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ICOM 6115©Manuel Rodriguez-Martinez ICOM 6115 – Computer Networks and the WWW Isidoro Couvertier, Ph.D. Lecture 11
ICOM 6115©Manuel Rodriguez-Martinez Lecture Objectives Understand the properties of telephone technologies used to implement the physical layer Major technologies –Modems (discussed in previous class) –T1, T2, T3 and T4 –SONET –ADSL –Wireless Local Loops
ICOM 6115©Manuel Rodriguez-Martinez The Problem Switching Element How to pass traffics from n slower lines into a higher bandwidth line?
ICOM 6115©Manuel Rodriguez-Martinez Time division multiplexing Allows each slower line to put a piece of data into higher speed link. –Piece could be 1 one byte (T1 carrier) 1 bit (T2 carrier) Time using the high speed link is shared Frames on High speed link carry parts of frames from slower links
ICOM 6115©Manuel Rodriguez-Martinez Time Division Multiplexing Switching Element Slower links Packets Faster Link Packets
ICOM 6115©Manuel Rodriguez-Martinez Multiplexing and De-multiplexing Switching Element Switching Element
ICOM 6115©Manuel Rodriguez-Martinez T1 Carrier Multiplex 24 64Kbps voice channels –Also called DS1 This is the first digital link on the phone system –Codec –switching element that maps analog to digital and vice-versa Bandwidth: Mbps –Each channel puts 8 bytes into frame –Frame has size 193 bits 192 bits of data (24 channels x 8 bits) 1 bit for synchronization (alternates between 0 and 1) –1 frame is sent every 125 usec.
ICOM 6115©Manuel Rodriguez-Martinez Example of T1 Carrier - Control bit is for synchronization of frames - Successive frames should alternate the bit value - Synchronization pattern Bit Value
ICOM 6115©Manuel Rodriguez-Martinez Let’s carry the idea of multiplexing T2 carrier – Bandwidth of 6.312Mbps –Multiplex 4 T1 links –Multiplexing bits rather than bytes T3 carrier – Bandwidth of Mbps –Multiplex 7 T2 links –Multiplexing bits T4 carrier – Bandwidth of Mbps –Multiplex 6 T3 links –Multiplexing bits Most people lease T1 and T3 lines –phone companies use T2 and T4 internally
ICOM 6115©Manuel Rodriguez-Martinez Multiplexing on T1, T2, T3 and T4 The idea is to maximize usage of high speed links
ICOM 6115©Manuel Rodriguez-Martinez Problem: How to standardize? T1, T2, etc. are used in North America and Japan Europe and rest of the world used other standards for multiplexing digital lines How can long distance carriers exchange data and voice? Solution: Make up a new standard –makes everyone more or less happy –Not perfect but get everyone on board
ICOM 6115©Manuel Rodriguez-Martinez SONET/SDH Synchronous Optical NETwork –Developed by Bellcore Synchronous Digital Hierarchy –European amendments to SONET Standard for how phone companies exchange data and voice on digital lines –Long distance trunks use SONET –T1, T2, …, T3 mainly for regional links –Traffic = data or voice moved over the links
ICOM 6115©Manuel Rodriguez-Martinez Design Goals for SONET Interoperability –Different carries (e.g. Sprint and AT&T) should be able to exchange traffic Backward compatible –Accept data from T1, …, T4 and from European standards Support Multiplexing of Digital Links –Must accommodate hierarchies of high speed links Built-in support for maintenance –Piggyback maintenance data along with regular traffic
ICOM 6115©Manuel Rodriguez-Martinez What Synchronous means? Switching elements must be synchronized to emit/receive frames –Called Clock-based framing Need a master clock to which every other switch synchronizes Every 125usec a SONET frame is sent –It might be full of data –It might be 50% filled with data –It might be 0% filled with data Just padding
ICOM 6115©Manuel Rodriguez-Martinez SONET Frames First link in the hierarchy is STS-1 (OC-1) –Synchronous Transport Signal 1 (Electrical carrier) –OC denotes the optical carrier –Bandwidth is 51.84Mbps Each frame can hold up to 810 bytes Logically it is viewed as a table –9 rows of 90 bytes (1 column is 1 byte) –First 3 bytes in each row are management signals Begin of frame, begin of data, etc. –First 2 bytes in the frame have a bit pattern indicating begin of frame
ICOM 6115©Manuel Rodriguez-Martinez SONET STS-1(OC-1) Frame Format 9 rows 80 columns Overhead Payload
ICOM 6115©Manuel Rodriguez-Martinez Multiplexing of SONET STS-3 (OC-3) – Bandwidth – Mbps –Multiplexes 3 OC-1 lines –Frame is 810 x 3 = 2430 bytes long STS-9 (OC-9) - Bandwidth – Mbps –Multiplexes 9 OC-1 lines STS-N (OC-N) –Multiplexes N OC-1 lines
ICOM 6115©Manuel Rodriguez-Martinez STS, OC and SDH STS – denotes the electrical signal used by the switching elements OC – denotes the actual optical carrier moved the fibers SDH – hierarchy from the Europeans –Their SDH-1 is equivalent to a OC-3 line
ICOM 6115©Manuel Rodriguez-Martinez SONET Hierarchy
ICOM 6115©Manuel Rodriguez-Martinez Switching on the Phone Lines Circuit Switching –Dynamically establishes a physical path between sender and receiver Must allocate lines at switches along path Message Switching –Dynamically moves variable-sized blocks of data between sender and receiver Virtual Circuit Packet Switching –Dynamically move size-bound blocks of data between sender and receiver Virtual Circuit with predictable packet size (enables QoS)
ICOM 6115©Manuel Rodriguez-Martinez Switching on the Phone Lines Circuit Switching Packet Switching
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