Multiplexing Methods Daubing the Information 2005/03/11 (C) Herbert Haas
“I think there is a world market for about five computers.” Thomas Watson, chairman of IBM 1943
Multiplexing Types TDM Most important Statistical and Deterministic SDM FDM and (D)WDM Will be covered in other chapters CDM 2005/03/11 (C) Herbert Haas 3
TDM (1) SDM 00110011001101000111100010010000101001010010101001110100010011001 User a User A 10011100010101001010101010011110001010001101011011100010101001011 User b User B 11000111000111100000000000000000000000000000000000000001000000000 User c User C 1011100111 1011100111 1011100111 1011100111 1011100111 User d User D Framed Mode Save wires User a User A User b User B 101010010111 0011100001101 1011100100100 1000011101101 User c User C TDM User d User D 2005/03/11 (C) Herbert Haas 4
TDM (2) Requires framed link layer Saves wires Is slower than SDM Requires multiplexers and demultiplexers Two fundamentally different methods: Two fundamentally different methods: Deterministic TDM Deterministic TDM Statistical TDM Statistical TDM 2005/03/11 (C) Herbert Haas 5
Deterministic TDM (1) Framing A A User A1 User A2 B B User B1 User B2 C D A B C D A B C D A B C D A C C User C1 User C2 D D User D1 User D2 "Trunk" 2005/03/11 (C) Herbert Haas 6
Deterministic TDM (2) A A User A1 User A2 64 kbit/s B B User B1 User B2 C D A B C D A B C D A B C D A 64 kbit/s 4 × 64 kbit/s + F ≅ 256 kbit/s C C User C1 User C2 64 kbit/s D D User D1 User D2 64 kbit/s • Trunk speed = Number of slots × User access rate • Each user gets a constant timeslot of the trunk 2005/03/11 (C) Herbert Haas 7
Deterministic TDM – Facts Order is maintained Frames must have same size No addressing information required Inherently connection-oriented No buffers necessary (QoS) Protocol transparent Bad utilization of trunk 2005/03/11 (C) Herbert Haas 8
Statistical TDM (1) Average date rates ≅ 64 kbit/s A User A1 User A2 B User B1 User B2 A D A C C C B C 256 kbit/s C User C1 User C2 D D D User D1 User D2 • Trunk speed dimensioned for average usage • Each user can send packets whenever she wants 2005/03/11 (C) Herbert Haas 9
Statistical TDM (2) User A1 User A2 User B1 User B2 D D A D 256 kbit/s User C1 User C2 D D User D1 User D2 • If other users are silent, one (or a few) users can fully utilize their access rate 2005/03/11 (C) Herbert Haas 10
Statistical TDM – Facts Good utilization of trunk Statistically dimensioned Frames can have different size Multiplexers require buffers Variable delays Address information required Not protocol transparent 2005/03/11 (C) Herbert Haas 11
Networking: Fully Meshed • Metcalfe's Law: User A n(n-1)/2 links • Good fault User F User B tolerance • Expensive User E User C User D 2005/03/11 (C) Herbert Haas 12
Networking: Switching • Only 6 links User A • Switch supports either User F User B deterministic or statistical TDM User E User C User D 2005/03/11 (C) Herbert Haas 13
Circuit Switching T1 T1 TA T2 T2 T4 T4 T3 . . . . . . TA(1) → T1(4) : A1-C9 User A2 T2(6) → T4(1) TA(2) → T2(7) : A2-B5 TA(2) → T2(7) : A2-B5 T2(7) → T3(18) T2(7) → T3(18) TA(3) → T2(6) : A3-D1 . . . . . . . . . . . . T3 T1 T4 T4 TB . . . . . . T3(18) → T4(5) T3(18) → T4(5) . . . . . . T3(19) → T1(1) User B5 T4(4) → TB(9) . . . . . . T4(5) → TB(5) T4(5) → TB(5) . . . . . . 2005/03/11 (C) Herbert Haas 14
Circuit Switching – Facts Based on deterministic TDM Minimal delay Protocol transparent Possibly bad utilization Good for isochronous traffic (voice) Switching table entries Static (manually configured) Dynamic (signaling protocol) Scales with number of connections! 2005/03/11 (C) Herbert Haas 15
Typical User-Configuration Channel Service Unit/ Data Service Unit (CSU/DSU PBX or "modem") Example: E1 or T1 circuit CSU/DSU Router Example: V.35/RS-530/RS-422 Switch Synchronous serial ports • CSU performs protective and diagnostic functions • DSU connects a terminal to a digital line 2005/03/11 (C) Herbert Haas 16
