Computer Networks. ENGG st Semester, Presented by: Ricky Kwok Department of Electrical and Electronic Engineering

Transcription

1 Computer Networks ENGG st Semester, 2011 Presented by: Ricky Kwok Department of Electrical and Electronic Engineering This sets of slides are based on those authored by Dr. Hayden So last year. All errors are mine! 1

2 Where are we in the semester? High Level Applications Systems Image & Video Processing Computer Systems Computer Networks Digital Logic Combinational Logic Boolean Algebra Circuits Basic Circuit Theory Low Level Electrical Signals Voltage, Current Power & Energy 2

3 Road Map Networks Internet Wireless 3

4 Introduction Computer Network: A collection of communicating computers Enabled by a set of interconnection devices 4

5 Categorizing Computer Networks LAN Local Area Network Network that covers a small geographical area A company, a university, home, etc WAN Wide Area Network Network that covers a relatively larger geometrical area Usually beyond a building, a city, or even a country Global network? Beyond the Earth? Many other ways to categorize networks 5

6 Direct Connections Computer C Computer D Computer A Direct (Physical) Connection Computer B Upside: No need to do sharing of networking resource (i.e., the links) Downside: Number of links proportional to number of computers 6

7 Switching Network Computer C Computer D switch Computer A Must share the physical connection to A to support data connection to/from C, D Computer B Analogy: A round-about in a road system 7

8 How to share resources (switch)? Two important ways of network communication: Circuit switching dedicated circuit per connection e.g., public switch telephone network (PSTN) Packet switching Data sent through the network in discrete packets e.g., Internet Basically, in both cases, resource is timeshared Main difference: Granularity of the time of sharing 8

9 Circuit Switching End-to-end resources reserved for exclusive use by each connection Network resources (e.g., bandwidth) divided into pieces Pieces allocated to each connection Guaranteed performance The link is solely occupied Resource piece idle if not used by owning call (no sharing) How to divide resources? frequency division time division 9

10 Packet Switching A 10 Mbs Ethernet statistical multiplexing C B queue of packets waiting for output link D 1.5 Mbs E Each end-end data stream divided into packets Different users share the same resource as needed Excessive queuing might cause packets to be dropped 10

11 Packet Switching vs. Circuit Switching 1 Mbit link each user: 100 kbps when active active 10% of time N users 1 Mbps link circuit-switching: 10 users packet switching: with 35 users, probability that >10 users active are less than.0004 Packet switching allows more users to use network! 11

12 Packet Implementation Header Payload Two main parts of a packet: Header Payload Header: Meta information about the content of the packet e.g., Source and destination address, type of packet, length etc. Payload: The actual content of the packet Length of payload specified in header, or it might be a fixed length In case of fixed length payload: padding is needed 12

13 Packet Routing The header of a packet states should contains enough information for the delivery of a packet Cf. address written on an envelope The process of determining and delivering packets is called routing Two common ways to route: Source routing Multi-hop routing 13

14 Source Routing Before a packet is sent, the source has already figured out the complete route the destination Store the calculated route in the header of a packet Similar to a direction to a location: Turn left at the 1 st traffic light, then Go straight, then Turn right at the post-office No address is needed 14

15 Multi-hop Routing Only need destination address in the packet Machines along the way determine how to deliver the packet to the destination Each computer-computer transfer is referred as a hop Usually designated machines are used only for routing purposes, we call them routers 15

16 Packet Forwarding Techniques A B Packets must travel from point A to point B via many intermediate points How should each intermediate point handle the packet? Two common switching methods: Store-and-forward Cut-through 16

17 Store-and-Forward A B Packets are forwarded out only after the entire packet has been received Easy to implement Allow error checking at each intermediate steps Need big buffers Transfer latency proportional to the number of hops 17

18 Store-and-Forward Performance L R R R Takes L/R seconds to transmit (push out) packet of L bits on to link of R bps Entire packet must arrive at router before it can be transmitted on next link delay = 3*L/R Example: L = 7.5 Mbits R = 1.5 Mbps delay = 15 sec 18

