Switching Fourth Meeting Switching Modes l Circuit Switching

Switching Fourth Meeting

Switching Modes l Circuit Switching l l l Continuous link Exclusive Packet Switching l l l No continuous link Data is divided into packets Not exclusive Circuit Switching` Packet Switching`

Incurred Delay l Circuit-switched l l fixed delay of transferring data Packet-switched l l Queuing delays Variable transmission delay because of the capacity of the link. Hi Mom

Multiplexing l l It improve the efficient use of the transmission capacity of physical media Two types of multiplexing l l l Deterministic Statistical Deterministic Multiplexing l transmission medium is divided between a fixed number of communication channels

Statistical Multiplexing l l The multiplexer and demultiplexer do not perform any switching The multiplexer stores the incoming packets in a buffer. Why? l l l Data rates of the input channel and the multiplexed channel are different; and Packets may arrive simultaneously on several input channels. In the multiplexer, the packets are stored with a header with input channel number. In the demultiplexer, the packets are retrieved in reverse order The packet header identify the output channel. Buffers are required for each output channel l The demultiplexer, over a short period of time, receive packets at a faster data rate than it can transmit.

Switching Structures l Cross-points l l l It connect input lines to output lines with a dedicated crosspoint It is non-blocking Switching Arrays l single-stage switch connects one input line to an output line using a single cross-point

Switching Structures: 3 -Stage-Switching l Three stage-switching l Blocking occurs N inputs = m groups * n inputs l N outputs = m groups * n outputs l l The first stage l l m array switches. Each array switches l l l The second stage l l k array switches Each array switch l l l n input lines k output lines m input lines m output lines The third stage l l m array switches. Each array switches l l k input lines n output lines

How Many Cross Points l The total number of cross-points = l l First stage l l l Total = k × m Since m = N/n, Then Total = k × (N/n)2. Third stage (same as first stage) l l Total = n × k × m Since N = n × m, then Total = N × k Second stage l l number of cross-points in the first stage plus number in the second stage plus number in the third. Total = N × k All three stages l .

Three Stage Switching Example l l 6 input lines that is blocking. four first-stage arrays l l Each array hasfour input lines l l n=4 Five second-stage arrays l l m=4 k=5 16 × 16 three-stage configuration has l

Digital Space Switching l Two types l l l Digital space switching and Digital time switching. Digital space switching l l The first automatic telephone exchanges Paths were set up using electromagnetic devices

Space Switch Carries A Time Division Multiplexed (TDM)

Space Switch Multiplexer

Space Switch Multiplexer

Example Frame 50 51 52 60 61 62 70 71 72 Multiplex X Control 0 Input 0 Selects 0 = 0 Selects 1 = 0 Input 1 Input 2 Time Slot 0 0 Output 0 0 Multiplex Y 0 1 Selects

Example Frame 50 51 60 61 62 70 71 72 Multiplex X Control 0 Input 1 Input 2 Time Slot 1 1 Output 0 1 Multiplex Y 0 1 Selects

Example Frame 70 50 51 60 61 71 72 Multiplex X Control 0 Input 1 Input 2 Time Slot 2 3 Output 1 1 Multiplex Y 0 1 Selects

Example Frame 70 50 51 60 61 71 72 Multiplex X Control 0 Input 1 Input 2 Time Slot 3 2 Output 1 0 Multiplex Y 0 1 Selects

Time Switching

Time Switching Implementation l Use two memory devices l Speech store l All incoming data octets are stored in their sequence of arrival, § § l Octet 0 → location 0 Octet 1 → location 1 : Cyclic counter Connection store, l l Contains the destination outgoing lines Has a cyclic counter

Example Input Time Slot Output Time Slot 0 3 1 2 2 1 3 4 0 0 1 2 3 4 0 Counter 1 0 Counter 2 0 Time 3 0 1 2 3 4

Example Input Time Slot Output Time Slot 0 3 1 2 2 1 3 4 0 1 2 3 4 1 Counter 1 1 Counter 2 0 1 Time 3 2 0 1 2 3 4

Example Input Time Slot Output Time Slot 0 3 1 2 2 1 3 4 0 1 2 3 4 2 Counter 1 2 Counter 2 0 1 2 Time 3 2 1 0 1 2 3 4

Example Input Time Slot Output Time Slot 0 3 1 2 2 1 3 4 4 0 0 2 3 0 1 2 3 4 3 Counter 1 3 Counter 2 0 1 2 3 Time 3 2 1 4 0 1 2 3 4

Example Input Time Slot Output Time Slot 0 3 1 2 2 1 3 4 4 0 2 3 4 0 1 2 3 4 4 Counter 1 4 Counter 2 0 1 2 3 Time 4 3 2 1 4 0 0 1 2 3 4

Space Time Space Switching k=1 n=3

Packet Switching l l In the past, the delay associated with packet switching meant that it was not considered suitable for real-time applications such as voice communication. Packet-switching exchanges (PSEs) have used the concept of stored program control (SPC) since the first generation of exchanges, in the late 1960 s

2 nd and 3 rd Generation PSE

Cell Switching Virtual Path Switch 2 Virtual Path Switch 1 l Cell switching ≈ packet switching l l Cell switching ≠ packet switching l l Cells have a fixed size Cell switching ≈ circuit-switching l l l breaks a data stream into packets connection-oriented service virtual circuit created. Cell switching ≠ circuit-switching l l circuit is virtual No reserving of network resources Picture source http: //www. sce. carleton. ca/netmanage/dcomm. Tutorials/scan 0603. gif