data communications and the internet - srm institute … conditions security elements of a wireless...
TRANSCRIPT
Required Reading
Chapter 6, Computer Networking: A Top-Down Approach Featuring the Internet, James F. Kurose and Keith W. Ross, Addison-Wesley, third edition, 2005.
Local Area Networks (LANs)
LAN
products
fibertwisted pair
coaxair (wireless)
busStarringtree
FDDIEthernetToken ringWireless LAN
token passing polling
IEEEISO, ATM Forum
MACprotocols
standardbodies
topologies
transmissionmedia
Wireless Networks The boom in wireless communications in recent
years has been exponential and the wireless communications landscape is changing rapidly.
Number of wireless (mobile) phone subscribers now exceeds the number of wired phone subscribers !!
computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet access
Challenges/Issues Mobility management QoS Management Channel conditions Security
Elements of a wireless network
network infrastruc
ture
wireless hosts laptop, PDA, IP
phone run applications may be stationary
(non-mobile) or mobile wireless does not
always mean mobility
base station typically
connected to wired network
relay -responsible for sending packets between wired network and wireless host(s) in its “area” e.g., cell
towers 802.11 access points
Elements of a wireless network
network infrastructure
wireless link typically used to
connect mobile(s) to base station
also used as backbone link
multiple access protocol coordinates link access
various data rates, transmission distance
Comparison of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps802.15
802.11b802.11{a,g}
IS-95 CDMA, GSM
UMTS/WCDMA, CDMA2000
.11 p-to-p link
2G
3G
Indoor
10 – 30m
Outdoor
50 – 200m
Mid rangeoutdoor
200m – 4Km
Long rangeoutdoor
5Km – 20Km
Wireless Networks
1001010.1 Mbps
Vehicle
Walk
Fixed
Walk
FixedIndo
or
O
utdo
or
GSM
, CD
MA
WLANWPAN
LAN
WAN3G2G
WPAN – Wireless Personal Area Network
Elements of a wireless network
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
Wireless Link CharacteristicsDifferences from wired link ….
decreased signal strength: radio signal attenuates as it propagates through matter (path loss)
interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well
multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times
…. make communication across (even a point to point) wireless link much more “difficult”
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access):
AB
C
Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each othermeans A, C unaware of their
interference at B
A B C
A’s signalstrength
space
C’s signalstrength
Signal fading: B, A hear each other B, C hear each other A, C can not hear each other
interferring at B
Wireless Local Area Networks (WLANs)
WLANs-Motivation Mobile users’ typical demands of information access is
characterized by heavy data files and applications; WLAN can provide mobility and speed at the same time
In major structured hot spots such as airports and rail stations, the mobile radio infrastructure support of data communications seems to be inadequate and expensive
For office users, mobility, simple and low-cost network scalability, and high-speed access are advantageous factors
For home users, advantages of mobility without new wiring and at the same time high-speed access are the key issues
WLAN provides network flexibility: No infrastructure (ad hoc); a single-cell network (BSS); a cellular topology (ESS)
Use of unlicensed spectrum reduces the user’s costInformation extracted from Advanced Communication Networks- Elec 5509
Wireless LAN Requirements Same as any LAN
High capacity, short distances, full connectivity, broadcast capability
Throughput: efficient use wireless medium Number of nodes:Hundreds of nodes across multiple cells Connection to backbone LAN: Use control modules to connect to both
types of LANs Service area: 100 to 300 m Low power consumption:Need long battery life on mobile stations
Mustn't require nodes to monitor access points or frequent handshakes
Transmission robustness and security:Interference prone and easily eavesdropped
Collocated network operation:Two or more wireless LANs in same area License-free operation Handoff/roaming: Move from one cell to another Dynamic configuration: Addition, deletion, and relocation of end
systems without disruption to users
Wireless LAN
Why Wireless LANs?
