Communication Networks Chapter 5. Switching Technology

Transcription

1 Communication Networks Chapter 5. Switching Technology o Circuit Switching o Message Switching o Virtual Circuit / Virtual Connection o Datagram Recapitulation: Telecommunications Network LAN 1 Backbone mobile users URL (wireless) LAN 3 LAN 2 Web-Page 156 Prof. Jochen Seitz 1

2 Connection-oriented and Connectionless Communication Connection-oriented Communication Connection has to be established before data can be transferred High complexity Quality of service can be guaranteed Connectionless Communication No context in the switches / routers No guarantee for quality of service Low complexity ( Shoot and pray ) 157 Addressing in Connection-oriented Communications All participating entities have to build a connection context. One end system may support several connections in parallel. Connection must be identifiable. Connection End Point X Y Service Access Point Connection 158 Prof. Jochen Seitz 2

3 Switching and Signaling Switching: Provides a temporary communication path through the communication network on demand Involves both end systems and a chain of switching systems Can be realized both connection-oriented and connectionless Signaling: Comprises information for switching Starts at the requesting end systems, traverses all involved switching systems and ends at the called end system Is (usually) implemented in a separate protocol stack 159 Routing Relevant for partly meshed networks Probably several paths to the destination Different metrics for the paths Goal: Finding the optimal path to the given destination Proactive or reactive routing Both for connection-oriented and connectionless communication Mainly done in the network layer 160 Prof. Jochen Seitz 3

4 Routing Example Sender Satellite Earth Ground Stations Receiver WAN 161 Network Layer Connection Connection of two network layer users over one or more switching systems Might be temporary or fixed Differentiation of users: Calling party Called party For connection establishment, called party must be identified Telephone number IP address Service number Connection is established based on signaling information 162 Prof. Jochen Seitz 4

5 Switching Technologies: Circuit Switching / Line Switching Currently still in use for public telephone networks Connection is established over an end-toend physical channel between end users Physical channel is on physical layer only and does not include any buffers Delay is only depending on the propagation delay of the physical signals and hence on the length of the channel itself Delivery is ordered 163 Circuit Switching: Circuit Switching Scheme Subscriber Line Long Distance Line Subscriber Line Local Exchange Local Exchange End Systems Long-haul Exchange 164 Prof. Jochen Seitz 5

6 Circuit Switching: PSTN Office Classification Hierarchy Source: Circuit Switching: The Beginning of Circuit Switching Caller turned the crank. In the central office, a flip fell down. The operator asked who the caller wanted to talk to. She plugged the according lines. When the talk was over, the operator had to disconnect the phones again. 166 Prof. Jochen Seitz 6

7 Circuit Switching: Evolution of Circuit Switching At the beginning, the network was created using analog voice connections through manual switchboards. As a next step, automated telephone exchanges replaced most switchboards. Later digital switch technologies were used. Most switches now use digital circuits between exchanges, with analog two-wire circuits still used to connect to most telephones. 167 Circuit Switching: Circuit Switching based on SDMA Crosspoints on crossbars are used for connection establishment Example: m incoming lines n outgoing lines Here, incoming line 1 is connected to outgoing line 2 Incoming Lines m n Outgoing lines 168 Prof. Jochen Seitz 7

8 Circuit Switching: Circuit Switching based on TDMA Incoming line is connected with the outgoing line only for a short time (time slot). A scheduler is responsible for setting the corresponding crosspoint. 1 Incoming 2. Lines.. m Scheduler Outgoing n Lines 169 Circuit Switching: Time-Division Multiplexing (TDM) I Buffers for each Slot Incoming Multiplexed Line A4 A3 A2 A1 t E4 t E3 t E2 t E1 Cyclic Input t E1 t E2 t E3 A1 Buffer 1 A2 Buffer 2 A3 t A4 t A1 t A2 Random output Outgoing Multiplexed A2 A4 A1 A3 Line t A4 t A3 t A2 t A1 125µs-Period t E4 Buffer 3 A4 t A3 125µs-Period Buffer 4 Scheduler Control addresses ( Buffer ID) 170 Prof. Jochen Seitz 8

9 Circuit Switching: Time-Division Multiplexing (TDM) II S 1 S 1 S 2 S 2 S 1 Frame S 1 S 2 S 2 A B S 3 S 3 S 3 S 4 S 4 S 3 S 4 t 4 t 3 t 2 t 1 t 4 t 3 t 2 t 1 t 4 t 3 t 2 t 1 Multiplexing Time Slot Demultiplexing S Switching Technologies: Store and Forward Firstly used in telegram switching. Switching systems equipped with buffer. No engaged lines, but data might have to wait or might get lost if there is no buffer available. Incoming data is stored in the buffer until the desired outgoing line is free. Since several data portions from different sources might be waiting for transmission, they can be re-ordered (e. g. according to priority). For two data portions sent after each other, there is no fix timing. Sender and receiver do not have to work at the same speed. 172 Prof. Jochen Seitz 9

