TCOM 513 Optical Communications Networks

1. TCOM 513Optical Communications Networks Spring, 2007 Thomas B. Fowler, Sc.D. Senior Principal Engineer Mitretek Systems

2. Topics for TCOM 513 • Week 1:Wave Division Multiplexing • Week 2:Opto-electronic networks • Week 3: Fiber optic system design • Week 4: MPLS and Quality of Service • Week 5: Optical control planes • Week 6: The business of optical networking: economics and finance • Week 7: Future directions in optical networking

3. Virtual Network Service Application Application Virtual Session Presentation Logical portion of code Presentation End-to-End Messages Session Session End-to-End Packets Transport Transport Packets Network Network Network Network Physical portion of code Frames DLC DLC DLC DLC Data Link Control Data Link Control Physical Physical Physical Physical Physical Physical Bits Physical Link, e.g. electrical signals Subnet node Originating site Terminating site Subnet node

4. Opto-electronic systems and networks • LAN protocols • Fiber distributed data interface (FDDI) • Fiber channel • Gigabit/10 Gigabit Ethernet • SONET/SDH • Ethernet over optical networks

5. LAN protocols • Layers 1 and 2 • Map into OSI reference model Souce: Cisco

6. FDDI • Developed by American National Standards Institute (ANSI) • Originally proposed as internal fiber optic I/O channel for computers • Later became generalized to high-speed LAN running at 100 Mbps • Can run on copper as well as fiber • Dual-ring is usual configuration • Can go up to 200 Mbps with single ring • Token ring architecture • Advantage of token-passing networks: deterministic • Possible to calculate maximum time before station can transmit • Popular in real-time environments

7. Characteristics of FDDI • Token ring architecture • Two countercirculating rings • Only one used for data; other for backup • Ring size • Up to 200 km (on multimode fiber, single ring) • Dual ring size up to 100 km • Maximum of 500 stations • Max distance between stations is 2 km • Packet switched: utilizes variable length frames • Max frame size is 4500 bytes • Frame header contains destination address

8. Characteristics of FDDI (continued) • Guaranteed bandwidth availability • Equality of access as in all token-ring systems • Guaranteed bandwidth for synchronous traffic • Token-ring protocol • Similar to IEEE 802.5 token-ring LAN • Differs in that it is dependent on timers • Ring stations • Each may connect to both rings or only primary ring • Ring monitor • Performed cooperatively by all stations rather than by single active monitor • All look for errors; if found any station can request reinitialization of ring • Each station does not have to have ring monitor function

9. FDDI ring structure, with/without break Source: Dutton

10. FDDI ring configuration Source: Dutton

11. FDDI token ring protocol operation • Ring access controlled by special frame called a “token” • Only one token present at any time • When a station receives the token it has permission to send • When station finishes sending it must place token back on ring • Each station on the ring receives and retransmits frames • Ring is not a node

12. Timing on FDDI • 3 timers required due to need to handle synchronous traffic • Token rotation timer (TRT) • Elapsed time since last token received • Target token rotation timer (TTRT) • Target maximum time between tokens  time for token to traverse ring • 4 msec < TTRT < 165 msec • Optimal value often around 8 msec • Token holding timer (THT) • Governs max amount of data station may send • Max time allocated for station to send

13. Operation • When station receives token it compares time since last token (TRT) with target time (TTRT) • Normal operation: TRT < TTRT • Station can send multiple frames until TTRT reached • TTRT-TRT = THT • Overload: 2xTTRT> TRT > TTRT • Synchronous data only permitted • Error: TRT > TTRT • Must be conveyed to LAN manager • Delays may occur • Stations must be capable of buffering data • Stations must remove data they send when it returns to them • May be many frames on ring, but only one token

14. Operation (continued) • When ring initialized, stations cooperate to determine TTRT value • Minimum of all requested TTRT values • Changed only if new station enters ring

