Τηλεπικοινωνιακα Δίκτυα Υψηλων Ταχυτητων

1. Τηλεπικοινωνιακα Δίκτυα Υψηλων Ταχυτητων • Τηλεφωνικα Δικτυα, Ιντερνετ, ΑΤΜ • Προχωρημενα θεματα θεωριας αναμονης • Οπτικες Τεχνολογιες • Οπτικα Δίκτυα • Μεταγωγεις (switches) • QoS routing • Θεματα χρονοδορολόγησης (scheduling),δικαιοσυνης, κλπ Α. Εργασια (term paper), B. Προφορικη εξέταση

2. Information economy • Today’s economy • manufacturing, distributing, and retailing items • but also: publishing, banking, CDs, film making, bills…. • main ‘product’ is creation and dissemination of information • Future economy likely to be dominated by information • e.g. smart coffee machines, wireless tags on groceries • Can represent in two ways: analog (items) and digital (bits) • Digital is better • computers manipulate digital information • infinitely replicable • networks can move bits efficiently • We need ways to represent all types of information as bits • Ways to move lots of bits everywhere, cheaply, and with quality of service

3. Common network technologies • Two successful computer networks • telephone network • Internet • What comes next? (“next-generation” Internet) • something like an ATM network or MPLS or IPv6 or?

4. The Telephone Network

5. Tηλεφωνικό δίκτυο 1920’s: A: αναλογικοί σύνδεσμοι επικοινωνίας. Η μεταγωγή (switching) γινόταν χειρωνακτικά. 1988: To φωνητικό δίκτυο είναι πλέον ένα ψηφιακό δίκτυο που προσπελαύνεται από τοπικά αναλογικά loops. A: αναλογικοί σύνδεσμοι επικοινωνίας. D: ψηφιακοί σύνδεσμοι επικοινωνίας. Η μεταγωγή γίνεται ηλεκτρονικά.

6. Is it a computer network? • Specialized to carry voice (also carries fax, modem calls) • Internally, uses digital samples • Switches and switch controllers are special purpose computers • Its design principles apply to more general computer networks Concepts • Single basic service: two-way voice • low end-to-end delay • guarantee that an accepted call will run to completion • Endpoints connected by a circuit • signals flow both ways (full duplex) • associated with bandwidth and buffer resources

7. Fully connected core • simple routing • telephone number is a hint about how to route a call • hierarchically allocated telephone number space The pieces 1. End systems 2. Transmission 3. Switching 4. Signaling

8. 1. End-systems • Transducers • Dialer • Ringer • Switchhook Since wires for reception and transmission are shared, the received signal is also transmitted, leading to echo. This is OK for short-distance calls, but for long distance calls, we need to put in echo cancellors . This is expensive and has other disadvantages

9. 2. Transmission • Link characteristics • information carrying capacity (bandwidth) • propagation delay • time for electromagnetic signal to reach other end • light travels at 0.7c in fiber ~5 microseconds/km • NY to SF => 20 ms; NY to London => 27 ms • attenuation • degradation in signal quality with distance • long lines need regenerators • dispersion

10. Multiplexing • Trunks between central offices carry 100s of conversations on the same wire • Frequency Division Multiplexing: bandlimit call to 3.4 KHz and frequency shift onto higher bandwidth trunk; this is now obsolete • Time Division Multiplexing • first convert voice to samples • each sample is rounded to the nearest quantization level (256 quantization levels, logarithmically spaced according to μ-law or A-law) => 1 sample = 8 bits of voice • 8000 samples/sec => call = 64 Kbps • output interleaves samples from n input streams (each with a 1-byte buffer) • need to serve all inputs in the time it takes one sample to arrive => output runs n times faster than input • overhead bits mark end of frame

11. Transmission: Link technologies • Many in use today • twisted pair • coax cable • terrestrial microwave • satellite microwave • optical fiber • Popular today: fiber, satellite • Cost is in installation, not in link itself. Builders can install twisted pair (CAT 5), fiber, and coax to every room. Even if only one of them used, still saves money. • For long distance, there is overprovision by up to ten times

12. Transmission: fiber optic links • Advantages: lots of capacity, nearly error free, very little attenuation, hard to tap. • Three types • step index (multimode) • graded index (multimode) • single mode • Multimode: cheap, use LEDs, for short distances (up to a few kilometers) • Single mode: moreexpensive, use lasers, for longer distances (up to hundreds of kilometers)

