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CSCI-370 C omputer Networks: Shrinking the globe one click at a time Lecture 6

CSCI-370 C omputer Networks: Shrinking the globe one click at a time Lecture 6. Khurram Kazi. Major sources of the slides for this lecture. Computer Networks: A Systems Approach, Larry Peterson The Internet and Its Protocol, Adrian Farrel’s book.

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CSCI-370 C omputer Networks: Shrinking the globe one click at a time Lecture 6

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  1. CSCI-370Computer Networks:Shrinking the globe one click at a timeLecture 6 Khurram Kazi CSCI 370

  2. Major sources of the slides for this lecture • Computer Networks: A Systems Approach, Larry Peterson • The Internet and Its Protocol, Adrian Farrel’s book. • http://www.pcc.qub.ac.uk/tec/courses/network/SDH-SONET/sdh-sonetV1.1a_1.html • http://electrosofts.com/sonet/index.html • SONET by Walter Goralski, McGraw-Hill • Optical Networking Standards: A Comprehensive Guide by K. Kazi, Springer. CSCI 370 2

  3. Reference Network: For discussion purposes CSCI 370 3

  4. Link State Keeps the volume of information passed along to other routers to a minimum Each router periodically checks on the status of neighboring routers, reporting which links are alive to all the other participating routers With the this information each router can then create its own map of the internetwork Vector/Distance vs. Link State Routing CSCI 370 4

  5. Autonomous Systems • Who owns the internet (one happy family) • Wide variety of organizations • National governments • Large Internet Service Providers (ISPs) • Telephone companies with wide geographic footprint • In the real world, each organization wants the largest possible amount of control and secrecy • Each organizational grouping of computers/servers defines itself as an Autonomous System (AS) • AS can operate in isolation from all other groupings • Within an AS, routing information is generally widely distributed • One router can clearly see the path through the AS network to another router within the same AS • Protocols that distribute routing information within as AS is referred as Interior Gateway Protocols (IGPs). • The word gateway is the old name for a router CSCI 370 5

  6. Autonomous Systems • Organizations and ASs require connectivity to make the Internet work • Connectivity operates in a largely hierarchical way • Home users and small companies paying smaller ISPs for private access (dial-up, wireless, leased lines, cable etc.) • Smaller ISPs and larger corporations buy access to the backbone network operated by larger ISPs • The larger ISPs create a peering agreement with each other to glue the whole thing together CSCI 370 6

  7. Autonomous Systems • Just the connectivity is not enough • Must have the ability to route from a router in one AS to a router in another AS • Key to this is the routers that sit on the links between ASs • These Autonomous Systems Border Routers (ASBRs) are responsible for leaking routing information from one AS to another AS • These routers do not divulge too much information about their internal network infrastructure • They reveal just enough information such that IP packets can be routed to the hosts that AS supports • Such routing protocols are called Exterior Gateway Protocols (EGPs) • EGPs distribute reachability information in terms of subnetted and aggregated IP addresses and unique AS identifiers called AS numbers CSCI 370 7

  8. Autonomous Systems within the Internet CSCI 370 8

  9. Moving onto Physical layer: Optical Transport Technologies CSCI 370 9

  10. Reference Network: For discussion purposes Physical Layer CSCI 370 10

  11. Prior to SONET/SDH: The need for Synchronous Optical Networks • Previous technology - PDH - Plesiochronous Digital Heirarchy was limited: • US and European systems had little in common - expensive translators required for transatlantic traffic • "Standard" equipment from different vendors was incompatible • No self checking - expensive manual check and repair system • No standard for high bandwidth links - proprietary • Not synchronous above US DS-1 bandwidth CSCI 370 11

  12. Prior to SONET/SDH: The need for Synchronous Optical Networks • Synchronous? • What does synchronous mean to a telephone engineer • "bits from one telephone call are always in the same location inside a digital transmission frame" • US telephone calls, DS-0, are multiplexed 24 per DS-1 channel • DS-0 refers to 64 Kb/s digitized voice signal that is carried over digital telephone networks • DS-1 lines are synchronous it is easy to remove or insert a call CSCI 370 12