Mobility
Flexibility
No cables, save cost
Fast installation
Problems:Limited bandwidth
Noisy channel
Multipath
Security
Power consumption
IEEE 802.11 Wireless LAN - WiFi
802.11b 2.4-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence spread
spectrum (DSSS) in physical layer
• all hosts use same chipping code
widely deployed, using base stations
802.11a 5-6 GHz range up to 54 Mbps
802.11g 2.4-5 GHz range up to 54 Mbps
All use CSMA/CA for multiple access
All have base-station and ad-hoc network versions
802.11 LAN architecture wireless host communicates
with base station base station = access point
(AP) Basic Service Set (BSS) (aka
“cell”) in infrastructure mode contains: wireless hosts access point (AP): base
station• “Infrastructure WLAN”
ad hoc mode: hosts only• No central control, no
connections to the outside world.
BSS 1
BSS 2
Internet
hub, switchor routerAP
AP
Wireless Cells 802.11 has 11 channels Channels 1, 6, and 11 are
non-overlapping Each AP coverage area is
called a “cell” AP admin chooses
frequency for AP interference
possible: channel can be same as that chosen by neighboring AP!
host: must associatewith an AP
Wireless nodes can roam between cells
AP
AP
AP AP
AP AP
Channel 1
Channel 6
Channel 1Channel 11
Channel 6
Channel 1
IEEE 802.11: Physical Layer
Radio• Experience gained from
building mobile phone andwireless WAN networkscan be reused.
• Large coverage and deeppenetration.
• Very limited license-freefrequency bands.
• Very noisy.
Infrared• Cheap.• No licenses needed.• Interference by
sunlight, heat sources.• Low bandwidth.• Signals may be blocked
by many surroundingobjects.
IEEE 802.11: multiple access Like Ethernet, uses CSMA:
random access carrier sense: don’t collide with ongoing transmission
Unlike Ethernet: no collision detection – transmit all frames to completion acknowledgment – because without collision detection, you
don’t know if your transmission collided or not Why no collision detection?
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)
can’t sense all collisions in any case: hidden terminal, fading Goal: avoid collisions: CSMA/C(ollision)A(voidance)
IEEE 802.11 MAC Protocol: CSMA/CA802.11 sender1 if sense channel idle for DIFS then
- transmit entire frame (no CD)2 if sense channel busy then
- start random backoff time- timer counts down while channel idle- transmit when timer expires- if no ACK, increase random backoff
interval, repeat 2802.11 receiverif frame received OK
- return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
DIFS: Distributed Inter-frame Space
SIFS: Short Inter-frame Space
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4seq
control
802.11 frame: addressing
Address 2: MAC addressof wireless host or AP transmitting this frame
Address 1: MAC addressof wireless host or AP to receive this frame
Address 3: MAC addressof router interface to which AP is attached
Address 3: used only in ad hoc mode
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4seq
control
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
802.11 frame: moreduration of reserved transmission time (RTS/CTS)
frame seq #(for reliable ARQ)
frame type(RTS, CTS, ACK, data)
Hidden Station Problem
When A is transmitting aframe to B, since D is notin A’s coverage, D is ahidden station that Ddoesn’t sense a busychannel, thus D may starta transmission thatcollides with A’stransmission
Hidden Station Problem in Wireless LANs:
A BC D
A’s coverageB’s coverage
Exposed Station problem
While A is transmitting aframe to C, B senses a busychannel and concludes thatit may not transmit anyframe to D which isincorrect
Exposed Station Problem in Wireless LANs:
A BC D
A’s coverageB’s coverage
RTS/CTS
idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames
optional; not typically used sender first transmits small request-to-send (RTS) packets
to AP using CSMA RTSs may still collide with each other (but they’re short)
AP broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets!
Solution to Hidden/Exposed Station Problem
Development of WANs - X.25 protocol X.25 is the standard protocol for the interface between an end
system and a packet-switching network. The X.25 protocol is a (ITU) recommendation that defines
connections of terminals and computers to packet-switching networks. X.25 is an example of a connection-oriented network, was the first
public data network. The protocol was standardized in 1976 and revised a number of times.
The CCITT (ITU) revised the standard in 1992 and boosted the speed to 2 Mbits/sec.
X.25 is a well-established (and now somewhat dated) packet-switching service traditionally used to connect remote terminals to host systems.
The service provides any-to-any connections for simultaneous users. Signals from multiple users on a network can be multiplexed through
the X.25 interface into the packet-switched network and delivered to different remote sites.