13. Transmission: satellites • Long distances at high bandwidth • Geosynchronous • 36,000 km in the sky • up-down propagation delay of 250 ms • bad for interactive communication • slots in space limited • Nongeosynchronous (Low Earth Orbit or Medium Earth Orbit) • appear to move in the sky • we need more of them • handoff is complicated

14. 3. Switching: what does a switch do? • Transfers data from an input to an output • many ports (up to 200,000 simultaneous calls)` • need high speeds • Some ways to switch: • space division • time division (time slot interchange or TSI) • If inputs are multiplexed, we need a schedule • To build larger switches we combine space and time division switching elements

15. 4. Signaling • Switching systems establish temporary circuits, and they have a switch and a switch controller. • Switch controller is in the control plane (it does not touch voice samples). • Manages the network: call routing (including call forwarding), billing (including collect calls), alarms (ring bell at receiver), directory lookup (for 800/888 calls) • Switch controllers are special purpose computers, linked by their own internal computer network [the Common Channel Interoffice Signaling (CCIS) network].Messages on CCIS conform to Signaling System 7 (SS7) spec. • The switch controller keeps track of the state of every call through a state transition diagram

16. Challenges for the telephone network • Multimedia • simultaneously transmit voice/data/video over the network • people want it but existing network can’t handle it • bandwidth requirements • burstiness in traffic (TSI can’t skip input) • Flexibility • Backward compatibility of new services (huge existing infrastructure) • Regulation/Competition (future telephone networks are no longer monopolies; how to manage the transition?)

17. The Internet

18. What does it look like? • The Internet has doubled in size every year since 1969 • Soon, everyone who has a phone will also have an email account • Loose collection of networks organized into a multilevel hierarchy • 10-100 machines connected to a hub or a router • service providers also provide direct dialup access • or over a wireless link • 10s of routers on a department backbone • 10s of department backbones connected to campus backbone • 10s of campus backbones connected to regional service providers • 100s of regional service providers connected by national backbone • 10s of national backbones connected by international trunks

19. Example of message routing # traceroute henna.iitd.ernet.in traceroute to henna.iitd.ernet.in (202.141.64.30), 30 hops max, 40 byte packets 1 UPSON2-NP.CIT.CORNELL.EDU (128.84.154.1) 1 ms 1 ms 1 ms 2 HOL1-MSS.CIT.CORNELL.EDU (132.236.230.189) 2 ms 3 ms 2 ms 3 CORE1-MSS.CIT.CORNELL.EDU (128.253.222.1) 2 ms 2 ms 2 ms 4 CORNELLNET1.CIT.CORNELL.EDU (132.236.100.10) 4 ms 3 ms 4 ms 5 ny-ith-1-H1/0-T3.nysernet.net (169.130.61.9) 5 ms 5 ms 4 ms 6 ny-ith-2-F0/0.nysernet.net (169.130.60.2) 4 ms 4 ms 3 ms 7 ny-pen-1-H3/0-T3.nysernet.net (169.130.1.121) 21 ms 19 ms 16 ms 8 sl-pen-21-F6/0/0.sprintlink.net (144.228.60.21) 16 ms 40 ms 36 ms 9 core4-hssi5-0.WestOrange.mci.net (206.157.77.105) 20 ms 20 ms 24 ms 10 core2.WestOrange.mci.net (204.70.4.185) 21 ms 34 ms 26 ms 11 border7-fddi-0.WestOrange.mci.net (204.70.64.51) 21 ms 21 ms 21 ms 12 vsnl-poone-512k.WestOrange.mci.net (204.70.71.90) 623 ms 639 ms 621 ms 13 202.54.13.170 (202.54.13.170) 628 ms 629 ms 628 ms 14 144.16.60.2 (144.16.60.2) 1375 ms 1349 ms 1343 ms 15 henna.iitd.ernet.in (202.141.64.30) 1380 ms 1405 ms 1368 ms

20. What lies at the heart: Packets • Self-descriptive data (packet = data + header) • Packets vs. samples (as in circuit switching) • samples are not self descriptive; to forward a sample, we have to know where it came from and when; we can’t store it! Store and forward • Headers allows us to forward packets when we want (e.g. letters at a post office) • Efficient use of critical resources • Three problems: a) hard to control delay within network, b) switches need buffers c) convergence of flows can lead to congestion.