Packet Switching T1 T1 TA T2 T2 T4 T4 T3 User A2 Address Information T3 T1 T4 T4 TB • Each switch must analyze address information User B5 • "Store and Forward" 2005/03/11 (C) Herbert Haas 17
Technology Differences Datagram Datagram Principle Global and routable addresses Connectionless Routing Table Virtual Call Virtual Call Principle Local addresses Connectionoriented Switching Table 2005/03/11 (C) Herbert Haas 18
Datagram Destination Next Hop A R1 B R4 C R3 R1 R2 R3 ..... ..... A2 B5 A2 B5 Destination Next Hop User A.2 A local B R2 C R2 ..... ..... Destination Next Hop A2 B5 A R4 B local C R4 ..... ..... R4 R5 Destination Next Hop A2 B5 A2 B5 A R2 B R5 C R2 ..... ..... User B.5 2005/03/11 (C) Herbert Haas 19
Datagram – Facts (1) Addresses contain topological information Must be globally unique Routing table is configured Static (manually) Dynamic (routing protocols) Endless circling in case of routing loops Important issue among routing protocols Requires "routable" or "routed" protocols 2005/03/11 (C) Herbert Haas 20
Datagram – Facts (2) No connection establishment necessary Faster delivery of first data No resource reservation (bad QoS) Sequence not guaranteed Rerouting on topology change Load sharing on redundant paths End stations must care 2005/03/11 (C) Herbert Haas 21
Datagram – Facts (3) Best effort service Router may drop packets Reliable data transport requires good transport layer ("Dumb network, smart hosts") Simple protocols Easy to implement (Internet's success) Proactive flow control difficult Since routes might change 2005/03/11 (C) Herbert Haas 22
Examples IP IPX Appletalk OSI CLNP 2005/03/11 (C) Herbert Haas 23
Virtual Call – CR Destination Next Hop A PS1 B PS4 P1 P1 PS1 PS2 C PS3 PS3 ..... ..... In Out P0 P2 P0 P2 P0 P0:10 P3:02 A2 B5 CR 44 A2 B5 CR 10 P3 Destination Next Hop Destination Next Hop User A.2 A local A PS4 A2 B PS2 B local C PS2 B5 C PS4 ..... ..... ..... ..... CR In Out In Out 02 P1 P0:44 P2:10 P0:69 P2:19 P0 P2 P0 P2 A2 B5 CR 69 A2 B5 IC 19 Destination Next Hop A PS2 PS4 PS5 B PS5 C PS2 ..... ..... User B.5 In Out P1:02 P2:69 2005/03/11 (C) Herbert Haas 24
Virtual Call – CA P1 P1 PS1 PS2 PS3 In Out P2 P0 P0:10 P3:02 P0 P2 P0 44 CC A2 B5 10 CA A2 B5 P3 In Out User A.2 P0:44 P2:10 02 CA A2 B5 In Out P1 P0:69 P2:19 P0 P2 P0 P2 69 CA A2 B5 19 CA A2 B5 In Out PS4 PS5 P1:02 P2:69 User B.5 2005/03/11 (C) Herbert Haas 25
Virtual Call – Data P1 P1 PS1 PS2 PS3 In Out P2 P0 P0:10 P3:02 P0 P2 P0 44 10 P3 In Out User A.2 P0:44 P2:10 02 In Out P1 P0:69 P2:19 P0 P2 P0 P2 69 19 In Out PS4 PS5 P1:02 P2:69 User B.5 2005/03/11 (C) Herbert Haas 26
Virtual Call – Facts (1) Connection establishment Through routing process (!) Globally unique topology-related addresses necessary Creates entries in switching tables Can reservate switching resources (QoS) Packet switching relies on local identifiers Not topology related Only unique per port 2005/03/11 (C) Herbert Haas 27
Virtual Call – Facts (2) Packet switching is much faster than packet forwarding of routers Routing process is complex, typically implemented in software Switching is simple, typically implemented in hardware 2005/03/11 (C) Herbert Haas 28
Virtual Call – Facts (3) Connection can be regarded as virtual pipe Sequence is guaranteed Resources can be guaranteed Network failures disrupt pipe Connection re-establishment necessary Datagram networks are more robust 2005/03/11 (C) Herbert Haas 29
Virtual Call – Facts (4) Virtual call multiplex Multiple virtual pipes per switch and interface possible Pipes are locally distinguished through connection identifier Other names for connection identifier LCN (X.25) DLCI (Frame Relay) VPI/VCI (ATM) 2005/03/11 (C) Herbert Haas 30
Example BANG 2005/03/11 (C) Herbert Haas 31
Two Service Types Switched Virtual Circuit (SVC) Dynamic establishment as shown At the end a proper disconnection procedure necessary Permanent Virtual Circuit (PVC) No establishment and disconnection procedures necessary Switching tables preconfigured by administrator 2005/03/11 (C) Herbert Haas 32
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