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CSCI-370/EENG-480 Computer Networks
Khurram Kazi
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Major sources of the slides for this lecture
Interworking with TCP/IP, M , Global knowledge, training manual, ( TCP/IP Clearly Explained, Pete Loshin. The Internet and Its Protocol, Adrian Farrel’s book. Tanenbaum’s webpage SONET by Walter Goralski, McGraw-Hill
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Reference Network: For discussion purposes
Physical Layer
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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
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Prior to SONET/SDH: The need for Synchronous Optical Networks
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
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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
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Time Division Multiplexing PDH (Plesichronous Digital Hierarchy) Networks
The T1 carrier (1.544 Mbps).
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Multiplexing T1 streams into higher carriers.
Time Division Multiplexing PDH (Plesichronous Digital Hierarchy) Networks Multiplexing T1 streams into higher carriers.
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Time Division Multiplexing PDH (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)”
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Time Division Multiplexing Precursor to SONET/SDH
Compromises Basic rate for SONET increased to 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
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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
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Time Division Multiplexing (5)
SONET and SDH multiplex rates.
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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
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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.
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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
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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 and A2 byte is 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 , first Z0 to , second Z0 to etc.) Where as in STS-192 frame each Z0 byte is set to the fixed pattern ‘ ’.
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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. 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.
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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 other communication needs between STE. This channel is used for internally generated, externally generated and supplier-specific messages. SDH uses this channel for the same purpose.
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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.
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Pointer Function
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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.
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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
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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.
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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)
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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.
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Client Signals of SONET/SDH
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SONET Multiplexing Structure
AU : Administrative Unit TUG: Tributary Unit Group
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Virtual Concatenation: Link sizes provided by VC
SDH SONET from to In steps of VC-11 (1-64) VT1.5 (164) 1.6 Mbit/s 102.4 Mbit/s VC-12 (1-64) VT2 (164) 2.2 Mbit/s 139.3 Mbit/s VC-3 (1-256) STS-1 (1256) 49 Mbit/s 12.7 Gbit/s VC-4 (1-256) STS-3c (1256) 150 Mbit/s 38.3 Gbit/s
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Virtual Concatenation: Link sizes provided by VC
SONET 89% 98% 99% 100% 95% VT-1.5-7v VT v VT v STS-1-2v STS-1-4v STS-1-21v STS-3-7v SDH 92% VC-12-5v VC-12-12v VC-2-4v VC-12-46v VC-3-2v VC-3-4v VC-4-7v Contiguous concatenation 67% 33% 42% none STS-3c STS-12c STS-48c VC-2-4c VC-4-4c VC-4-16c No concatenation 20% 50% STS-1 VC-3 VC-4 Service / bitrate Ethernet / 10 Mbit/s ATM / 25 Mbit/s Fast Ethernet / 100 Mbit/s ESCON / 200 Mbit/s Gigabit Ethernet / 1 Gbit/s
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