1. UNIT-II DIGITAL SWITCHING

2. What is it all about?
How do we move traffic from one part of the network to another?
Connect end-systems to switches, and switches to each other
Data arriving to an input port of a switch have to be moved to one or more of the output ports

3. Types of switching elements
Telephone switches
switch samples
Datagram routers
switch datagrams
ATM switches
switch ATM cells

4. Classification Packet vs. circuit switches
packets have headers and samples don’t
Connectionless vs. connection oriented
connection oriented switches need a call setup
setup is handled in control plane by switch controller
connectionless switches deal with self-contained datagrams

5. Crossbar Simplest possible space-division switch
Crosspoints can be turned on or off
For multiplexed inputs, need a switching schedule (why?)
Internally nonblocking
but need N2 crosspoints
time taken to set each crosspoint grows quadratically
vulnerable to single faults (why?)

6. Multistage crossbar
In a crossbar during each switching time only one crosspoint per row or column is active
Can save crosspoints if a crosspoint can attach to more than one input line (why?)
This is done in a multistage crossbar
Need to rearrange connections every switching time

7. Multistage crossbar Can suffer internal blocking
unless sufficient number of second-level stages
Number of crosspoints < N2
Finding a path from input to output requires a depth-first-search
Scales better than crossbar, but still not too well
120,000 call switch needs ~250 million crosspoints

8. Time-space switching Precede each input trunk in a crossbar with a TSI
Delay samples so that they arrive at the right time for the space division switch’s schedule

9. Dealing with blocking Overprovisioning Buffers Backpressure
internal links much faster than inputs
Buffers
at input or output
Backpressure
if switch fabric doesn’t have buffers, prevent packet from entering until path is available
Parallel switch fabrics
increases effective switching capacity

10. Three generations of packet switches
Different trade-offs between cost and performance
Represent evolution in switching capacity, rather than in technology
With same technology, a later generation switch achieves greater capacity, but at greater cost
All three generations are represented in current products

11. First generation switch
Most Ethernet switches and cheap packet routers
Bottleneck can be CPU, host-adaptor or I/O bus, depending

12. Example First generation router built with 133 MHz Pentium Copy loop
Mean packet size 500 bytes
Interrupt takes 10 microseconds, word access take 50 ns
Per-packet processing time takes 200 instructions = µs
Copy loop
register <- memory[read_ptr]
memory [write_ptr] <- register
read_ptr <- read_ptr + 4
write_ptr <- write_ptr + 4
counter <- counter -1
if (counter not 0) branch to top of loop
4 instructions + 2 memory accesses = ns
Copying packet takes 500/4 * = µs; interrupt 10 µs
Total time = µs => speed is Mbps
Amortized interrupt cost balanced by routing protocol cost

13. Second generation switch
Port mapping intelligence in line cards
ATM switch guarantees hit in lookup cache
Ipsilon IP switching
assume underlying ATM network
by default, assemble packets
if detect a flow, ask upstream to send on a particular VCI, and install entry in port mapper => implicit signaling

14. Third generation switches
Bottleneck in second generation switch is the bus (or ring)
Third generation switch provides parallel paths (fabric)

15. Third generation (contd.)
Features
self-routing fabric
output buffer is a point of contention
unless we arbitrate access to fabric
potential for unlimited scaling, as long as we can resolve contention for output buffer

17. The importance of switching in communication Definition:
The cost of switching is high
Definition:
Transfer input sample points to the correct output ports at the correct time
Terminology
Switching
Digital switching (sample points amplitudes are 0's and 1's)
PABX
Circuit
Circuit switching
Packet switching

18. Space division

20. Voice digitization: Telephone switching
W=3KHz, sampling at 2*3=6 or 8KHz
256 levels for quantization (8 bits)
Bit rate=64Kb/s
Telephone switching
Time division multiplexing: time slot (0.1 ms), field, frame;
125ms/0.8=150 channels + time for synchronization and control

21. Switch architecture
Sampling input signals, storing values in memory, placing values in the proper field and frame of the output sequence
Need for more channels: hierarchical switching
Combining time and space switching

22. Switching techniques and networking
Switching is the technology allowing to get a message between the nodes of a network
Crossbar switching: mechanical (in the past) or electronic.
Bus and cable switches: computer buses or cables (switching + transportation = network)
Token passing approach (similar to the locks used by multiprocessors connected by a bus)
Ethernet approach: cable or ring, packets, conflicts, resending
Synchronization and Hub switch: star networks (no conflicts)

25. Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.
Ethernet Cabling
Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.