19 Packet Switching: Message Segmenting Now break up the message into 5000 packets Each packet 1,500 bits 1 msec to transmit packet on one link pipelining: each link works in parallel Delay reduced from 15 sec to sec 19

20 Cut-Through Forwarding Packets are forwarded as soon as its destination address is known Destination address preferred to be at the front of a packet Cannot perform error checking Error can only be detected once the entire packet has arrived at the router No need for large packet buffers Can cause queue-blocking or even deadlock in some networks Lower latency than store-and-forward, but throughput the same 20

21 Is packet switching always better? Great for bursty data resource sharing simpler, no call setup Excessive congestion: packet delay and loss protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior? bandwidth guarantees needed for audio/video apps still an unsolved problem 21

22 22

23 Measuring Network Performance Latency Throughput Measures the time to get the first byte of data across a network Unit: seconds (s) Measures the aggregated amount of data transported over a period of time Unit: byte per second (Bps) or bit per second (bps) 23

24 Example: Mailing DVDs Mailing a DVD takes 1 day = 24 hours = 86400s Each DVD is 4GB Latency is 86,400s Throughput is an average value If we mail 1 DVD everyday, then the throughput is: 4GB / 86400s 46.2 kbps 370 kbps 24

25 Example: Scaling Up What if we mail 100 DVDs at the same time? Latency remains the same: 86400s Bandwidth increases by 100 times 37 Mbps In other words, mailing 100 DVDs at the same has higher throughput than your average broadband internet access at home 25

26 Physical Mail vs. Network When the data transfer is large, traditional postal mail has much higher bandwidth than any Internet connection E.g., Amazon AWS Import/Export allows company to send data to the Internet cloud by physically sending hard drives. When is physical mail faster? Available Internet Connection Theoretical Min. Number of Days to Transfer 1TB at 80% Network Utilization When to Consider AWS Import/Export? T1 (1.544Mbps) 82 days 100GB or more 10 Mbps 13 days 600GB or more T3 (44.736Mbps) 3 days 2TB or more 100 Mbps 1-2 days 5TB or more 1000 Mbps < 1day 60TB or more 26

27 Which one to use? Both are important Latency is more important when small amount of data is needed to be sent back-and-forth very quickly Interactive applications such as: Web surfing, online game, remote log in, etc. Microprocessor network Throughput is more important when streaming data are considered Video streaming Music streaming Large file transfer 27

28 Which network is better? time time time Network A has low latency and low throughput Network B has high latency, high bandwidth Network C has moderate latency, moderate throughput, and unpredictable performance 28

29 Which network is better? (A) time time Network A is the best for short, bursty traffic, such as web browsing. Think of the first yellow packet as all you need for 1 web page , IM (MSN messenger, gtalk, etc.) are all examples of short bursty traffic time 29

30 Which network is better? (B) time time time Network B has the longest latency (longest wait time) But once the data arrive, it has the highest bandwidth Network B is best for streaming data E.g., youtube, and any other streaming data 30

31 Which network is better? (C) time time time Network C is difficult to classified Network C is the most realistic model of today s ISP s capability 31

32 How do loss and delay occur? packets queue in router buffers packet arrival rate to link exceeds output link capacity packets queue, wait for turn packet being transmitted (delay) A B packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers 32

33 Four sources of packet delay 1. nodal processing: check bit errors determine output link Order of microseconds in high-speed routers A transmission 2. queueing propagation time waiting at output link for transmission depends on congestion level of router Order of microseconds to milliseconds B nodal processing queueing 33

34 Delay in packet-switched networks 3. Transmission delay: Store-and-forward delay R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R Order of microseconds to milliseconds A transmission 4. Propagation delay: d = length of physical link s = propagation speed in medium (~2x10 8 m/sec) propagation delay = d/s Note: s and R are very different quantities! propagation B nodal processing queueing 34