21. Τι κρατάει το Internet μαζί? 1. Η διευθυνσοποίηση (addressing): πως δηλ. aναφερόμαστε σε μια μηχανή στο δίκτυο 2. Η δρομολόγηση (routing): πως να φτάσουμε εκεί. 3. To Internet Protocol (IP): πως να μιλάμε μεταξύ μας ώστε να καταλαβαινόμαστε. Για να μπείς στο Internet πρέπει να πάρεις μια διεύθυνση από τον administrator. Αν έχεις μόνο έναν σύνδεσμο στο δίκτυο τότε ΟΚ, αλλιώς χρειάζεσαι αλγόριθμο δρομολόγησης.Τα πακέτα σου πρέπει να τα φορμάρεις σύμφωνα με το IP πρωτόκολλο για να ξέρουν οι routers τι να τα κάνουν.

22. Κλάσεις ΙΡ διευθύνσεων Το prefix δίνει τον αριθμό δικτύου, και το suffix δίνει τον αριθμό του υπολογιστή. Ο αριθμός δικτύου απαιτεί διεθνή συνεννόηση, αλλά ο αριθμός υπολογιστή δίδεται τοπικά. Η διεύθυνση που έχει όλα 1, είναι για limited broadcast.

23. Eνας router είναι ένας κόμβος μεταξύ δικτύων. Οι routers έχουν μιά IP διεύθυνση για κάθε δίκτυο στο οποίο ανήκουν. Αυτήν την στιγμή υπάρχουν πάνω απο 80000 δίκτυα. Πρόβλημα: αν θέλεις να βάλεις πάνω από 256 μηχανές, χρειάζεσαι δίκτυο τύπου Β, το οποίο επιτρέπει μέχρι και 64K μηχανές => wasted address space

24. Τι τύπου διεύθυνση είναι η 135.104.53.100?

25. Πως τα LANs χρησιμοποιούν hardware (ή physical) addresses για να φιλτράρουν τα πακέτα Π.χ. Ethernet (τα πεδία είναι σε bytes; οι διευθύνσεις στα πλαίσια είναι hardware διευθύνσεις) Γενικά, οι υλικές διευθύνσεις μπορεί να είναι στατικές ή δυναμικές

26. Αddress Resolution Techniques H IP διεύθυνση πρέπει να μετατραπεί σε hardware διεύθυνση για να σταλεί το πακέτο στο LAN. 1. Table Lookup: 2. Closed-Form Computation:Είναι δυνατή όταν οι hardware διευθύνσεις είναι δυναμικές. Π.χ. hardware_address = ip_address & 0xff

27. 3. Address Resolution με ανταλλαγή μηνυμάτων Π.χ. Το ΑRP πρωτόκολλο Το address resolution γίνεται κάθε φορά τοπικά για ένα δίκτυο.

28. Μορφή ARP μηνύματος Εναλλακτικά μπορεί αν χρησιμοποιηθεί κάποιος server για Address Resolution. Επίσης μπορεί να χρησιμοποιείται caching για μείωση του αριθμού των μηνυμάτων που στέλνονται.

29. Eπικεφαλίδα ενός IP datagram Δρομολόγηση ενόςIP datagram

30. Επικεφαλλίδα για την επόμενη γενιά του ΙΡ πρωτοκόλλου (IPv6)

31. Routing • How to get to a destination given its IP address? • Strictly speaking, you need next hop information for every node in the network (10’s of millions). • With hierarchical design, we need next hop information for the nodes in the same sub-network (that’s OK), and also next hop information for every network in the Internet (> 80,000 now) • Instead, keep detailed routes only for local neighborhood; for unknown destinations, use a default router • Reduces size of routing tables at the expense of non-optimal paths

32. Endpoint control • Key design philosophy • do as much as possible at the endpoint • relatively dumb/unreliable network • exactly the opposite philosophy of telephone network • Layer above IP compensates for network defects • Transmission Control Protocol (TCP) • Can run over any available link technology • but no quality of service • modification to TCP requires a change at every endpoint

