National Taiwan University
Computer Science and Information Engineering
(Spring 2008)
Wireless Multimedia Systems
Final
Name:_______________陳正偉_______________
ID____________________B93902050____________________
Good Luck !!
1. (14 Points) The mobile multicast protocol must deal not only with dynamic group
management but also with dynamic member location. The current version of mobile
IP proposed two approaches to support mobile multicast, i.e. bidirectional tunneling
and remote subscription. (A) Can these two approaches (bidirectional tunneling and
remote subscription) handle source mobility and recipient mobility? (B) A direct
mechanism for achieving multicast reception on MHs is to let HA handle multicast
routing by executing IGMP and delivering multicasts to the MHs as if it was at home.
In current IETF mobile multicast, is it qualified as remote subscription or bi-direction
tunnel multicast? Advantage? Disadvantage? (C) What is the duplication problem? (D)
What is the tunnel convergent problem? (E) Can you describe what is “combined
routing”? Advantage? Disadvantage? (F) MoM uses the DMSP (Designated Multicast
Service Provider) to avoid duplicate packets being tunneled to the common FA.
Describe the situations in which DMSP handoff might occur. (G) What is the scoping
problem?
Answer:
(A)Yes to both.
(B)Bi-direction tunnel multicast:
Advantage: Allow both sending and receiving of multicast datagrams,
with the same delivery guarantees given to fixed hosts.
Disadvantage: If multiple mobile hosts on the same foreign network
belong to the same multicast group then duplicate copies
of the multicast packets will arrive at that foreign
network. Multiple encapsulation increases the packet
size substantially and can cause fragmentation.
Remote subscription:
Advantage: Providing the most efficient delivery of multicast datagrams
Disadvantage: May come at a high price for the networks involved the
multicast routers that must manage the multicast tree
(C)If bi-directional tunneling is used, all multicast packets are
forwarded individually to each MH by its HA. If many MHs belong to the
same FA and HA and they ask the same multicast packet, the number of
multicast packets sent to FA is the same as the number of MHs. Thus the
same packets will be duplicate.
(D)The foreign agent selects one home agent as the designated multicast
service provider (DMSP) if a mobile host is the first mobile host to
request subscription to group G at the foreign network Our scheme is
complicated by the tunnel convergence problem resulting from the fact
that multiple Mobile IP tunnels (from different home agents) can
terminate at a particular foreign agent.
For example, multiple home agents (at different parts of the
internetwork) all happen to have mobile hosts (and members of the same
multicast group) at the same foreign network, managed by the same
foreign agent. Thus one copy of every multicast packet would be
forwarded to the foreign agent by each home agent that is serving
interested mobile hosts. Since the foreign agent would locally deliver
every multicast datagram forwarded to it, the problem of duplicate
multicast packet delivery to the mobile hosts must be dealt with. While
duplicate datagrams do not constitute a violation of the service
assumptions, they again constitute unnecessary network load. In the
worst case, the number of duplicate copies delivered increases with the
number of mobile hosts present, a serious concern.
(E)The FA gathers membership information and arranges for unique or More
tunnels to be set up for each group
Advantage: Using combine routing can solve the duplication problem.
Disadvantage: If the only packet sent from HA to FA is lost, then there
will no copies.
(F) a DMSP handoff may occur in. two situations. One is that a new mobile host enters a new
network and new
(G)A small scope group may be defined as a group such that multicasts
addressed to that group are constrained (using the IP Time-To-Live (TTL) field)
to remain within a relatively small region of an internetwork. In IPv6, group
scope is controlled by a flag in the multicast address itself. The existence of small
scope groups introduces a scoping problem. This problem has two dimensions.
First, small scope groups are not unique across networks; thus a naïve foreign
agent may designate DMSPs to handle a small scope multicast group, ignorant of
the fact that this may eliminate the ability of mobile hosts that are not from the
same network as the DMSPs to receive local scope multicasts from their own
networks. Second, when a packet addressed to one of these groups is received by
the foreign agent, it will be broadcast onto the foreign network and picked up by
all hosts subscribing to that group. Since these groups are not unique across
networks, hosts on the foreign network may receive packets that were never
meant for them.