10 LL Switching Network End Systems (Users) A B LL IL2 IL1 IL3 End Systems (Users) D E C LL Switching System LL IL = Local Loop = Internal Data Link 173 Data Link Communication from buffer to buffer Data link consists of Sender buffer Physical link Receiver buffer Communication is finished, when the data frame has been completely received, checked against errors and stored in the receiver buffer Sender with data frames in buffer Non-buffering physical channel Receiver with buffer for incoming frames Data Link 174 Prof. Jochen Seitz 10

11 LL Model of a Switching Network Terminal Equipment A B LL IL1 IL2 IL3 LL Terminal Equipment C D E Terminal Equipment LL IL = Local Loop = Internal Data Link 175 Message Switching Definition Message : a self-contained object of communication Variable length Depending of the contained information Message might be too long to be transferred in one data frame only 176 Prof. Jochen Seitz 11

12 Segmenting Messages Partitioning the message (layer 3) into several data frames (layer 2) According to the layer 2 entity, data frames might have a minimum length, which has to be achieved by padding Layer 3 Message Header Message Body Data Frame 1 Data Frame 2 Data Frame 3 DF 4 Padding Layer 2 Data Frames Message 177 Segmenting over Several Data Links According to the characteristics of the layer 2 entities, data frames may have different sizes on different data links: Layer 3 Entity of Layer 3 Message Entity of Layer 3 Message Data Link Data Link1 1 2 Layer 2 1, 2, 3, 4: Data frames on the according data link 178 Prof. Jochen Seitz 12

13 Segmenting and Reassembling Segmenting and reassembling the layer 3 message to consider the maximum frame size of different data links Entity of Layer 3 Entity of Layer 3 Layer 3 PCI SDU PCI SDU Reassembling PCI SDU PCI SDU PCI SDU PCI SDU PCI SDU Data Link 2 Segmenting Data Link 1 Segmenting Layer 2 PCI = Protocol Control Information of the according layer SDU = Service Data Unit of the according layer 179 Message Switching The message hast to be completely received and reassembled, before it can be transmitted over the next data link. A message consists of message header Source and destination address Message type Message length Priority Timing information (e.g. sending time) message body User information to be transferred 180 Prof. Jochen Seitz 13

14 Packet Switching Layer 3 deals with packets that have a maximum length might have a fixed length Packets are quite short Goal is that a layer 3 packet completely fits into a layer 2 frame Each packet contains a header with layer 3 protocol control information, especially for packet forwarding 181 Packet Forwarding Packets are forwarded independently of each other No reassembling needed in the forwarding nodes Most important difference to message switching Temporal overlap of he packets results in shorter throughput times compared to message switching 182 Prof. Jochen Seitz 14

15 Packet Switching Alternatives Connection-oriented: Virtual Connection / Virtual Circuit Permanent Virtual Circuit (PVC) Switched Virtual Circuit (SVC) Connectionless: Datagram Switching 183 Virtual Circuits Characteristics Bidirectional fixed transmission path (full duplex) Virtual Circuit established between two connection end points in the data terminating equipment of sender and receiver Virtual Circuit can comprise several switches Permanent Virtual Circuit Established for a longer time Comparable to a leased line Switched Virtual Circuit Established on demand Establishment must precede data exchange phase Comparable to Circuit Switching 184 Prof. Jochen Seitz 15

16 Establishment of a Virtual Circuit During establishment phase, a path is defined for all packets belonging to the virtual circuit Thus, all packets are forwarded over the same path Switches need to store information about all active virtual circuits End User A A Packets Packets End User B B Packets End User Packet Switch 185 Datagram Switching Datagrams are packets that contain complete addresses Source address Destination address No connection establishment (and release) required Datagram switches do not need to store information about ongoing communication associations Routing decision is done separately for each datagram Advantages Information transfer is quicker (no connection establishment!) Network resources can be better utilized Disadvantages Quality of service cannot be guaranteed Reordering of datagrams is possible 186 Prof. Jochen Seitz 16

17 Process of Datagram Switching Each datagram may have its own path No reservation of resources possible End User A End User B Sequence of datagrams sent Received sequence Datagrams End User Packet Switch 187 Overview of the Different Switching Technologies Switching Technology Circuit Switching Store and Forward Space Multiplex Time Multiplex Packet Switching Message Switching Virtual Circuit Datagram permanent switched 188 Prof. Jochen Seitz 17

18 Literature FLOOD, J.E. (1995). Telecommunications Switching, Traffic and Networks. New York; London; Toronto; Sydney; Tokyo; Singapore, Prentice Hall. HALSALL, F. (1996). Data Communications, Computer Networks, and Open Systems. Harlow, England; Reading, Massachusetts; Menlo Park, California, Addison-Wesley. TANENBAUM, A. S. (2002). Computer Networks. Upper Saddle River, New Jersey, Pearson Education. 189 Prof. Jochen Seitz 18