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Wireless Communication Systems
Background of Wireless Communication
Wireless Communication Technology
Wireless Networking and Mobile IP
Wireless Local Area Networks
Wireless Personal Area Networks
Wireless Metropolitan Area Networks
Wireless Wide Area Networks
Multi-hop Ad hoc Wireless
Networks, Wireless TCP and
Overview
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Introduction to Multi-hop Ad hoc Networks
Bluetooth Piconets and Scatternets
Mobile IP
TCP for Wireless
 Indirect TCP
 Snooping TCP
 Mobile TCP
 Transaction oriented TCP
Introduction (Multihop Ad hoc networks)
 Mobile Multi-hop Ad Hoc Networks are collections
of mobile nodes connected together over a wireless
medium.
 These nodes can freely and dynamically selforganize into arbitrary and temporary, “ad-hoc”
network topologies, allowing people and devices to
seamlessly internetwork in areas with no pre-existing
communication infrastructure, (e.g., disaster recovery
environments).
Introduction (Single-hop Ad hoc Networks)
 Multi-hop ad hoc networking is not a new concept
having been around for over twenty years, mainly
exploited to design tactical networks.
 Recently, emerging wireless networking technologies
for consumer electronics are pushing ad hoc
networking outside the military domain.
 The simplest ad hoc network is a peer-to-peer
network formed by a set of stations within the range
of each other that dynamically configure themselves
to set up a temporary single-hop ad hoc network.
Introduction (Bluetooth Piconets)
 Bluetooth piconet is the most widespread example of
single-hop ad hoc networks.
 802.11 WLANs can also be implemented according
to this paradigm, thus enabling laptops’
communications without the need of an access point.
 Single-hop ad hoc networks just interconnect devices
that are within the same transmission range.
 This limitation can be overcome by exploiting the
multi-hop ad hoc paradigm.
Introduction (MANETs)
 In this new networking paradigm, the users' devices
must cooperatively provide the functionalities that
are usually provided by the network infrastructure.
 Nearby nodes can communicate directly by
exploiting a single-hop wireless technology (e.g.,
Bluetooth, 802.11, etc.), while devices that are not
directly connected communicate by forwarding their
traffic via a sequence of intermediate devices.
 Generally, these users’ devices are mobile, therefore
these networks are often referred to as Mobile Ad
hoc NETworks (MANETs).
Introduction (MANETs)
 In Being completely self organizing, MANETs are
attractive for specialized scenarios like disaster
recovery, vehicle-to-vehicle communications, and
home networking.
 Unfortunately, nowadays they have a very limited
penetration as a network technology for mass-market
deployment.
 To turn mobile ad hoc networks in a commodity, we
should move to a more pragmatic scenario in which
multi-hop ad hoc networks are used as a flexible and
“low cost” extension of Internet.
Introduction (Mesh Networks)
 Indeed, a new class of networks is emerging from
this view:
 The mesh networks
 Unlike MANETs, where no infrastructure exists and
every node is mobile, in a mesh network there is a set
of nodes, the mesh routers, which are stationary and
form a wireless multi-hop ad hoc backbone.
Introduction
 Some of the routers are attached to the Internet, and
provide connectivity to the whole mesh network.
 Mesh routers are not users’ devices but they represent
the infrastructure of a mesh.
 Routing protocols running on mesh routers allow the
backbone to be self configuring, self healing, and
easy to set up.
 Client nodes connect to the closest mesh router, and
use the wireless ad hoc backbone to access the
Internet.
Introduction (Opportunistic Networking)
 Mesh networks are moving multi-hop ad hoc
networks from emergency-disaster-relief and
battlefield scenarios to the main networking market.
 While mesh networks represent a short-term
direction for the evolution of MANETs, opportunistic
networking constitutes a long-term direction for the
evolution of the ad hoc networking concept.
 The bottom line of this paradigm is providing end-toend communication support also to very dynamic ad
hoc networks, in which users disconnection is a
feature rather than an exception.
Introduction (Opportunistic Networking)
 Nodes can be temporarily disconnected and/or the
networks can be partitioned, and the mobility of
nodes creates the communication opportunities.