35 Caravan Analogy 100 km 100 km ten-car caravan toll booth Cars propagate at 100 km/hr Toll booth takes 12 sec to service a car (transmission time) car~bit; caravan ~ packet Q: How long until caravan is lined up before 2nd toll booth? toll booth Time to push entire caravan through toll booth onto highway = 12*10 = 120 sec Time for last car to propagate from 1st to 2nd toll both: 100km/ (100km/hr)= 1 hr A: 62 minutes 35

36 Caravan analogy (more) 100 km 100 km ten-car caravan toll booth Cars now propagate at 1000 km/hr Toll booth now takes 1 min to service a car Q: Will any car(s) arrive to 2nd booth before all cars serviced at 1st booth? toll booth Yes! After 7 min, 1st car at 2nd booth and 3 cars still at 1st booth. 1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router! 36

37 Nodal delay d proc = processing delay typically a few microsecs or less d queue = queuing delay depends on congestion Vary from packet to packet d trans = transmission delay = L/R, significant for low-speed links d prop = propagation delay a few microsecs to hundreds of msecs 37

38 Queueing delay: simple analysis R=link bandwidth (bps) L=packet length (bits) a=average packet arrival rate (packets/s) traffic intensity = La/R La/R ~ 0: average queueing delay small La/R -> 1: delays become large La/R > 1: more work arriving than can be serviced, average delay infinite! 38

39 39

40 40

41 Internet 41

42 Internet Basics The Internet is a network that connects millions of computing devices throughout the world The Internet is formed by connecting many different networks. Inter -Net Distributed management What s so special about the Internet? 42

43 Back in time Before the Internet, each University and military unit has their own network Different standards Some of them work only when the computers are physically close to each other Simple point-to-point connections Networking was a niche Src: DEC s PDP-10 43

44 ARPANET Advanced Research Projects Agency Network Project active during early 70 s Commonly referred as the predecessor of the Internet A large-scale research effort involved multiple Universities on building networks that Connect computers remotely in long distance Scalable First operational packet-switch network 44

45 What s the Internet: nuts and bolts view Now, Internet is a network that interconnects millions of computing devices throughout the world Computer devices can be a PC, a workstation, or a PDA, TV, even a toaster! These devices are called hosts or end systems and are running network applications router local ISP company network server workstation mobile regional ISP 45

46 What s the Internet: nuts and bolts view Hosts are connected by communication links fiber, copper, radio, satellite transmission rate = bandwidth Most of the time, hosts are connected through intermediate switching devices called routers A router forwards packets (chunks of data) router local ISP company network server workstation mobile regional ISP 46

47 What s the Internet: nuts and bolts view Internet Service Providers (ISP) provides network access to hosts protocols control sending, receiving of messages e.g., TCP, IP, HTTP, FTP router server local ISP workstation mobile regional ISP company network 47

48 What s a protocol? human protocols: what s the time? I have a question introductions specific msgs sent specific actions taken when msgs received, or other events network protocols: machines rather than humans all communication activity in Internet governed by protocols protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt 48

49 What s a protocol? a human protocol and a computer network protocol: Hi Hi Got the time? 2:00 time TCP connection req TCP connection response Get <file> 49

50 What s the Internet: nuts and bolts view Internet Service Providers (ISP) provides network access to hosts protocols control sending, receiving of messages router server local ISP workstation mobile e.g., TCP, IP, HTTP, FTP The Internet is implemented following Internet standards called RFC (Request for comments) developed by IETF (Internet Engineering Task Force) regional ISP company network 50

51 What s the Internet: a service view communication infrastructure enables distributed applications: Web, , games, e- commerce, database., voting, file (MP3) sharing communication services provided to apps: connectionless connection-oriented However, current Internet does not provide delay (Quality of Service) guarantee! 51

52 A closer look at network structure: network edge: applications and hosts network core: routers network of networks access networks, physical media: communication links 52

53 3 Big Ideas about Internet The end-to-end design principle The core network is dumb Layering Modular design Division of labor Packet switching Data are transmitted as packets Compared to circuit switching networks 53