  13. Prior to SONET/SDH: The need for Synchronous Optical Networks • Plesiochronous? • Plesiochronous means • "almost synchronous because bits are stuffed into the frames as padding and the calls location varies slightly - jitters - from frame to frame" • 4 DS-1 lines are multiplexed for DS-2 • 7 DS-2s are multiplexed to DS-3 • To isolate a particular call from DS-3 it must be demultiplexed to DS-1 • Very expensive equipment is needed at every exchange to demultiplex and multiplex high speed lines CSCI 370 13

  14. Time Division MultiplexingPDH (Plesichronous Digital Hierarchy) Networks The T1 carrier (1.544 Mbps). CSCI 370 14

  15. Time Division MultiplexingPDH (Plesichronous Digital Hierarchy) Networks Multiplexing T1 streams into higher carriers. CSCI 370 15

  16. Time Division MultiplexingPDH (Plesichronous Digital Hierarchy) Networks • Bellcore originally proposed SONET - Synchronous Optical NETwork • 1985 ANSI T1X1 committee • 1986 CCITT SDH standards published: G.707, G.708, G.709 • 1987 Bellcore submitted SONET to CCITT - much European opposition • G.709 was reassigned to “Interfaces for Optical Transport Network (OTN)” CSCI 370 16

  17. Time Division MultiplexingPrecursor to SONET/SDH • Compromises • Basic rate for SONET increased to 51.840 Mbs to permit more bandwidth for OAM (operation, administration and maintenance functions) - concession to Europeans - a good move • Europeans dropped demand for level 2 and 3 rates to be directly supported • SDH/SONET merged on DS-3 and CEPT-4 rates CSCI 370 17

  18. SONET/SDH • SDH/SONET would: • Improve on existing DS-3 multiplexing standard • Provide a non-proprietary solution • Establish a hierarchy of digital standards compatible with European and US systems CSCI 370 18

  19. Time Division Multiplexing (5) SONET and SDH multiplex rates. CSCI 370 19

  20. SONET/SDH Model 4 layers • Photonic - physical characteristics of the optical equipment • Section - frame format and electro-optic conversion • Line - synchronization and multiplexing onto SONET frames • Path - end to end transport • Physical realization: • Section - single run of fibre optic cable • Line - one or more sections • Path - end to end circuit CSCI 370 20

  21. SONET/SDH Model • SONET/SDH networks are configured as linear networks, where SONET/SDH nodes knows as Add Drop Multiplexers (ADMs) are hooked together in a line as shown in the figure. There may be two or four fibers between the two consecutive ADMs with one set serving as “protection” or “back up”. • Add/drop multiplexers (ADMs) are places where traffic enters and leaves. The traffic can be at various levels in the SONET/ SDH hierarchy • Also SONET network elements can receive signals from a variety of facilities such as DS1, DS3, ATM, Internet, and LAN/MAN/WAN. They can also receive signals from a variety of network topologies • ADMs drop some timeslots from the receive path and add timeslots to the transmit path • In an STS-192, there could be 192 STS-1 timeslots that can be added or dropped at an ADM CSCI 370 21

  22. An example of adding/dropping of timeslots CSCI 370 22

  23. SONET Frame Structure • STS-1 Frame Format • SONET is based on the STS-1 frame • STS-1 consists of 810 octets • 9 rows of 90 octects • 27 overhead octets formed from the first 3 octets of each row • 9 used for section overhead • 18 used for line overhead • 87x9 = 783 octets of payload • one column of the payload is path overhead - positioned by a pointer in the line overhead • Transmitted top to bottom, row by row from left to right • STS-1 frame transmitted every 125 us: thus a transmission rate of 51.84Mbps A1 and A2 are framing bytes and consist of F6 28 (hex). MSB is transport out first. CSCI 370 23

  24. SONET Frame Structure • STS-3 Frame Format • STS-3 is based on byte interleaving of 3 STS-1 frames • STS-s frame transmitted every 125 us: thus a transmission rate of 155 Mbps CSCI 370 24