2. (14 Points) The Session Initiation Protocol (SIP) provides advanced signaling and
control functionality for a wide variety of multimedia services. (A) Explain why and
how SIP could support personal mobility. (B) Explain how caller could influence can
influence the routing of the SIP message. (C) Explain how SIP could support forking
function? (D) While almost all phone calls are between two parties, signaling also
would be required to accommodate multiparty calls (such as “PUSH TO TALK”).
Describe how SIP could be enhanced to support multiparty calls. (E) Explain the
motivations for IP multimedia subsystem (IMS) from the network operator
perspective? (F) Describe the application-level registration and session setup in the
IMS. (G) Explain the usage of PRACK (provision response acknowledgement)
message.
Answer:
(A)SIP uses Proxy server and registrar to find the dynamic IP of the caller,
who can use multiple IP address and move everywhere. The SIP user can
register at the SIP registrar. Registrar can maintain current IP of
the caller, and we can find one person by querying the registrar. The
use of registration enables SIP to support personal mobility.
(B)Callers could set Route and Record-Route field in the header to
influences the routing. All mobile terminal originated session setup
attempts must transit the server proxy in the home network so that proxy
can properly trigger the SIP services allocated to the user. This
implies a requirement for some sort of source-routing mechanism to
ensure these proxies are transited correctly.
(C)A proxy can fork requests. This would happen if a particular user is
registered at several locations. An incoming INVITE for the user would
be sent from the proxy to each of the registered locations. If one of
the locations answered, then the proxy could issue a CANCEL to the other
locations. In order to handle such forking, a proxy must be state.
(D)The multiparty IP telephony calls may be established with SIP. SIP
carrying SDP will convey the necessary address and transport
information to the terminals of the call participants so that these
will be able to construct the xcast header information. The combination
of SIP and xcast introduces the benefits of a simple multicast scheme
that does not suffer from problems that are introduced by the classical
multicast schemes.
(E) The motivation for IMS is the provision of easy and efficient ways to integrate
different services, combining web browsing, e-mail, instant messaging, VoIP,
video conferencing, telephony, multimedia content delivery, even from third
parties and enables the seamless integration of legacy services and is designed for
consistent interactions with circuit switched domains.
(F) In order to request services of the IMS domain, a user must perform an application
level registration. This is done once a user has an activate PDP context with
appropriate QoS parameters for transfer of IMS-related SIP signalling. The QoS
parameters specified in PDP context activation must be appropriate for IMS
signalling. Figure below shows the flow of messages for registration of the UE
with its Serving CSCF, assuming the UE was not previously registered. As shown,
the S-CSCF authenticates the mobile before registration is successful.
(G ) Those retransmissions cease when a PRACK message is received. The PRACK
request plays the same role as ACK, but for provisional responses. There is an
important difference, however. PRACK is a normal SIP message, like BYE. As
such, its own reliability is ensured hop-by-hop through each stateful proxy. Also
like BYE, but unlike ACK, PRACK has its own response.
The PRACK messages contain an RAck header field, which indicates the sequence
number of the provisional response that is being acknowledged. The acknowledgments
are not cumulative, and the specifications recommend a single outstanding provisional
response at a time, for purposes of congestion control.
3. (12 Points) (A) TCP is a reliable transport protocol tuned to perform well in
traditional networks made up of links with low bit-error rates. Can you explain
how TCP-Probing could achieve higher throughput rates while consuming less
energy? (B) Can you propose some scenarios that TCP-probing might not work
well? (C) By looking at wireless TCP problem from a different perspective, and
considering the memory usually present in the error process induced by a fading
channel, some research efforts argue that in some cases, the window adaptation
algorithm used in TCP may in fact be more efficient than predicted by those
early studied, most of which neglected the actual characteristics of wireless
propagation. Design your own wireless TCP protocol based on different wireless
error characteristics and round trip propagation delay of the connection.
Answer:
(A) TCP-Probing achieves energy and throughput efficiency by implementing a self
-adjusting strategy which is responsive to the nature of errors.Probing enables
TCP to go beyond a circumscribed functionality exclusively focused on
congestion control, and to move towards a universal error control with energy
-conserving capabilities.