54 54

55 History of Packet Switching Though it sounds trivial now, the concept of packet switching was first introduced back in the time when ARPANET was still at its infant stage To provide reliable connection in case a network link is down as long as smart protocol (TCP) is used To improve the system capacity To make a scalable system 55

56 Internet Packet Routing Surprisingly, the Internet routing protocol is relatively simple Each router only knows the general direction on where to route a packet A hierarchal routing idea Example: Send a package from the USA to the address Rm 516, Dept of EEE, HKU, HK 1. Send to HK 2. Postman (router) at HK sends to HKU 3. Router at HKU sends to EEE 4. Router at EEE sends to Rm 516 The simple design of IP routing makes the Internet extremely scalable! 56

57 Internet structure: network of networks a packet passes through many networks! local ISP Tier 3 ISP Tier-2 ISP local ISP local ISP Tier-2 ISP local ISP Tier 1 ISP NAP local ISP Tier 1 ISP Tier-2 ISP local ISP Tier 1 ISP Tier-2 ISP local ISP Tier-2 ISP local ISP 57

58 Visualizing Internet Routes Traceroute A standard tool to look for all the intermediate routers to a remote site Interesting website that provides visualization of routes 58

59 59

60 Layered Architecture (1) Divides the task of data communication into layers of smaller modules Each layer has a well defined subtask Each module has restricted interaction with each other Each layer provides services to the layer(s) above utilizes services from the layer(s) below Uses services of Uses services of Layer 2 Layer 1 Layer 0 Provides services to Provides services to 60

61 Layered Architecture (2) The module in a particular layer only communicates with its counterpart on the other side of the network at the same layer Any module can be replaced with another module at the same layer as long as it provides the exact same interface This is the reason, e.g., your web browser works identically regardless of whether you are using WiFi network or a wired network, or 3G mobile network 61

62 Network Protocol A common language for different networks to communicate Protocol defines the format and order of messages being sent and received among network entities It also specifies actions that should be taken by the entity upon message transmission and receipt Human analogies: say hello when you pick up a phone When you are asked what is the time?, you would answer The time is instead of My shirt is white 62

63 Layers, protocols, and interfaces. 63

64 Protocol Hierarchies The philosopher-translator-secretary architecture 3 I like rabbits Message Philosopher J'aime bien les lapins 2 L: Dutch Ik vind konijnen leuk Information for the remote translator Translator L: Dutch Ik vind konijnen leuk 1 Fax #--- L: Dutch Ik vind konijnen leuk Information for the remote secretary Secretary Fax #--- L: Dutch Ik vind konijnen leuk 64

65 The OSI Network Model Open Systems Interconnection Basic Reference Model OSI 7 layer model A standard layering model Abstract the a network into 7 layers Unfortunately (fortunately) the Internet has its own notion of layering Internet layer slightly different from the OSI model But may be mapped back to 5 layers in the OSI model Application Presentation Session Transport Network Data Link Physical 65

66 Internet protocol stack application: supporting network applications FTP, SMTP, HTTP transport: host-host data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements PPP, Ethernet physical: bits on the wire application transport network link physical 66

67 How Layering Works? Each layer hides the layers above from any detail about lower layers Each layer focuses on its core functions, assuming other layers will handle the rest data application transport network link physical data ack network link physical data e.g., A logical connection at transport layer application transport network link physical 67

68 Layering: physical communication data application transport network link physical application transport network link physical application transport network link physical network link physical data application transport network link physical 68

69 Protocol layering and Data Each layer takes data from above adds header information to create new data unit passes new data unit to layer below source destination Ht HnHt Hl HnHt M M M M application transport network link physical application transport network link physical Ht HnHt Hl HnHt M M M M message segment datagram frame 69

70 Summary: layering abstracts details Each computer thinks it is directly connected to another one through a magic cloud 70

71 71

72 End-to-End Design Principle In laymen s term: What happens at the end host is what really matters Slightly more technical: Some information are only known to the end hosts, so there should be a distinction in responsibility on what the end hosts should perform and what the network should perform. Jump to conclusion: The core network should be dumb. 72

