, K13, K15C

K3, K9, K5, K11B

K1, K2, K5, K6A


Deterministic Solution –Square Grid [Ref. 4]

[0,0] [0,1] [0,2] [0,3]

[1,0] [1,1] [1,2] [1,3]

[2,0]

[3,0]

[2,1] [2,2] [2,3]

[3,1] [3,2] [3,3]

User Placement



Deterministic Solution –Square Grid

[0,0] [0,1] [0,2] [0,3]

[1,0] [1,2]

[2,0]

[3,0]

[2,2]

[3,2]

Kg(0,0)

Kg(2,2)[2,3][2,1]

Grid Secrets




[0,0]

[0,1] [0,2] [0,3]

[1,0]

[2,0]

[3,0]

Direct Secrets




[0,0] [0,1] [0,2] [0,3]

[1,0] [1,2]

[2,0]

[3,0]

[2,2]

[3,2]

[2,3][2,1]

Communication

Along Same Row/Column




[0,0] [0,1] [0,2] [0,3]

[1,0] [1,2]

[2,0]

[3,0]

[2,2]

[3,2]

Kg(0,2)

Kg(2,0)[2,3][2,1]

Communication Among Users of Different Rows/Columns



Square Grid Features and Issues

Mobility has no effect on key establishment –always guaranteed by design

Failure tolerant –failure of links hardly matters

Storage is high, but comparable to random KPS

Collusion resistance is slightly weak Two users are sufficient to compromise session key

Scalability is weak as the grid size is fixed before hand Optimizations possible, by choosing higher grid size and allowing for

some additional users


Security Solutions

Can reduce storage further by considering a k – dimensional grid

User belongs to multiple grids with lower dimension: n1/k

number of keys stored per node decreases to kn1/k.

At k = log n, this reduces to log n.

But collusion resistance decreases with increasing k

Best case storage is around: 12log2n

Lower values are possible but multiplication constant is higher

Security Solutions-Hierarchical Solution

B D

A C

•Stands for any P2P key distribution

•E.g. (A,C) could be given a unique shared key

•Better key distributions are possible

Security Solutions-Hierarchical Solution for Reducing Storage

AB

CD

EF

GH

Nodes Treated as Single Entity

• E.g. (A,B) and (C,D) could share a common key

• If B, needs to communicate with C, this key can be used

• Collusion resistance is an issue

Outline

Introduction




Security Solutions

Open Problems

Open Problems

Problem 1: Secure Admission Control

For fixed infrastructure networks, how to decide admitting a new node

into the network?

EAP-TLS, EAP-TTLS are expensive in terms of computation and do not

work well in ad hoc mode

Access points should be able to handle more decisions to enable easy

roaming

Need for a scalable but practical solution for admission control especially for

roaming accessibility

If key management is used dynamics and storage become issues

Open Problems

Problem 2 : Application Layer Security for fixed infrastructure

networks

Equivalent notions of wired networks.

Require Light-weight sand boxing mechanisms

Privacy-preserving light-weight content filtering techniques

Existing solutions: J2ME KVM, DownloadFun, QualComm

BREW

Open Problems

Problem 3: Real-time Cell Communication Security

Key management solutions may not work due to real-

time voice data

Hacking/tapping cell phones is possible depending on

the encoding scheme used

Open Problems 4 Certificate mechanisms for nodes

Certificates in wired networks are

well understood.

Users typically have better user

interfaces e.g., PC Monitor, allowing

them to examine things like

certificates

Certificate verification/validation is tolerable on desktops and even laptops.

Open Problem 4

Problem: Not the same for mobile users say, cell phones

Integrating such features into a cell-phone is difficult

Expensive to verify certificates due long certification path.

Solution more difficult for devices with no display or limited display or

regular monitoring of the device, such as sensors.

Need a different way of handling certificates.

Conclusions

Situations are more complex in wireless networks, even with infrastructural support.

Threats exist at various layers of operation.

Present solutions to address these threats are not scalable or not strong enough.

Simple key management solutions can help.

But not always.

Still, lots of interesting and open issues to be solved.

Thank You!

References

Jean-Pierre Hubaux, Levente, Buttyan and Srdan Capkun “The Quest for Security in Mobile Ad Hoc Networks”, ACM MobiHOC 2001

Laurent Eschenauer and Virgil D. Gligor “A Key Management Scheme for Distributed Sensor Networks” ACM CCS 2002

Haowen Chan, Adrian Perrig and Dawn Song “Random Key Predistribution Schemes for Sensor Networks” IEEE Symposium on Security and Privacy 2003

S.S.Kulkarni, M.G.Gouda and A.Arora “Secret Instantiation in Ad Hoc Networks” Special Issue of Elsevier Journal of Computer Communication on Dependable Wireless Sensor Networks, 2006

Amitanand S. Aiyer, Lorenzo Alvisi, Mohamed G. Gouda “Key Grids: A Protocol Family for Assigning Symmetric Keys” IEEE International Conference on Network Protocols, 2006

B.Bruhadeshwar and Sandeep Kulkarni “An Optimal Symmetric Secret Distribution for Secure Communication” Michigan State University Technical Report 2008 MSU-TR-08-196

References

Bezawada Bruhadeshwar, Kishore Kothapalli: A Family of Collusion Resistant Symmetric Key Protocols for Authentication. ICDCN 2008: 387-392

Kishore Kothapalli, Christian Scheideler, Melih Onus, Andréa W. Richa: Constant density spanners for wireless ad-hoc networks. SPAA 2005: 116-125

Edmund L. Wong, Praveen Balasubramanian, Lorenzo Alvisi, Mohamed G. Gouda, Vitaly Shmatikov: Truth in advertising: lightweight verification of route integrity. PODC 2007: 147-156

Ran Canetti, Adrian Perrig, Dawn Song and Doug Tygar “The TESLA Broadcast Authenitcation Protocol” RSA Cryptobytes 2002

Chalermek Intanagonwiwat, Ramesh Govindan, Deborah Estrin, John S. Heidemann, Fabio Silva: Directed diffusion for wireless sensor networking. IEEE/ACM Trans. Netw. 11(1): 2-16 (2003)

Arshad Jhumka, Sandeep S. Kulkarni: On the Design of Mobility-Tolerant TDMA-Based Media Access Control (MAC) Protocol for Mobile Sensor Networks. ICDCIT 2007:

General: Wikipedia, WiFi Forum, WiMAX Forum, IETF Website

security issues and challenges in wireless networks

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