(B) Probing represents a strategy wherein transmission effort and time are invested in
robe cycles in order to determine the nature of prevailing error conditions. This
cost of probing” is recouped and made to yield effective returns by adopting
appropriately conservative and aggressive transmission tactics in response to the
conditions detected. The 1-second phase tests indicate that the decision-making
component of the probing mechanism is amenable to localized, heuristic
mprovement. The two tests at error rates 10% and 20% are noteworthy for the fact
that TCP-Probing performance falls well below the best of the three standard TCP
versions, which happens to be Reno in this case. An examination of the data in the
Table shows that the shortfall comes not so much from extra overhead expended
by TCP-Probing, but rather from extended connection times: TCP-Probing
connection times come in very close to Tahoe’s in both cases. At these error rates
probing cycles are not particularly extended, so their impact on both overhead and
connection times is minimal. The problem seems to be the unduly conservative
decision-making criteria whereby Immediate Recovery is entered at the end of
probing only if both probe-cycle RTTs are less than the last estimated RTT.
Simply stated, TCP-Probing behavior is insufficiently aggressive under the
circumstances. This is currently the subject of ongoing research.
(C)
4. (10 Point) (A) Explain how IEEE 802.11 could address the power conservation
issue effectively. (B) The EC-MAC (Energy Conserving-Medium Access Control)
protocol, on the other hand, was developed with the issue of energy efficiency as a
primary design goal. The EC-MAC protocol is defined for an infrastructure network
with a single base station serving mobiles in its coverage area. Can you describe cases
that EC-MAC perform better power conservation than IEEE 802.11 power saving? (C)
Explain how PAMAS PAMAS (Power Aware Multiple Access) protocol addresses
the power conservation issue.
Answer:
(A)The IEEE 802.11 standard recommends the following technique for power
conservation. A mobile that wishes to conserve power may switch to
sleep mode and inform the base station of this decision. The base
station buffers packets received from the network that are destined
for the sleeping mobile. The base station periodically transmits a
beacon that contains information about such buffered packets. When the
mobile wakes up, it listens for this beacon, and responds to the base
station which then forwards the packets. This approach conserves power
but results in additional delays at the mobile that may affect the
quality of service (QoS).
(B)New users registering with the base station may have to use some form
of random access protocol. In this case, using a small packet size for
registration and bandwidth request may reduce energy consumption. The
EC-MAC protocol is avoids collisions during reservation and data
packet transmission.
(C)Power conservation is achieved by requiring mobiles that are not able
to receive and send packets to turn off the wireless interface. The
idea is that a data transmission between two mobiles need not be
overheard by all the neighbors of the transmitter. The use of a separate
control channel allows for mobiles to determine when and for how long
to power off. A mobile should power itself off when: (I) it has no
packets to transmit and a neighbor begins transmitting a packet not
destined for it, and (II) it does have packets to transmit but at least
one neighbor-pair is communicating. Each mobile determines the length
of time that it should be powered off through the use of a probe protocol.
Theoretical bounds on power savings for random, line, and fully
connected topologies are also presented.
5. (10 Points) (a) Can you describe two main motivations for The Terminal Project:
Toward Self-Organized Mobile Ad Hoc Networks? (b) Can you describe Ad hoc
network evolutions from early stage to the future? (c) Can you describe the
characteristics of Ad hoc networks? (d) In the early stage, two kinds of routing
schemes: (1) table driven routing scheme (such as DSDV) (2) on demand routing
scheme (such as AODV). Can you explain the differences between them? (3)
Different from traditional routing schemes, recently, geographical methods have been
utilized for routing. Can you describe how geographic information can be helpful for
the routing function? (4) For large scale ad hoc networks, what are the problems for
the routing schemes? (e) What are the similarities and differences between ad hoc
networks and sensor networks? (f) What are the similarities and differences between
ad hoc networks and vehicular Ad hoc networks (VANET)?
Answer:
(A)1. Try to capture the business and societal potential.
2. All layers and interlay interactions. That is, from physical layer
up to software architecture and applications.
(B)In these early generations, each node was considered to be an
individual, hand-held device. Because of their very nature, mobile ad
hoc networks have a certain number of peculiarities. First of all, as
already mentioned, they can act independently from any provider.
Moreover, they have to be highly cooperative: the tasks are distributed
over the nodes and any operation is the result of the collaboration
of a group of them. In addition, the nodes rely on batteries for their
energy; therefore, energy savings are an important system design
criterion; furthermore, nodes have to be power-aware: the set of
functions offered by a node depends on its available power. The
topology can be highly dynamic, hampering the stability of the links
and of the routes. The links themselves are wireless; their
dependability and capacity have to be carefully scrutinized. Finally,
security is difficult to implement, because of the vulnerability of
the links, and of the limited physical protection of each of the nodes.