73 End-to-End: 2 versions According to van Shewick s interpretation, there are two related version of end-to-end arguments as proposed by the original author: Narrow Version: The function in question can completely and correctly be implemented only with the knowledge and help of the application standing at the endpoints of the communication system; Therefore, providing that questioned function as a feature of the communication system itself is not possible [i.e. it is not possible to provide a complete and correct implementation of the function within the communication system only]. Broad Version: a function or service should be carried out within a network layer only if it is needed by all clients of that layer, and it can be completely implemented in that layer. Barbara van Schewick, Internet Architecture and Innovation, pp

74 Example: Reliable File Transfer While you are uploading your photo to Facebook on your mobile phone, you entered into an elevator and lost your mobile phone connection Q: which of the following makes more sense? 1. The network is smart, so that when the connection comes back (after you exit the elevator), it should retry the upload of the photo 2. The network is dumb, and terminate the connection to Facebook without uploading the picture 74

75 File Upload (cont d) According to E2E principle, the Internet design picks (2) as the answer Reason: the mobile phone network should not retry the upload because it does not, and should not understand the high-level requirement of the user e.g., You may decide to redo the upload if it has failed, and if the photo upload was really that important Or, you may just give up and upload another photo If the network (think your mobile phone carrier) retries the upload, AND you perform another upload at the same time, you will end up with two different versions of photo uploaded 75

76 Quick Summary Internet is a collection of global networks End-to-End architecture allows the core of the network be constantly changing without affecting the end nodes Packet-switch network architecture allows many more nodes to share the same resource than circuit switch Standardized network protocols allow different computers to communicate Protocol stacks provide layer abstractions 76

77 77

78 Introduction The Internet Protocol (IP) layer, as well as TCP, are the two most important protocols of the Internet Roughly corresponds to the Network layer in the OSI 7-layer model Currently IPv4 deployed on most of Internet IPv6 starting to be deployed in some ISPs 78

79 Service Model A best effort service model It tries its best to deliver datagrams, but it makes no guarantees Connectionless Two parts of service: Addressing Scheme: Allows node in the Internet to be identified Datagram delivery: Allows data to be transmitted over wide varieties of underlying networks 79

80 Data Delivery Data are delivered in a best effort, connectionless manner Relax the requirements of the underlying network Layering abstracts the difference between underlying networks to higher layer -- TCP Data are transmitted using predefined packet Fragmentation+Reassembly is used to break long packets into smaller pieces if needed Usually as a result of underlying physical network 80

81 IPv4 Packet Header Version: version of protocol. 4 or 6 Length: length including header TTL: Time to live Checksum: checksum of header SourceAddr: Source IP address DestinationAddr: Destination IP Address 81

82 Addressing Scheme A node on the Internet is identified by its IP address In IPv4 standard 32 bits Usually represented as 4 integers connected with dots E.g., A hierarchical addressing scheme Host part + network part The higher order bits (bits on the left) are used to identify network The lower order bits are used to identify hosts within the network 82

83 Sample Addressing Scheme E.g. 3 IP networks x x x The first 24 bits are network part Last 8 bits are host part Q: How many hosts can you have within 1 network? Q: Why 24 bits? LAN

84 Address Classes IP addresses are divided into 3 classes Class A: First bit is 0 7 bits for network id 16M hosts Class B: First 2 bits are bits for network id Max 64k hosts Class C First 3 bits are bits for network id Max 256 hosts 84

85 Subnetting Problems with the 3 classes classification: Many IP address not usable even if the owner of the network id does not need all numbers And IPv4 IP numbers are running out! Subnetting allows fine grain division of an IP address in to network+subnet+host parts Real network identifier = IP & Subnetmask 85

86 Private vs. Public Original IPv4 assumes all hosts on the Internet have their own IP addresses Public IP address Uniquely identify a host on the Internet Three groups of special private addresses for private LAN uses. 10.x.x.x x.x x.x They should not be used publicly Not unique over the entire Internet One group of auto assigned private IPs x.x Should not be used normally E.g., assigned automatically by OS if no reply from DHCP server 86