  25. SONET Frame Overhead Explained: Section Overhead • Framing Bytes (A1 and A2): These bytes are used to indicate the start of SONET/SDH frame. A1 byte is 1111 0110 and A2 byte is 0010 1000. These values remain the same in all STS-1s in an STS-N. SDH uses the same values for framing • Section Trace (J0)/Section Growth: This byte is used to trace the origin of an STS-1 frame as it travels across the SONET networks.  It allows two connected sections to verify the connections between them by transmitting a sixteen-byte message. This message is transmitted in sixteen consecutive frames with first byte carried in first frame, second byte in second frame and so on. If no such section trace message is defined or being transmitted, then in STS-48 or lower bit rate the, J0 and each Z0 shall be set corresponding to its order of appearance in the STS-N frame (i.e. J0 shall be set to 000000001, first Z0 to 0000010, second Z0 to 00000011 etc.) Where as in STS-192 frame each Z0 byte is set to the fixed pattern ‘11001100’. CSCI 370 25

  26. SONET Frame Overhead Explained • Section BIP-8 (B1): B1 byte indicates bit error rate to the receiving terminal. This byte is known as Bit Interleaved Parity (BIP-8). The first bit in all the bytes in the previous frame are taken and then B1 is set so that the parity is even. Similarly all the other bits in B1 are set. The parity is calculated after scrambling and placed before scrambling. Scrambling is explained in later sections. The parity represented by this octet is the parity of the previous frame. It is used to estimate the bit error rate (BER) on the line. Note that the B1 parity is computed over all the bytes in the frame, no matter how large the frame. Because of this, the B1 byte does not provide a good BER estimation for large frames (perhaps STS-48 and larger) under adverse error conditions. SDH uses this byte for the same purpose. CSCI 370 26

  27. SONET Frame Overhead Explained • Orderwire (E1): The E1 byte is located in the first STS-1 of an STS-N. It is called Local Orderwire (LOW). The corresponding byte locations in the second through Nth STS-1s are currently undefined. This byte is used for a voice channel between two technicians as they installed and tested an optical link. It has a bit rate of 64kb/s. SDH uses this octet for the same purpose. CSCI 370 27

  28. SONET Frame Overhead Explained • Section User Channel (F1): The F1 byte is located in the first STS-1 of an STS-N, and is used by the network provider. The corresponding byte locations in the second through Nth STS-1s are currently undefined. This byte is passed from Section to Section within a Line and can be read, written, or both at each Section Terminating Equipment (STE) in that line. The use of this function is optional. SDH also uses this byte for the same purpose. • Section Data Communication Channel (D1, D2 and D3): These are the bytes, which form communication channel. These bytes are defined only for first STS-1 of an STS-N frame. These three bytes are considered as one 192-kb/s, message-based channel for alarms, maintenance, control, monitoring, administering and othercommunication needs between STE. This channel is used for internally generated, externally generated and supplier-specific messages. SDH uses this channel for the same purpose. CSCI 370 28

  29. SONET Frame Overhead Explained:Line Overhead • Pointers (H1 and H2): The processing of H1 and H2 bytes in SONET and SDH is a beautiful concept. The Synchronous Payload Envelop (SPE) can be floating in a SONET frame. It can start in one frame and end in the next frame. Now these two bytes are allocated to a pointer that indicates the offset in bytes between the pointer and the first byte of the STS SPE. The pointer bytes are used in all STS-1s within an STS-N to align the STS-1 Transport Overheads in the STS-N, and to perform frequency justification. SDH handles these pointer bytes in the same way. • Pointer Action Byte (H3): The pointer action byte is allocated to compensate for the SPE timing variations. The value carried by H3 is not defined when there is no negative frequency justification. SDH handles this byte in the same way. CSCI 370 29

  30. Pointer Function CSCI 370 30

  31. SONET Frame Overhead Explained:Line Overhead • Line BIP-8 (B2): The operation of this B2 byte is same as that of B1 byte in the SOH except that B2 is calculated over Line Overhead and Synchronous Payload Envelope of the previous frame before scrambling and placed in the current STS-1 frame before scrambling. SDH uses this byte for the same purpose. • Automatic Protection Switching (APS) Channel (K1, K2): Set of fibers is used for protection. These K1 and K2 are the bytes, which are transmitted over these protection channels for Automatic Protection Switching (APS) signaling between line level entities. These bytes are defined only for first STS-1 of an STS-N. In the remaining STS-1s it is undefined. These bytes are used to indicate a number of defects, alarms etc. detected at the receiving terminal back to the corresponding transmitting terminal through protection channels. SDH  uses these bytes for the same purpose. There is lot more explanation to be done on this concept of APS. CSCI 370 31