(C)Ad hoc mode
No base stations
Nodes can only transmit to other nodes within link coverage
Nodes organize themselves into a network: route among themselves
(D)In DSDV, route information is labeled with an increasing sequence
number and route info with greatest number will be update. Route info
of broken link is broadcast with odd sequence one greater than the
original sequence number.
In AODV, route maintenance is by using table entry and the route info
is invalid if life time in table entry is expired or receiving RERR
packet.
(E)Similarities: Self-organized. Without the aid of any centralized
administration.
Differences: In sensor network, sensors are equipped on each node for
signal processing and networking of the data. However,
ad hoc network is a connection method of wireless LANs
so that it is the internet adapter to do signal processing
and data networking.
(f ) Similarity: VANET can be considered as one of concrete applications of MANETs in
the future.The difference between VANET and MANET:
(i ) VANET have vehicles as network nodes and their main characteristics are highly
mobility and speed
(ii) VANET nodes move non-randomly along specific paths (roads)
(iii) VANET nodes are vehicles, so there are less power and storage constraints
6. (10 Points) Providing real time services such as voice or video over ad hoc network
is a challenge. Usually QoS routing will be required for real time connections. (a)
Describe the difference between QoS routing and shortest path routing. (b) Usually
bandwidth constraint must be satisfied for the required voice and video. How do you
calculate the available bandwidth for an ad hoc node if you have 2M bits per second
bandwidth and each requested video stream require 50 K bit per second bandwidth.
As Figure 1, the video connection is constructed from sender to receiver, how many
bandwidth resources could be consumed for each node (sender, R1, R2, and receiver).
(c) If you need to employ two alternative paths, how would you extend DSR
algorithm? (d) Multipath routing has been proposed to satisfy QoS requirements. Can
you describe the advantages and disadvantages for mutlipath routing?
Figure 1.
Answer:
(A)QoS routing is to find a path that has best connection condition.
Shortest path routing is to find a shortest path but may not be the
path with best connection condition.
(B)1. 50 × 4 = 200 (Kps).
2. For sender and receiver:
50 x 2 = 100 (Kbs)
For R1 and R2:
50 x 3 = 150 (Kbs)
(C)SMR (split multipath routing) extends DSR in the way that the
destination can discover two paths for each route request, in which
one is the shortest path, and the other is the maximally disjoint path.
There are two approaches in SMR route maintenance. The first scheme
builds a new pair of routes when any existing route of the session is
disconnected. The second scheme performs rerouting only when both
routes are broken.
(D)
Advantages
(1)The reduction in route
computing time
(2)The high resilience to path
breaks
(3)High call acceptance ratio
(4)Better security
Disadvantages
(1)Overhead on finding multiple
paths and maintenance of
large routing table.
(2)Complexity
7. (10 points) (A) Multiple carrier modulation techniques, specifically Orthogonal
Frequency Division Multiplexing (OFDM), are capable of operating with severe
multipath and can avoid ISI problem associated with SC (Single Carrier) modulation
techniques. Can you explain why? (B) Describe and explain the downlink sub-frame
structure and uplink sub-frame structure for IEEE 802.16. (C) Describe the downlink
sub-frame structure and uplink sub-frame structure for IEEE 802.16e. (D) Describe
the procedures of channel acquisition and initial ranging and negotiation of SS
capabilities. (E) Describe how IEEE 802.16 realize CBR (such as VoIP) and VBR
(such as Video over IP) services?
Answer:
(A)A single stream of data is split into parallel streams each of which is coded and
modulated on to a subcarrier, a term commonly used in OFDM systems. Thus the
high bit rates seen before on a single carrier is reduced to lower bit rates on the
subcarrier. In OFDM, the sub-carrier frequencies are chosen so that the
sub-carriers are orthogonal to each other, meaning that cross-talk between the
sub-channels is eliminated and inter-carrier guard bands are not required.This
greatly simplifies the design of both the transmitter and the receiver; unlike
conventional FDM, a separate filter for each sub-channel is not required. It is easy
to see that ISI will therefore be reduced dramatically.