87 IP Routing Each router on the Internet has a limited information about what the rest of the Internet looks like A router understands what is the next hop of a packet based on its routing table and forwarding table Special inter-router protocol that updates routing tables of routers Cons: It is possible to have loop Pros: Routes are not always optimal Scalable` Destination x.x en0 en1 Output Interface en x.x en en1 87

88 IPv6 The next generation IP standard Designed to address issues in IPv4 IP address running out Security, real-time service support, etc. IPv6 Address: 128 bits Even a toaster can have an IP address! x:x:x:x:x:x:x:x Each x is a 16 bits hexadecimal number E.g., 47CD:1234:4422:AC02:0022:1234:A456:0124 Like in IPv4, IPv6 defines many address spaces depending on the prefix of the address Deployment much slower than expected Mostly because IPv4 has not run out as fast as expected Network Address Translation (NAT) hides many private IPs from the public internet 88

89 Physical Network IP protocol runs on top of many different physical networks Common physical networks Ethernet Wireless LAN 89

90 Ethernet Most common LAN technology Common standards: 10 Mbps Ethernet, 100 Mbps Gigabit Ethernet and 10 Gigabit Ethernet Uses a shared medium Originally shared a physical wire 10Base2, 10Base5 Modern Ethernet networks use switches/hubs to connect multiple segments 90

91 MAC An excellent example of the general Carrier Sense Multiple Access with Collision Detect (CSMA/CD) technology Multiple node on a Ethernet LAN may want to transmit at the same time If more than 1 host transmit at the same time, collision occurs The algorithm to determine when/how to transmit/ retransmit is called is media access control (MAC) An Ethernet node do the following thing: It senses the media to see if it is idle or someone else is transmitting (CS) It transmit opportunistically While it transmits, it detects if collisions occur (CD) If collisions have occurred, it wait for a random time and retransmit again 91

92 Frame Format and Address Data sent on a Ethernet link as frames simple header min 46 bytes Need minimum length for collision detection max 1500 bytes Each Ethernet adaptor has a 48 bits globally unique address. The Ethernet address Sometimes called MAC address Denoted as x:x:x:x:x:x Each x is a 8 bit number written as 2 hex numbers Top bits: manufacturer id; lower bits: serial # Need to be unique to avoid name crash within a shared LAN 92

93 93

94 Elements in Wireless Network Wireless host End-system that runs application, e.g. laptop, PDA, or even desktop computer Wireless link Base station Responsible for sending and receiving data to and from a wireless host E.g. cell tower in cellular network, access point in an wireless LAN Network infrastructure The larger network that the wireless hosts wants to communicate 94

95 Differences between wired and wireless Signal strength Signal in free space decreases as the propagation distance increases Interference Several technologies use the same frequency band Multipath propagation Portions of the electromagnetic wave reflect off objects and the ground, taking paths of different lengths Conclusion: bit errors will be more common in wireless links than in wired links 95

96 Radio Wave Propagation 96

97 Radio Wave Propagation Radio waves propagate through space as traveling electromagnetic waves Continuous flow of energy from electric field to magnetic field, and vice versa Energy is lost over a distance Radio waves arrive at a mobile receiver from different directions with different time delays Combined via vector addition (superimposed) at the receiver antenna Resultant signal strength depends on whether the incoming waves reinforce or cancel out each other Thus, there could be substantial amplitude and phase fluctuations, i.e., fading 97

98 Radio Wave Propagation: Fading 98

99 Radio Wave Propagation: Fading Long-term fading (slow fading): Associated with a propagation distance of a few kilometers Short-term fading (fast fading): Associated with a propagation distance of a few hundred meters Resulted from movements of the transmitter, receiver, and surrounding objects Rayleigh distribution can capture the statistical variations for locations heavily shadowed by surrounding buildings Rician distribution can capture the statistical variations for locations where there is one dominant propagation path (e.g., base station is visible to the mobile) 99

100 IEEE Wireless LAN b 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 a 5-6 GHz range up to 54 Mbps g GHz range up to 54 Mbps All use CSMA/CA for multiple access All have base-station and ad-hoc network versions 100