26. Time division switching
Key idea: when demultiplexing, position in frame determines output link
Time division switching interchanges sample position within a frame:
Time slot interchange (TSI) M U X D E
TSI

27. Time Slot Interchange (TSI) : example
sessions: (1,3) (2,1) (3,4) (4,2) 1 2 3 4 1 2 2 4 3 1 4 3
Read and write to shared memory in different order

28. TSI Simple to build. Multicast: easy (why?)
Limit is the time taken to read and write to memory
For 120,000 telephone circuits
Each circuit reads and writes memory once every 125 ms.
Number of operations per second : 120,000 x 8000 x2
each operation takes around 0.5 ns => impossible with current technology
Need to look to other techniques

29. Time-Space: Example time 1 time 2 2 1 2 1 TSI 3 4 4 3 3 2 1 4
2 1
3 4 3 1 2 4
2 1
4 3
TSI
TSI
Internal speed = double link speed

30. From space switch to T-S-T switchk 1 2
.... n 1 2
.... n 1 2
.... k
TSI 1
TSI 1 k 1 2
.... k 1 2
.... n 1 2
.... n 1 2
....
TSI 2
TSI 2 k 1 2
.... n 1 2
.... k 1 2
.... n 1 2
....
TSI N/n
TSI N/n
Replace each first-stage and every last-stage switch by a TSI. Replace multiple
intermediate-stage switch with a single time-shared space switch.
The intermediate stage switch is a single switch of size:
The intermediate stage switch is reprogrammed for each of the k time slots.
last updated March 8, 04
M. Veeraraghavan

31. Time-space-time (TST) switching
Allowed to TSI both on input and output
Gives more flexibility => lowers call blocking probability
TSI

32. Internal Non-Blocking Types
Re-arrangeable
Can route any permutation from inputs to outputs.
Strict sense non-blocking
Given any current connections through the switch.
Any unused input can be routed to any unused output.
Wide sense non-blocking.
There exists a specific routing algorithm, s.t.,
for any sequence of connections and releases,
Any unused input can be routed to any unused output,
assuming all the sequence was served by the routing algorithm.

33. Circuit switching - Space division
graph representation
transmitter nodes
receiver nodes
internal nodes
Feasible schedule
edge disjoint paths.
cost function
number of crosspoints (complexity of AxB is AB)

34. Crossbar - example 1 2 3 4 4 1 2 3

35. Another Example
inputs
outputs

36. Another Example outputs inputs
sessions: (1,3) (2,6) (3,1) (4,4) (5,2) (6,5)
inputs
outputs

37. Clos Network Clos(N, n , k) : N - inputs/outputs;
cross-points: 2 (N/n)nk + k(N/n)2
nxk
(N/n)x(N/n)
kxn
2x2
3x3
2x2
N=6
n=2
k=2 N
2x2
3x3
2x2 k
N/n
N/n

38. Clos Network - strict sense non-blocking
Holds for k  2n-1
Proof Methodology:
Recall: IF [A,B  S and |A|+|B| > |S|] then A∩ B≠Ø
S= The k middle switches
A = middle switches reachable from the inputs
B = middle switches reachable from the outputs
Our case:
|S|=k
|A| ≥ k-(n-1)
|B| ≥ k-(n-1)

39. No.4 ESS TOLL SWITCH Basic Concepts
We will define basic telecommunication terms, such as:
analog
digital
bandwidth
compression
protocols
codes and bits

40. The architecture or protocol suite is the umbrella under which the devices communicate with each other

41. Congestion
All networks are limited in how many peripherals they can support without experiencing too much degradation
Today more and more peripherals are being added to networks
New ways to eliminate congestion on a network have been developed