  32. SONET Frame Overhead Explained:Line Overhead • Line Data Communication Channel (D4-D12): These bytes form a communication channel to send administrative messages just as D1 to D3. These nine bytes are considered as one 576-kb/s, message-based channel for alarms, maintenance, control, monitoring, administering and other communication needs. This channel is available for internally generated, externally generated and supplier-specific messages. These bytes are defined only for STS-1 number 1 of an STS-N signal. SDH uses these bytes for the same purpose but with additional codes. • Synchronization Status (S1): This byte is allocated for transporting synchronization status messages. S1 is defined only for first STS-1 of an STS-N signal. Currently only bits 5-8 of S1 are used to transport synchronization status messages. Bits 1-4 are undefined.  These messages contain clock quality labels that allow a SONET NE to select the most suitable synchronization reference from the set of available references. The purpose of these messages is to allow SONET NEs to reconfigure their synchronization references autonomously while avoiding the creation of timing loops. As an example for bits 5-8 in S1. Bits 5-8 are 0001 for stratum 1 traceable, 0111 for stratum 2 traceable, 0000 Synchronized traceability unknown etc. SDH uses this byte for the same purpose CSCI 370 32

  33. SONET Frame Overhead Explained:Line Overhead • Growth (Z1): Z1 byte is located in second through Nth STS-1s of an STS-N. This byte is undefined. • STS-1 REI (M0): The M0 byte is defined only for the STS-1 in an OC-1 or STS-1 electrical signal. Bits 5 through 8 of the M0 byte are allocated for a Line Remote Error Indication function (REI-L), which conveys the error count detected by LTE (using the B2 code) back to its peer LTE. Bits 1 through 4 of the M0 byte are currently undefined. The error count shall be a binary number from zero (i.e., ‘0000’) to 8 (i.e., ‘1000’). The remaining seven values represented by the four REI-L bits (i.e., ‘1001’ through ‘1111’) shall not be transmitted, and shall be interpreted by receiving LTE as zero errors. Since there is no rate in SDH equivalent to STS-1, SDH does not define an M0 value for this byte. • Growth (Z2): These bytes are allocated for future growth, and their use is currently undefined. Note that STS-1 signal does not contain a Z2 byte. • Orderwire (E2): This byte has the same purpose for line entities as the E1 byte has for section entities. It is called Express Orderwire (EOW) channel. The corresponding bytes in the second through the Nth STS-1s of an STS-N frame are currently undefined. SDH uses this byte for the same purpose. CSCI 370 33

  34. SDH Frame Structure • STM-N Frame Format • STM - "Synchronous Transmission Module" • STM-N general format • Originally the basic frame STM-1 consists of • 270x9=2430 octets • 9x9=81 octets section overhead • 2349 octets payload • Higher rate frames are derived from multiples of STM-1 according to value of N • Later STM-0 was standardized by ITU (which corresponds to STS-1 rate) CSCI 370 34

  35. Scrambling in SONET/SDH:as an Aid to Clock Recovery on the Rx Side • Scrambling of outgoing data ensures enough 1 to 0 and 0 to 1 transitions • Helps in clock recovery on the receiver • The framing bytes A1 and A2, Section Trace byte J0 and Section Growth byte Z0 are not scrambled to avoid possibility that bytes in the frame might duplicate A1/A2 and cause an error in framing. The receiver searches for A1/A2 bits pattern in multiple consecutive frames, allowing the receiver to gain bit and byte synchronization. Once bit synchronization is gained, everything is done, from there on, on byte boundaries – SONET/SDH is byte synchronous, not bit synchronous. CSCI 370 35

  36. Client Signals of SONET/SDH CSCI 370 36

  37. SONET Multiplexing Structure AU : Administrative Unit TUG: Tributary Unit Group CSCI 370 37

  38. Virtual Concatenation: Link sizes provided by VC CSCI 370 38

  39. Virtual Concatenation: Link sizes provided by VC CSCI 370 39

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