 The main idea is thus to opportunistically exploit, for
data delivery, nodes’ mobility and contacts with other
nodes/networks.
 In opportunistic networks the communication is still
multi-hop, with intermediate nodes acting as routers
but, in this case, forwarding is not necessarily “onthe-fly”.
Introduction
 Intermediate nodes store the messages when no
forwarding opportunity exists (e.g., no other nodes
are in the transmission range, or neighbours are not
suitable for that communication), and exploit any
contact opportunity with other mobile devices to
forward the data toward the destination.
 In this view, the existence of a simultaneous path
between sender and receiver is not mandatory (as in
traditional MANET) to communicate.
Introduction
 This networking paradigm is well suited for a world
of pervasive devices equipped with various wireless
networking technologies (802.11 family, Bluetooth,
ZigBee, etc.) which are frequently out of range from
a global network but are in the range of other
networked devices, and sometime cross areas where
some type of connectivity is available (e.g. Wi-Fi
hotspots).
Introduction
 Among multi-hop ad hoc networks, wireless sensor
networks have a special role.
 A sensor network is composed by a large number of
small sensor nodes, which are typically densely (and
possibly randomly) deployed inside the area in which
a phenomenon is being monitored.
 Wireless multi-hop ad hoc networking techniques
constitute the basis for sensor networks, too.
Introduction
 However, the special constraints imposed by the
unique characteristics of sensing devices, and by the
application requirements, make the solutions
designed for multi-hop wireless networks (generally)
not suitable for sensor networks.
 First of all, power management is a “pervasive” issue
in the overall design of a sensor network.
 Sensor networks utilize on-board batteries with
limited energy that cannot be replenished in most
application scenarios.
Introduction
 Furthermore, sensor networks produce a shift in the
networking paradigm from a node-centric to a datacentric view.
 The aim of a sensor network is to collect information
about events occurring in the sensor field rather than
supporting the communications between users’
devices.
 Multi-hop ad hoc network technologies have big
potentialities for innovative applications of great
impact on our everyday life.
Introduction (Research direction)
 However, after almost a decade of research, ad hoc
networking technologies are rarely used and have not
yet affected our way of using wireless networks.
 It is believed that this is due to a wrong approach in
the research, which was dominated by simulation
modeling and theoretical analyses with only few
attempts to build network prototypes to understand
how well MANETs work in reality.
Introduction (Research direction)
 In the last few years, this stimulated a new
community of researchers combining theoretical
research on ad hoc networking with
experiences/measurements obtained by implementing
ad hoc network prototypes.
Bluetooth Piconets and Scatternets
Piconets and Scatternets
 Piconet
 Basic unit of Bluetooth networking
 Master and one to seven slave devices
 Master determines channel and phase
 Scatternet
 Device in one piconet may exist as master or slave in
another piconet
 Allows many devices to share same area
 Makes efficient use of bandwidth
Wireless Network Configurations
Network Topology
Piconet 1
Piconet 2
Slave Master
Master
Scatternet
 Piconet = set of Bluetooth nodes synchronized to a master node
 The piconet hopping sequence is derived from the master MAC address
 Scatternet = set of piconet
 Master-Slaves can switch roles
 A node can only be master of one piconet. Why?
Frequency Hopping
 Total bandwidth divided into 1MHz physical channels
 FH occurs by jumping from one channel to another in
pseudorandom sequence
 Hopping sequence shared with all devices on piconet
 Piconet access:
 Bluetooth devices use time division duplex (TDD)
 Access technique is TDMA
 FH-TDD-TDMA
Scatternets
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Each piconet has one master and up to 7 slaves
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Master determines hopping sequence, slaves have to synchronize
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Participation in a piconet = synchronization to hopping sequence
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Communication between piconets = devices jumping back and forth between the piconets
piconets
Mobile IP
Motivation for Mobile IP
 Routing
 based on IP destination address, network prefix (e.g. 129.13.42)
determines physical subnet
 change of physical subnet implies change of IP address to have a
topological correct address (standard IP) or needs special entries in the
routing tables
 Specific routes to end-systems?