15. Physical media for FDDI • Multimode fiber • Originally defined mode of operation • Single mode fiber • Included in standard but little used • Twisted pair copper wire • STP = shielded twisted pair • Not as good as fiber, but cheaper • UTP-5 (=cat 5) unshielded twisted pair standard in 1994

16. Media specifications

17. Data encoding and clocking • Four data bits encoded as five bit group • 100 Mbps actually 125 Mbaud on ring • Allows adding of more transitions into bit stream to allow for problem of too many 1s or 0s • Uses Non Return to Zero Inverted (NRZI) encoding • Each station has own clock • Specification is accuracy of 0.005% • Max difference between stations 0.01% • 10 bit buffer inside each station to allow for differences in clocks between stations • Gives average of 4.5 bit times to smooth out timing differences • Determines max frame size 4.5 bits/0.01% = 45,000 bits = 9,000 symbols = 4,500 bytes

18. Physical layer operation Source: Dutton

19. Comparison with standard token ring networks • Standard TRN uses Manchester encoding • Allows exact recovery of clock, but at cost of doubling frequency • FDDI uses optical signals at higher speed than TRN • Does not have exact clock recovery, substitutes buffer

20. FDDI layers Source: Dutton

21. FDDI layers (continued) • Physical Medium Dependent layer (PMD) • Optical link parameters • Cables and connectors • Optical bypass switch • Power levels • Physical Layer Protocol (PHY) • Access to ring • Clocking, synchronization, buffering • Code conversion • Ring continuity

22. FDDI layers (continued) • Media Access Control (MAC) • Tokens and timers • Frame check sequence • Station Management (SMT) • Ring Management (RMT) • Ensures valid token circulating • Connection Management (CMT) • Physical connections and topology • Operational Management • Monitors timers and parameters • Interfaces to external network management software

23. SONET overview • SONET = Synchronous Optical Network • Should have been called Synchonous Opto-electronic network (SOENET) • Technology developed in 1980s for long-haul trunks needed by Telcos • Formulated by Exchange Carriers Standards Association (ECSA) • Industry group which sets standards for telecoms • 1984 work began • Expected to serve as basis for Telcos for 20-30 years • Designed from ground up based on 64kbps channels (DS0—voice channels) • Everything a multiple of this

24. SONET (continued) • Emphasis on qualities important to Telcos • Reliability • Availability • Millisecond recovery from outages • Optimal use of bandwidth of secondary concern • Not originally intended as bulk data carrier or carrier for asychronous packets • Serves as transport only • Does not do switching • Utilizes optical components only because copper not fast enough • Otherwise copper or fiber could transmit SONET

25. Advantages of SONET • Reduction in equipment • Standardization of equipment to allow for plug and play • Increased network reliability • Provision of overhead and payload bytes • Synchronous multiplexing format • Allows carrying of different loads • Simplifies interfacing to switching equipment

26. Basic structure of SONET • Utilizes time division multiplexing to combine large number of individual signals • Structured in fixed-length frames • Entire network operates synchronously • Synchronous operation requires extremely precise clocking throughout network • Utilizes Stratum atomic clock • Known as “Primary Reference Clock” (PRC) • Accurate to 1 part in 1011

27. Basic structure of optical part of SONET Input SONET signal (time multiplexed individual signals)

28. l1 end user services end user services SONET D W D M SONET D W D M end user services end user services SONET SONET ln Evolving SONET network architecture Encoder(Time Division Multiplexer) Modulator/ transmitter (Wavelength multiplexer) Receiver/ demodulator (Demux) Decoder(Demux) Source Receiver Link

29. SONET structure • First step in SONET multiplexing process: generation of lowest level or base signal • Referred to as Synchronous Transport Signal level-1, or STS-1 • 51.84 Mbits/second • Higher level signals are multiples of this, giving rise to STS-N • N is not arbitrary, but restricted to certain values • STS-N signals composed of N byte-interleaved STS-1 signals • Optical counterpart known as “Optical Carrier level-1” or OC-1