The X.25 interface supports line speeds up to 64 Kbits/sec.
Frame Relay Designed to be more efficient than X.25. Frame relay offers many of the same connection-oriented, packet-
switched network services as X.25, but with better performance. Frame relay is a metropolitan and wide area networking solution
that implements a form of packet-switching technology. It routes frames of information from source to destination over a
switching network owned by a carrier such as Telstra. The essence of frame relay is that it is a connection oriented
network with no error control and no flow control. Frame relay was developed to utilize the higher data rates and
lower error rates of contemporary networking facilities Developed before ATM (Asynchronous Transfer Mode) and larger
installed base than ATM. ATM now of more interest on high speed networks.
Frame Relay: Protocol Architecture
• Control Plane: establishment and termination of logical connections
• these protocols are between a scriber and the network
• User Plane: transfer of user data between subscribers
• these protocol provide end-to-end functionality.
Frame Relay: Control Plane Between subscriber and network. Separate logical channel used.
Similar to common channel signaling for circuit switching services.
At data link layer, LAPD (Link Access Procedure for Data services) protocol is used for: Provide reliable data link control. Error and flow control between user and network.
Frame Relay: User Plane Actual transfer of information between end users. LAPF (Link Access Procedure for Frame Mode Bearer
Services) protocol is used for: Frame delimiting, alignment and transparency. Frame multiplexing/demultiplexing using addressing field. Ensure frame is integral number of octets. Ensure frame is neither too long nor short. Detection of transmission errors. Congestion control functions.
User Data Transfer One frame type used for carrying user data.
No control frame. No in-band signaling. No sequence numbers.
No flow control and no error control.
Frame Relay: User Data Transfer
only have one frame type which carries user data
no control frames means no inband signaling no sequence numbers
flag and FCS function as in HDLC address field carries DLCIDLCI (Data Link Connection Identifier) has
local significance only
ATM a streamlined packet transfer interface similarities to packet switching
transfers data in discrete chunks supports multiple logical connections over a single physical
interface ATM uses fixed sized packets called cells with minimal error and flow control data rates of 25.6Mbps to 622.08Mbps ATM is a high-speed network technology that is
designed for both LAN and WAN use.
Relation to Frame Relay and ATM Frame Relay and ATM have displaced X.25 for
enterprise WAN packet network connections Both are based on packet switching Frame Relay:
Is a very simple Layer 2 protocolHas low overhead
• Short 2-byte frame header• No numbering and ACKing
Operates at high speeds Its price is largely independent of distance Does not require special CPE hardware
Relation to Frame Relay and ATMATM is a Layer 1 and 2 protocolData is packed into an ATM frame, which
has no header The Layer 2 frame is segmented into cells The cells are transmitted across the provider
network• >10% overhead due to cell headers
ATM operates at high speeds Price is largely independent of distance
Relation to Frame Relay and ATM ATM requires special CPE equipment An ATM DTE can participate in many concurrent
virtual circuits ATM supports PVCs (permanent virtual circuit) and
SVCs (switched virtual circuit) ATM handles quality of service very well. It can
support voice and video as well as data. Many Frame Relay service networks are
implemented by converting incoming Frame Relay traffic to ATM This enables Frame Relay service providers to
offer quality of service that approximates ATM QoS
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
802.15: Personal Area Network less than 10 m diameter replacement for cables (mouse,
keyboard, headphones) ad hoc: no infrastructure master/slaves:
slaves request permission to send (to master)
master grants requests 802.15: evolved from Bluetooth
specification 2.4-2.5 GHz radio band up to 721 kbps
UWB (802.15.3) RFID (Radio Frequency Identification)
Broadband Wireless - WiMax WiMAX (Worldwide interoperability for Microwave Access) is
an exciting and unique technology that delivers high-speed, broadband fixed and mobile services wirelessly to large areas with much less infrastructure than is needed today.
What's WiMAX all about?