 change of all routing table entries to forward packets to the right
destination
 does not scale with the number of mobile hosts and frequent changes in
the location, security problems
 Changing the IP-address?
 adjust the host IP address depending on the current location
 almost impossible to find a mobile system, DNS updates take too much
time
 TCP connections break, security problems
Mobile IP Requirements
 Transparency
 mobile end-systems keep their IP address
 continuation of communication after interruption of link possible
 point of connection to the fixed network can be changed
 Compatibility
 support of the same layer 2 protocols as IP
 no changes to current end-systems and routers required
 mobile end-systems can communicate with fixed systems
 Security
 authentication of all registration messages
 Efficiency and scalability
 only little additional messages to the mobile system required
(connection typically via a low bandwidth radio link)
 world-wide support of a large number of mobile systems in the whole
Internet
Terminology
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Mobile Node (MN)
 system (node) that can change the point of connection
to the network without changing its IP address
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Home Agent (HA)
 system in the home network of the MN, typically a router
 registers the location of the MN, tunnels IP datagrams to the COA
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Foreign Agent (FA)
 system in the current foreign network of the MN, typically a router
 forwards the tunneled datagrams to the MN, typically also the default
router for the MN
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Care-of Address (COA)
 address of the current tunnel end-point for the MN (at FA or MN)
 actual location of the MN from an IP point of view
 can be chosen, e.g., via DHCP

Correspondent Node (CN)
 communication partner
Example network
HA
MN
router
home network
mobile end-system
Internet
(physical home network
for the MN)
FA
foreign
network
router
(current physical network
for the MN)
CN
end-system
router
Data transfer to the mobile
HA
2
MN
home network
Internet
receiver
3
FA
1
CN
sender
foreign
network
1. Sender sends to the IP address of MN,
HA intercepts packet (proxy ARP)
2. HA tunnels packet to COA, here FA,
by encapsulation
3. FA forwards the packet
to the MN
Data transfer from the mobile
HA
1
home network
sender
Internet
FA
foreign
network
1. Sender sends to the IP address
of the receiver as usual,
FA works as default router
CN
receiver
MN
TCP for Wireless Networks
Motivation
 Transport protocols typically designed for
 Fixed end-systems
 Fixed, wired networks
 TCP congestion control
 Packet loss in fixed networks typically due to (temporary) overload
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situations
Routers discard packets as soon as the buffers are full
TCP recognizes congestion only indirectly via missing
acknowledgements
Retransmissions unwise, they would only contribute to the congestion
and make it even worse
Slow-start algorithm as reaction
TCP Slow Start
 Sender calculates a congestion window for a receiver
 Start with a congestion window size equal to one segment
 Exponential increase of the congestion window up to the congestion
threshold, then linear increase
 Missing acknowledgement causes the reduction of the congestion threshold
to one half of the current congestion window
 Congestion window starts again with one segment
TCP Fast Retransmit/Recovery
 TCP sends an acknowledgement only after receiving a packet
 If a sender receives several acknowledgements for the same packet, this is
due to a gap in received packets at the receiver
 However, the receiver got all packets up to the gap and is actually receiving
packets
 Therefore, packet loss is not due to congestion, continue with current
congestion window (do not use slow-start)
Influences of mobility on TCP
 TCP assumes congestion if packets are dropped
 typically wrong in wireless networks, here we often have packet loss
due to transmission errors
 furthermore, mobility itself can cause packet loss, if e.g. a mobile node
roams from one access point (e.g. foreign agent in Mobile IP) to
another while there are still packets in transit to the wrong access point
and forwarding is not possible
 The performance of an unchanged TCP degrades severely
 however, TCP cannot be changed fundamentally due to the large base
of installation in the fixed network, TCP for mobility has to remain
compatible
 the basic TCP mechanisms keep the whole Internet together