42. Eliminating Congestion
Multiplex:
to transmit two or more signals over a single channel
Compression:
reducing the representation of the information, but not the information itself
reducing the bandwidth or number of bits needed to encode information or a signal

43. Multiplexing Several devices can share a telephone line
T-1 telephone line will carry 24 communication paths on one high-speed link
T-3 provides 672 communication paths on one link

44. Compression
Applications such as: graphics, x-ray images, video are bit intensive
Thus require high bandwidth when transmitting
Compression reduces the number of bits needed to transfer

45. Analog and Digital Telephone system developed to transmit speech
Spoken words are transmitted as analog sound waves
People speak in an analog format, in waves
Telephone system was completely analog until 1960

46. Analog Signals Move down telephone lines as electromagnetic waves
The way it travels is expressed in frequency
Frequency refers to the number of times per second that a wave oscillates or swings back and forth in a complete cycle from its starting point to its ending point

47. Analog Signals
A complete cycle occurs when a wave starts at a zero point of voltage, goes to the highest positive point of the wave, down to the negative voltage portion, and then back to zero voltage

48. Analog Signals
The higher the speed or frequency, the more complete cycles of a wave are completed in a period of time
This speed or frequency is measured in Hertz
Hertz: a measurement of frequency in cycles per second, 1 hertz is 1 cycle p/sec

49. Hertz
A wave that oscillates or swings back and forth 10 times per second has a speed of 10 hertz or cycles per second
Bandwidth or range of frequencies a service occupies is determined by subtracting the lower range from the higher range
Hz (voice)= = 3000Hz

50. Analog Services Voice (300 -3300 Hz) Microwave Radio (2-12 GHz)
Analog cable TV signals ( MHz)
Oscillate between a specific range of of frequencies

51. Analog versus Digital
Analog system can no longer handle the increase in the number of calls that are being generated, was designed for lower volume
Digital networks are faster, have more capacity, and are more reliable

52. Impairments on Analog Services
Analog signals loose their power the longer they travel
Signal meets resistance in the media (copper, coaxial cable, air), causes fading of the signal or attenuation of the signal
Analog signals also pick up noise or electrical energy while travelling from power lines, light sources, and electrical machinery
Requires: amplification to inhibit attenuation

53. Amplification
To overcome resistance in a signal, analog signals are amplified while they travel over a medium
Drawbacks amplification:
also increases level of noise in signal

54. Digital Signals Advantages digital signals: higher speeds
clearer voice quality
fewer errors
less complex peripheral equipment required

55. Digital Signals No waves are transmitted
Digital signals are transmitted in the form of binary bits
Binary = being composed of two parts
In telecommunications this means only on or off, one or zero piece(s) of information transmitted

56. Applications Wideband: Narrowband: TV Cable
Connections between telephone offices
Narrowband:
phone connection to end users

57. Protocols Enable computers to communicate with each other
Spell out the rules of interaction between two or more computers
Handle error detection and correction and file transmission

58. Examples of Protocols Who transmits first?
What is the structure of the addresses of devices such as computers?
How are errors fixed?
How long to wait before disconnecting?
How to package data to be sent?

59. Architecture
Ties computers and peripherals together into a coherent whole
Forms the network which connects all devices together
Layers within architectures have protocols to define functions such as routing, error checking and addressing

60. OSI Not widely implemented
Laid foundation for the concept of open communications among vendors
Basic concept of layering of groups of functions into 7 layers
Each layer can be changed and developed independently

61. Bridges Used to connect a small number of LAN’s
Provide one common path to connect several LAN’s
Easy to configure, all data sent to all devices on a network, appropriate device picks it up, broadcast feature
Lack routing and congestion control

62. Routers
Used to connect multiple LAN’s over large distances (differing buildings, cities)
More sophisticated than bridges
Can handle differing protocols from various LAN’s

63. Switching Routers Faster than non-switching routers
Do not look up in tables where to send data
Address placed in the pack sent