30. SONET hierarchy Source: Tektronix

31. SONET frame format • 810 bytes • Logically a 90 column by 9 rows • Order of transmission: row by row, L to R within rows, most significant byte first 9 rows Source: Tektronix 90 columns

32. SONET frame format (continued) • One frame per 125 msec = 8,000 frames/sec • 8,000 frames/sec x 810 bytes/frame x 8 bits/byte = 51,840,000 bits/sec • Column = 9 bytes x 8000 per second x 8 bits/byte = 576K bits SONET frame Synchronous Payload Envelope (SPE)—783 bytes TransportOverhead Payload756 bytes (84 cols.) STS PathOverhead(POH)—9 bytes Fixedstuff18 bytes

33. SONET frame structure: SPE Source: Tektronix

34. SONET frame structure (continued) • SPE does not have to be aligned with STS frame • Can begin anywhere in STS frame • Starting location designated by STS payload pointer in transport overhead Source: Tektronix

35. Overhead structure • Two types • Transport (27 bytes) • Section (9 bytes) • Line (18 bytes) • Path (9 bytes, embedded in SPE)

36. Overhead structure (continued) Source: Tektronix

37. Detailed structure of overhead Source: Tektronix

38. Function of overhead • Section (9 bytes) • Performance monitoring (STS-N signal) • Local orderwire • Datacomm channels to carry info for OAM&P • Framing • Line overhead (18 bytes) • Locating SPE in frame • Multiplexing or concatenating signals • Performance monitoring • Automatic protection switching • Line maintenance

39. Function of overhead (continued) • Path overhead (9 bytes) • Performance monitoring (STS SPE) • Signal label (contents of STS SPE) • Path status • Path trace

40. SONET alarms • Three levels to allow close monitoring of deteriorating conditions • Anomaly: discrepancy between observed and expected • Does not constitute interruption in service • Defect: density of anomalies reached level where service is interrupted • May be correctable • Failure: Inability of function to perform required action (defect) persisted beyond allowable time span

41. SONET alarms

42. SONET Alarms (continued) Source: Tektronix

43. Tributaries and Virtual Tributaries (VTs) • Need exists to transmit channels slower than full STS • Called tributaries or virtual tributaries • Only certain channel speeds allowed • Tributaries may occupy a number of consecutive columns within payload or be interleaved (time multiplexed) (usual) • US T-1 (1.544 Mbps) uses 3 columns • Only requires 24 slots, given 27 = 3 slots wasted • Recall that each slot is 64 kbits, x 24 = 1.544 Mbps • European E-1 (2.048 Mbps) uses 4 columns • Only requires 32, given 36 = 4 slots wasted • Benefit is that single tributary can be demultiplexed without need to demultiplex entire stream

44. VT sizes Used for T1 Used for E1 Source: Tektronix

45. Tributaries (continued) • An SPE carrying VTs is divided into 7 VT groups • Each group consists of 12 columns • 12 x 7 = 84 columns = payload capacity • Columns for each VT type are all factors of 12 • Each VT group can carry only one VT type • Cannot mix VT1.5 and VT3, even though they would fit • Separate VT groups within frame can carry different VT types • Allowed combinations within a VT group • 4 VT1.5 • 3 VT2 • 2 VT3 • 1 VT6 • Within group, VTs are interleaved (time multiplexed)

46. Multiplexing of VTs within group Source: Tektronix

47. Multiplexing of VT groups Source: Tektronix

48. Pointers • Used to compensate for frequency and phase variation • Allow transport of synchronous payloads across plesiosynchronous (almost synchronous) network boundaries • Avoid delays and losses of having to use 125 msec slip buffers • Dynamically and flexibly aligning payloads • Dropping • Inserting • Cross-connecting • Effects of jitter can also minimized

49. Pointers (continued) • Byte stuffing used to fix alignment dynamically • Positive: byte added • Negative: byte deleted • Does not affect data

50. Pointers (continued) Source: Tektronix