Indirect TCP (1)
 Indirect TCP or I-TCP segments the connection
 no changes to the TCP protocol for hosts connected to the wired
Internet, millions of computers use (variants of) this protocol
 optimized TCP protocol for mobile hosts
 splitting of the TCP connection at, e.g., the foreign agent into 2 TCP
connections, no real end-to-end connection any longer
 hosts in the fixed part of the net do not notice the characteristics of the
wireless part
mobile host
access point
(foreign agent)
“wireless” TCP
wired Internet
standard TCP
I-TCP socket and state migration
access point1
socket migration
and state transfer
access point2
mobile host
Internet
Indirect TCP (2)
 Advantages
 no changes in the fixed network necessary, no changes for the hosts
(TCP protocol) necessary, all current optimizations to TCP still work
 transmission errors on the wireless link do not propagate into the
fixed network
 simple to control, mobile TCP is used only for one hop between, e.g.,
a foreign agent and mobile host
 therefore, a very fast retransmission of packets is possible, the short
delay on the mobile hop is known
 Disadvantages
 loss of end-to-end semantics, an acknowledgement to a sender does
not any longer mean that a receiver really got a packet, foreign agents
might crash
 higher latency possible due to buffering of data within the foreign
agent and forwarding to a new foreign agent
Snooping TCP (1)
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Transparent extension of TCP within the foreign agent
buffering of packets sent to the mobile host
lost packets on the wireless link (both directions!) will be retransmitted
immediately by the mobile host or foreign agent, respectively (so called “local”
retransmission)
the foreign agent therefore “snoops” the packet flow and recognizes
acknowledgements in both directions, it also filters ACKs
changes of TCP only within the foreign agent (+min. MH change)
local retransmission
correspondent
host
foreign
agent
„wired“ Internet
mobile
host
snooping of ACKs
buffering of data
end-to-end TCP connection
Snooping TCP (2)
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Data transfer to the mobile host
 FA buffers data until it receives ACK of the MH, FA detects packet loss via
duplicated ACKs or time-out
 fast retransmission possible, transparent for the fixed network
Data transfer from the mobile host
 FA detects packet loss on the wireless link via sequence numbers, FA answers
directly with a NACK to the MH
 MH can now retransmit data with only a very short delay
Advantages:
 Maintain end-to-end semantics
 No change to correspondent node
 No major state transfer during handover
Problems
 Snooping TCP does not isolate the wireless link well
 May need change to MH to handle NACKs
 Snooping might be useless depending on encryption schemes
Mobile TCP
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Special handling of lengthy and/or frequent disconnections
M-TCP splits as I-TCP does
 unmodified TCP fixed network to supervisory host (SH)
 optimized TCP SH to MH
Supervisory host
 no caching, no retransmission
 monitors all packets, if disconnection detected
 set sender window size to 0
 sender automatically goes into persistent mode
 old or new SH reopen the window
Advantages
 maintains semantics, supports disconnection, no buffer forwarding
Disadvantages
 loss on wireless link propagated into fixed network
 adapted TCP on wireless link
Mobile TCP
Transaction oriented TCP
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TCP phases
 connection setup, data transmission, connection release
 using 3-way-handshake needs 3 packets for setup and 3 for release,
respectively
 thus, even short messages need a minimum of 7 packets!
Transaction oriented TCP
 RFC1644, T-TCP, describes a TCP version to avoid this overhead
 connection setup, data transfer and connection release can be combined
 thus, only 2 or 3 packets are needed
Advantage
 efficiency
Disadvantage
 requires changed TCP
 mobility no longer transparent
References
 Multi-hop Ad hoc
Networks from Theory to
Reality
 Editors: Marco Conti (Inst
for Informatics and
Telematics, Pisa Italy) ; Jon
Crowcroft and Andrea
Passarella (Univ. of
Cambridge)
https://www.novapublishers.com/catalog/product_info.php?products_id=5556
Q&A
 ?
Assignment #5
 Write Note on Fast TCP
 What factors can degrade the performance of Fast
TCP?
 What are the problems of using Fast TCP over
Multi-hop Ad hoc networks?
 How can Fast TCP be implemented in practical?
 Write note on text colored in Green in the slides