101 Base station approach Wireless host communicates with a base station base station = access point (AP) Basic Service Set (BSS) (a.k.a. cell ) contains: wireless hosts access point (AP): base station BSS s combined to form distribution system (DS) 101

102 Ad Hoc Network approach No AP (i.e., base station) wireless hosts communicate with each other to get packet from wireless host A to B may need to route through wireless hosts X,Y,Z Applications: laptop meeting in conference room, car interconnection of personal devices battlefield IETF MANET (Mobile Ad hoc Networks) working group 102

103 IEEE : multiple access Collision if 2 or more nodes transmit at same time CSMA makes sense: get all the bandwidth if you re the only one transmitting shouldn t cause a collision if you sense another transmission Collision detection doesn t work: hidden terminal problem 103

104 Collision Avoidance Although wireless is a shared medium, carrier sense/collision detection is difficult because not all nodes can see all other nodes Hidden node problem Collision happens due to a hidden node that the sender is not aware of E.g., since A,C don t see each other, they may send to B at the same time and collide Exposed node problem A node fail to send thinking the medium is being used when it is not E.g., B is transmitting to A and C wants to transmit to D. C hears B is transmitting so it wait, but in fact, it is ok for C to send to D 104

105 IEEE MAC Protocol: CSMA/CA CSMA: sender - if sense channel idle for DIFS sec. then transmit entire frame (no collision detection) -if sense channel busy then random backoff CSMA receiver - if received OK return ACK after SIFS (ACK is needed due to hidden terminal problem) 105

106 Collision avoidance mechanisms Problem: two nodes, hidden from each other, transmit complete frames to base station wasted bandwidth for long duration! Solution: small reservation packets nodes track reservation interval with internal network allocation vector (NAV) 106

107 Collision Avoidance: RTS-CTS exchange sender transmits short RTS (request to send) packet: indicates duration of transmission receiver replies with short CTS (clear to send) packet notifying (possibly hidden) nodes hidden nodes will not transmit for specified duration: NAV 107

108 Collision Avoidance: RTS-CTS exchange RTS and CTS short: collisions less likely, of shorter duration end result similar to collision detection IEEE allows: CSMA CSMA/CA: reservations polling from AP 108

109 More on Multiple Access 109

110 Multiple Access: FDMA Merit: Hardware simplicity Demerit: Not very flexible Not efficient use of bandwidth 110

111 Multiple Access: TDMA Merit: Flexible bit-rate Demerit: Synchronizatio n is a must 111

112 Multiple Access: DSSS 112

113 Multiple Access: DSSS An example: three users communicating with the BS: A s code: (1, -1, -1, 1, -1, 1); B s code: (1, 1, -1, -1, 1, 1); C s code: (1, 1, -1, 1, 1, -1) 113

114 Multiple Access: Comparison 114

115 Cellular Communication 115

116 Cellular Frequency Reuse Adjacent cells assigned different frequencies to avoid interference Objective is to reuse frequency in nearby cells Transmission power controlled to limit power at that frequency escaping to adjacent cells The question is: deciding the reuse factor, N In hexagonal cell pattern, only the following values of N are possible: N = I 2 + J 2 + (I x J), where I, J = 0, 1, 2, 116

117 Cellular Frequency Reuse 117

118 Cellular Capacity Enhancement Adding more channels, or Cell splitting: Increasing reuse 118

119 Cellular Capacity Enhancement 119

120 Cellular Interference Subject to interference from nearby cells using the same frequencies 120

121 Cellular Capacity Reducing q, number of cells per cluster is reduced Then, number of channels per cell is increased Overall capacity also increased But co-channel interference is higher 121

122 General Cellular Call Processing 122

123 General Cellular Call Processing 123

124 General Cellular Call Processing 124

125 Future Wireless Systems A seamless integration of: Cellular systems WLANs for indoor applications Microwave access (e.g., WiMAX, LTE) for metropolitan areas WPANs for short-range and low mobility applications All based on TCP/IP 125