1. Switching Technologies & Data Link Layer Prof. Anish Goel

2. Network: Links & switches Circuit consists of dedicated resources in sequence of links & switches across network Circuit switch connects input links to output links 2 Switching Technologies and DLL Anish Goel

3. Circuit Switch Types Space-Division switches Provide separate physical connection between inputs and outputs Crossbar switches Multistage switches Time-Division switches Time-slot interchange technique Time-space-time switches Hybrids combine Time & Space switching 3 Switching Technologies and DLL Anish Goel

4. Crossbar Space Switch N x N array of crosspoints Connect an input to an output by closing a crosspoint Nonblocking: Any input can connect to idle output (definition of nonblocking) Complexity: N² crosspoints 4 Switching Technologies and DLL Anish Goel

5. Multistage Space Switch Large switch built from multiple stages of small switches The n inputs to a first-stage switch share k paths through intermediate crossbar switches Larger k (more intermediate switches) means more paths to output In 1950s, Clos asked, “How many intermediate switches required to make switch nonblocking?” 5 Switching Technologies and DLL Anish Goel

6. Clos Non-Blocking Condition: k=2n-1 Request connection from last input to input switch j to last output in output switch m Worst Case: All other inputs have seized top n-1 middle switches AND all other outputs have seized next n-1 middle switches If k=2n-1, there is another path left to connect desired input to desired output 6 Switching Technologies and DLL Anish Goel

7. Minimum Complexity Clos Switch 7 Switching Technologies and DLL Anish Goel

8. Time-Slot Interchange (TSI) Switching Write bytes from arriving TDM stream into memory Read bytes in permuted order into outgoing TDM stream Max # slots = 125 msec / (2 x memory cycle time), where memory cycle time is the time required for reading or writing a frame per slot. 8 Switching Technologies and DLL Anish Goel

9. Time-Space-Time Hybrid Switch Use TSI in first & third stage; Use crossbar in middle Replace n input x k output space switch by TSI switch that takes n slot input frame and switches it to k-slot output frame 9 Switching Technologies and DLL Anish Goel

10. Flow of time slots between switches The first slot corresponds to the first output line out of each of the first stage switch. Only one space switch active in each time slot 10 Switching Technologies and DLL Anish Goel

11. Time-Share the Crossbar Switch Interconnection pattern of space switch is reconfigured every time slot Very compact design: fewer lines because of TDM & less space because of time-shared crossbar 11 Switching Technologies and DLL Anish Goel

12. Example: A->2, B->4, C->1, D->3 12 Switching Technologies and DLL Anish Goel

13. Example: T-S-T Switch Design For N = 960 Single stage space switch ~ 1 million crosspoints T-S-T Let n = 120 N/n = 8 TSIs k = 2n – 1 = 239 for non-blocking Pick k = 240 time slots Need 8x8 time-multiplexed space switch For N = 96,000 T-S-T Let n = 120 k = 239 N / n = 800 Need 800x800 space switch 13 Switching Technologies and DLL Anish Goel

14. Data Link Protocols 14 Switching Technologies and DLL Anish Goel

15. DLL Framing 15 Switching Technologies and DLL Anish Goel

16. Character-based Framing Synchronization Used when frames consist of integer number of bytes Asynchronous transmission systems using ASCII to transmit printable characters Octets with HEX value <20 are nonprintable Special 8-bit patterns used as control characters STX (start of text) = 0x02; ETX (end of text) = 0x03; Byte used to carry non-printable characters in frame DLE (data link escape) = 0x10 DLE STX (DLE ETX) used to indicate beginning (end) of frame Insert extra DLE in front of occurrence of DLE STX (DLE ETX) in frame All DLEs occur in pairs except at frame boundaries 16 Switching Technologies and DLL Anish Goel

17. Framing & Bit Stuffing Frame delineated by flag (01111110) character HDLC uses bit stuffing to prevent occurrence of flag 01111110 inside the frame Transmitter inserts extra 0 after each consecutive five 1s inside the frame Receiver checks for five consecutive 1s 17 Switching Technologies and DLL Anish Goel

18. Example: Bit stuffing & de-stuffing 18 Switching Technologies and DLL Anish Goel

19. Frame & Byte Stuffing PPP uses similar frame structure as HDLC, except Protocol type field Payload contains an integer number of bytes PPP uses the same flag, but uses byte stuffing Problems with PPP byte stuffing Size of frame varies unpredictably due to byte insertion Malicious users can inflate bandwidth by inserting 7D & 7E 19 Switching Technologies and DLL Anish Goel

20. Byte-Stuffing in PPP PPP is character-oriented version of HDLC Flag is 0x7E (01111110) Control escape 0x7D (01111101) Any occurrence of flag or control escape inside of frame is replaced with 0x7D followed by original octet exclusive-ORed with 0x20 (00100000) At the receiver, remove 7D and XOR the next character with 0x20 20 Switching Technologies and DLL Anish Goel

21. Generic Framing Procedure GFP combines frame length indication with CRC PLI indicated length of frame (the size of GFP payload area), then simply count characters cHEC (includes CRC-16) protects against errors in count field (single-bit error correction + multiple bits error detection) GFP designed to operate over octet-synchronous physical layers (e.g. SONET) Frame-mapped mode for variable-length payloads: Ethernet Transparent mode carries fixed-length payload: storage devices 21 Switching Technologies and DLL Anish Goel

22. GFP Synchronization & Scrambling Synchronization in three-states Hunt state: examine 4-bytes to see if CRC ok If no, move forward by one-byte (assuming octet synchronous transmission is given at the physical layer). If yes, move to pre-sync state Pre-sync state: tentative PLI indicates next frame If N successful frame detections, move to sync state If no match, go to hunt state Sync state: normal state Validate PLI/cHEC, extract payload, go to next frame Use single-error correction Go to hunt state if non-correctable error Scrambling Payload is scrambled to prevent malicious users from inserting long strings of 0s which cause SONET equipment to lose bit clock synchronization (as discussed in line code section) 22 Switching Technologies and DLL Anish Goel