33. Challenges • IP address space shortage • because of free distribution of inefficient Class B addresses • decentralized control => hard to recover addresses, once handed out • even small devices will soon need an IP address • Decentralization • allows scaling, but makes reliability next to impossible • cannot guarantee delay, bandwidth or buffer resources • hard to guarantee security: there is no control over who can join! encryption is a partial solution, but who manages keys? • no uniform solution for accounting and billing (can’t even reliably identify users) • no equivalent of yellow pages (hard to reliably discover a user’s email address) • nonoptimal routing • Multimedia • requires network to support quality of service of some sort (hard to integrate into current architecture; store-and-forward => shared buffers => traffic interaction => hard to provide service quality) • requires user to signal to the network what it wants • but Internet does not have a simple way to identify streams of packets • nor are routers required to cooperate in providing quality • and there is no pricing!

34. ATM Networks

35. Why ATM networks? • Different information types require different QoS • Telephone networks support a single QoS (and at a high cost) • Internet supports no QoS (but it is flexible and cheap) • ATM networks are meant to support a range of service qualities at a reasonable cost. Potentially can replace both the telephone network and the Internet Design goals • Providing end-to-end QoS • High bandwidth • Scalability • Cost-effective

36. How far along are we? • Basic architecture has been defined • But delays have resulting in ceding desktop to IP • We may never see end-to-end ATM • but its ideas continue to powerfully influence design of next-generation Internet • Internet technology + ATM philosophy • Note--two standardization bodies • ATM Forum • International Telecommunications Union-Telecommunications Standardization Sector (ITU-T)

37. Concepts 1. Virtual circuits 2. Fixed-size packets (cells) 3. Small packet size 4. Statistical multiplexing 5. Integrated services Together can carry multiple types of traffic with end-to-end quality of service

38. 1. Virtual circuits • Telephone network operates in synchronous transmission mode • the destination of a sample depends on where it comes from, and when it came • idle users consume bandwidth • links are shared with a fixed cyclical schedule => quantization of link capacity (can’t ‘dial’ bandwidth) • ATM uses packets (header indicates destination =>arbitrary schedule and no wasted bandwidth) • Two ways to use packets • carry entire destination address in header • carry only an identifier Data Sample ATM cell Datagram VCI Data Addr. Data

39. Virtual circuits (contd.) • VC id’s save on header space • But need to be pre-established • We also need to switch Ids at intermediate points • Need translation table and connection setup

40. Features of virtual circuits • All packets must follow the same path • Switches store per-VCI state • can store QoS information • Signaling => separation of data and control • Small Ids can be looked up quickly in hardware • harder to do this with IP addresses • Setup must precede data transfer • delays short messages • Switched vs. Permanent virtual circuits • Ways to reduce setup latency • preallocate a range of VCIs along a path (Virtual Path) • send data cell along with setup packet • dedicate a VCI to carry datagrams, reassembled at each hop

41. 2. Fixed-size packets • Advantages • Simpler buffer hardware • Simpler line scheduling • Easier to build large parallel packet switches • Disadvantages • segmentation and reassembly cost • last unfilled cell after segmentation wastes bandwidth 3. Small packet size • At 8KHz, each byte is 125 microseconds • The smaller the cell, the less an end user has to wait to fill it • packetization delay • The smaller the packet, the larger the header overhead • Standards body balanced the two to prescribe 48 bytes + 5 byte header = 53 bytes • => maximal efficiency of 90.57%

42. 4. Statistical multiplexing • Suppose cells arrive in bursts • each burst has 10 cells evenly spaced 1 second apart • gap between bursts = 100 seconds • Average cell rate=0.09 cells/sec. Peak cell rate=1 cell/sec • What should be service rate of output line? We can trade off worst-case delay against speed of output trunk • Statistical Multiplexing Gain (SMG)= sum of peak input / output rate • Whenever long term average rate differs from peak, we can trade off service rate for delay

43. 5. Integrated service • Traditionally, voice, video, and data traffic on separate networks • How do ATM networks allow for integrated service? • lots of bandwidth: hardware-oriented switching • support for different traffic types • Signaling and resource reservation • admission control • easier scheduling

44. Challenges • Quality of service (defined, but not used) • Scaling (little experience) • Standardization (political and slow) • IP • a vast, fast-growing, non-ATM infrastructure • interoperation is difficult, because of fundamentally different design philosophies • connectionless vs. connection-oriented • resource reservation vs. best-effort