Next-generation wireless technology that delivers significant improvements in speed, throughput and capacity
Expected to be in widespread use within the next three to five years
Based on the IEEE 802.16 standards for wireless metropolitan-area networks (MANs) to extend coverage to today's existing networks
WIRELESS TECHNOLOGIES - COMPARISON
3G Cellular WLAN/802.11n WiMAX/802.16e
Coverage Ubiquitous Hotspot/Campus Zone/regionalQoS Controlled Shared spectrum ControlledMobility Full Very limited Fixed/FullRange Miles 100-500 ft MilesUser Speed 50–500 kbps 1-10 Mbps 1-10 MbpsArchitecture Hierarchical Flat, IP Flat, IP
Residential Area Networks
PSTN: Public Switched Telephone Network
ISDN: Integrated Service Digital Network
ADSL: Asymmetric Digital Subscriber
Cable modem: CATV
HFC: Hybrid Fiber Coax
Public Switched Telephone Network
• PSTN provides dedicated communication link between the computer and the server.
• Using an existing telephone line with a modem to provide a data communication between a computer in the residential area and a WAN server.
PSTNserver
56kbps
The Internet
home
• ISDN (Integrated Service Digital Network) provides dedicated end-to-end digital communication link between the computer and the server.
• Same as PSTN but the data communication is a digital one.
Integrated Switched Digital Network
PSTN
server
n*64kbps
The Internet
home
ISDN Features
PBX
LAN
Subscriber loop with ISDN channel
structure
Customer ISDN interface
ISDN Circuit switching network
Other networks
Packet switching network
Digital pipes to other networks and
services
...
...
...
ISDN• ISDN is intended to be a worldwide public telecom network to
replace existing PSTN.
• Narrowband ISDN: First generation specification (contributed to Frame relay).
• Broadband ISDN: Second generation specification (contributed to ATM) and supports very high data rate.
• Support of voice and non-voice applications using a limited set of standardized facilities.
• Support for switched and non-switched applications.
• Reliance on 64-kbps connections.
• Layered protocol architecture.
• Variety of configurations.
Asymmetric Digital Subscriber Line (ADSL)
• Modem technology designed to provide high-speed digital data transmission over ordinary telephone wire.
• Link between subscriber and a network (local exchange).– Local loop.
• Uses currently installed twisted pair cable.– Can carry broader spectrum (1 MHz or more).
• Asymmetric.– Greater capacity downstream than upstream.
• Frequency division multiplexing (FDM).– Lowest 25kHz for voice
• Plain old telephone service (POTS)– Use echo cancellation or FDM to give two bands.
• Range of up to 5.5km.
Asymmetric Digital Subscriber Line: Using existing telephone lines, but with a pair of special devices at the computer and the server.
ATU: ADSL Transmission Unit POTS: Plain Old Telephone Service
The Internet
server
PSTNPOTS
Home
16-640kbps
1.544M-6MbpsPCM
ATU-CATU-R
Asymmetric Digital Subscriber Line
Community Antenna TV (CATV)
Coax
Coax
CATVHead-End
Trunk splitter
…
Residential area
• Distribute broadcast television (and radio) programs.
• Based entirely on coaxial cable.
• TV programs are multiplexed using FDM.
Cable modem: Using part of the CATV bandwidth to provide data service.
CATV Bandwidth Usage
52MHz 550MHz 750MHz
frequency
Control
Video Channels
Delivered Bandwidth
Unused
Hybrid Fiber Coax (HFC)CATV
Head-End
server
TheInternet
fiber
coax
RF Amplifiers
Fiber Node…
Residential area• A network which incorporates both optical fiber along with coaxial
cable to create a broadband network• HFC provides a relatively high data rate for users.• The use of existing infrastructure leads to a cheap solution (even
cheaper than ADSL) for high data rate access to the Internetfrom residential areas.
• The cable is to be shared by all users. The data rate per user maydrop during peak hours.
Network Technologies
PSTN
Internet
Cellular Networks
LAN
PBX
WirelessData Networks
WLAN
CableNetworks
References Computer Networking: A Top-Down Approach Featuring the
Internet, James F. Kurose and Keith W. Ross, Addison-Wesley, third edition, 2005.
Computer Networks, 4/e, Andrew S. Tanenbaum, Prentice Hall. Data and Computer Communications, William Stallings, Prentice
Hall. www