(B) #Proposed Frame Structure
A frame structure in IEEE Std. 802.16 is divided into two subframes for a
downlink and a uplink transmission. In this proposal, a frame structure is
extended to support a relay link.
The frame structure is composed of:
‧ A downlink subframe, a uplink subframe, a TTG, and a RTG.
‧ The downlink subframe is composed of preamble, FCH, DL-MAP, UL-MAP,
1-hop
region, and multihop region.
‧ In the downlink, the 1-hop region includes the MMR-BS → MS related
traffic and the MMRBS → RS related control and traffic. The multi-hop
region includes the RS → MS related traffic and the RS → lower RS
related control and traffic.
‧ The uplink subframe is composed of a ranging subchannel, 1-hop region,
and multi-hop region.
‧ In the uplink, the 1-hop region includes the MS → MMR-BS related traffic
and the MS → RS related control and traffic. The multi-hop region
includes the MS → RS and the lower RS → RS related control and traffic.
‧ The boundary between 1-hop region and multi-hop region is logically divided.
‧ RS related control includes the data to control next hop related traffic
and RS related traffic includes the traffic data of next hop.
(C)
(D)
The MAC CPS is used to provide the core MAC functionality of system access,
bandwidth allocation, connection establishment, and connection maintenance:
a) System Access
b) Bandwidth Request/Allocation
c) Connection Establishment/Maintenance
d) Quality of Service
(E)
CID maps to a service flow identifier (SFID), which defines the Quality of Ser\
vice parameters of the
service flow associated with that connection.
8. (10 Points) The approach employed by UWB radio devices is based on sharing
already occupied spectrum resources by means of the overlay principle, rather than
looking for still available but possibly unsuitable new bands. (A) Describe the general
definition used within the FCC’s First Report and Order and widely accepted by the
industry for a UWB device. (B) Explain the meaning of the spectral efficiency. (C)
Describe the difference between 802.15.3a and Bluetooth. (D) Describe how IEEE
802.15.3 accommodate continuous (e.g. voice) and bustry traffic (e.g. TCP). Explain.
(E) Describe the possible interference problem when a great number of UWB radio
devices are being adopted within a certain area.
Answer:
(A)UWB signals are typically modulated pulse trains
1. Very short pulse duration (<1 ns)
2. Uniform or non-uniform inter-pulse spacing
Pulse repetition frequency (PRF) can range from hundreds of thousands
to billions of pulses/second Modulation techniques include
pulse-position modulation, binary phase-shift keying and others
(B)Spectral efficiency is a measure of the performance of encoding methods
that code information as variations in an analog signal. Any such
encoding will occupy an amount of bandwidth, and within that bandwidth
it is capable of transmitting bits at a certain rate. The number of
bits per second that can be transmitted per Hz of bandwidth defines
the encoding's spectral efficiency. The number of bits per second that
can be transmitted per Hz of bandwidth defines the encoding's spectral
efficiency. The maximum spectral efficiency one can hope for in a
channel with a certain signal to noise level, is given by the
Shannon-Hartley theorem. Low spectral efficiency does not necessarily
mean that a coding scheme is inefficient. As an example, consider Code
Division Multiplexed Access (CDMA) spread spectrum which is not
particularly spectrally efficient encoding scheme, when considering
a single channel, but the fact that one can "layer" multiple channels
on the same spectrum means that the spectrum utilization for a
multi-channel CDMA system can be very good nonetheless.
(C)
The efforts of IEEE 802.15 are divided up into four main areas :(1) Task
Group 1 (TG1) is creating a WPAN standard based on Bluetooth? to operate
in the 2.4 GHz ISM band; (2) TG2 is concerned with the coexistence of
unlicensed spectrum devices; (3) TG3 is responsible for high data rate
(in excess of 20 Mbps) WPAN standards; and (4) TG4 is creating a low data
rate, low power WPAN standard. An additional group, TG3a, was created
to
investigate physical layer alternatives for high data rate WPAN systems
(i.e. alternatives for the IEEE 802.15.3 physical layer).
IEEE 802.15.3a was an attempt to provide a higher speed UWB PHY enhancement
amendment to IEEE 802.15.3 for applications which involve imaging and
multimedia.
The IEEE 802.15.3a most commendable achievement was the consolidation of
23 UWB PHY specifications into two proposals using : Multi-Band Orthogonal
Frequency Division Multiplexing (MB-OFDM) UWB, supported by the WiMedia
Alliance, and Direct Sequence - UWB (DS-UWB), supported by the UWB Forum.
(D)The super frame has 3 parts:
Beacon: used only by the PNC to control the piconet timing.
Contention Access Period (CAP): Uses CSMA/CA with back-off procedure
Communication during this part of the frame uses the CSMA/CA
protocol with back-off procedure. This period is used for
stream-less data, channel time requests, authentication,
association request/response, asynchronous data and other commands
in the system. The PNC governs what can be sent in the Contention
Access Period.
Contention Free Period (CFP):
This part of the frame is composed of GTS (Guaranteed Time Slot)
and MTS (Management Time Slot). Each device makes a channel time
request during the CAP. The PNC responds by allocating GTS/MTS units
to the each DEV depending on the QoS requirements of the DEV. The
information concerning the assignment of GTS/MTS to a DEV is
transmitted during the BEACON portion of the frame. This is a
TDMA-based scheme to guarantee the DEVs their negotiated QoS. This
is very important to support real time video/audio applications as
they have very stringent requirements on timing jitter, end-to-end
delay etc.
(E) Large numbers of UWB transmitters working together have the potential to
cause interference to users across the spectrum (from around 1 to around 10
GHz for most currently planned devices). This frequency range includes a
number of economically and politically important users such as mobile phones
(GSM 1800, CDMA 1900 and UMTS), aeronautical and maritime radar, wireless
LAN
and HiperLAN, governmental and private fixed links, satellite links and in
some countries wireless cable services.
9. (10 Points) A critical design issue for future wireless ad hoc networks is the
development of suitable communication architecture, protocols and services that
efficiently reduce power consumption thereby increasing the operational lifetime of
network enable devices. For small communication devices (such as sensors, PDAs,
etc), reducing the transmission power may significantly extend the operational
lifetime of a device, thus enhancing the overall user experience. A distance between
two nodes could be determined by propagation model or GPS. (I) Can you enhance
DSR (Dynamic Source Routing) to minimize the total transmission power needed to
forward packets between devices in ad hoc wireless networks either through (A)
propagation model or (B) GPS location information? (II) Can you enhance DSR
(Dynamic Source routing) to maximize the time to network partition either through (A)
propagation model or (B) GPS location information?
Answer:
(I)
(A)Prior to transmitting a packet, a node updates its packet header to
indicate the power required to transmit the packet. A node overhearing
another node’s transmission can then use this information plus, a
localized measure of the received power, to compute the minimum
transmission power necessary to reach the overheard node. In this
simple manner, nodes can learn the minimum transmission power toward
neighboring nodes.
(B)The approach to routing in wireless ad hoc networks requires, at the
least, that every mobile have knowledge of the locations of every other
mobile and the links between them. This creates significant
communication overhead and increased delay. Research completed in
addresses this issue by proposing localized routing algorithms which
depend only on information about the source location, the location of
neighbors, and location of the destination. This information is
collected through GPS receivers which are included within each mobile.
Use the position of the neighboring mobile nodes and takes advantage
of the GPS to reduce the number of route query packets that are
generated to find a route from source node to the destination node.
DSR broadcasts the route requests to all its neighbors causing a route
request flood. By selecting and limiting the number of nodes to which
a query is forwarded, our scheme reduces power consumption to transmit
packet, and also reduce the network traffic thereby increasing the
bandwidth utilization and the efficiency of the network.
(II)
Since an early death of nodes will cause the network to partition, we can
maximize time to network partition through some power conservation
approaches.
(A) For propagation model, we can use Power Aware Routing protocol, which in
mobile ad-hoc networks aims at increasing node and network life by using
power-aware metrics for routing. The key to choosing the optimum metric for
power conservation (i.e. to increase individual node and hence, network life) is to
carefully share the cost of routing packets to avoid some critical nodes to be
dead.
(B) For GPS location information, we can use Minimum Energy Routing
protocol. By defining updated relay regions corresponding to every node, the
search for the next-hop node is restricted to a localized search, which leads to
lowest total energy consumption. This protocol requires strong connectivity, so it
is geared towards GPS.