ATM Switching

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ATM Switching

• Presented by
• Jesús Ríos
• May 2-3, 2002
• ELEE 6399

2
Presentation Outline

• Definition
• Characteristics
• Generic Block Diagram
• Classification
• Space Division
• Time Division
• Definition - Blocking
• Definition Buffering
• Approaches
• Pros Cons
• Sharing

• Space Division
• Crossbar
• Banyan Network
• Others
• Time Division
• Shared Memory
• Shared Medium

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ATM Switch

• Definition
• An ATM switch of size N can be regarded as a box
  with N input ports and N output ports, that
  routes cells arriving at its input ports to their
  desired output ports (destinations)
• The means by which to allocate the limited
  transmission facilities to users so as to provide
  a certain degree of connectivity among them.

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ATM Switch Characteristics

• High-speed interfaces (50 Mbps to 2.4 Gbps)
• Transports various types of traffic, each with
  different requirements
• cell loss
• errors
• delay an delay jitter (variance in delay)
• Relays cells (routes and buffers cells) from
  input ports to output ports
• Provides control and management functions
• Supports a set of traffic control requirements

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Generic ATM Switch
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Generic Switch Architecture
Functional Block Model
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Switch Classification

• Space Division Fabrics
• Time Division Fabrics
• Combination of the two

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Switch Classification

• Space-Division Fabrics
• All cells flow through the switch at the same
  time, but do so on different paths
• Multiple concurrent paths are established between
  input and output ports
• Many cells cross the switching fabric
  simultaneously
• Constructed from basic switching blocks called
  switching elements (SE) interconnected in a
  specific topology by links
• Examples are crossbar switch, banyan switch,
  batcher-banyan switch, and many more

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Space Division Switch
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Switch Classification

• Time-Division Fabrics
• All cells flow through a single resource shared
  by all input and output ports, but do so at
  different times
• Resource may be either a shared-memory or a
  shared medium such as a bus
• Most common types are shared-memory switch and
  shared-medium switch

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Time-Division Switch
OUTPUT 1
INPUT 1
AF
S/P
FIFO
OUTPUT 2
INPUT 2
S/P
AF
P/S
FIFO
TIME DIVISION BUS
...
...
...
...
...
...
INPUT N
OUTPUT N
AF
S/P
FIFO
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Definition - Blocking

• Internal Blocking
• When two or more cells addressed to different
  outputs compete for the same internal link
• Output Blocking
• When two or more cells addressed to the same
  external output port arrive within the same cell
  time slot
• Both result in cell loss due to cell conflict in
  delivery

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Definition - Buffering

• Buffers - Storage locations utilized to store
  cells on a temporary basis to prevent cell loss
  whenever port contention occurs
• Classified based on the location of buffers
• Input Buffering, Internal Buffering
• Output Buffering, Re-circulating Buffering
• Classified based on the memory sharing policy
• Dedicated buffers
• Shared buffers
• Used to prevent blocking

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Buffering Approaches
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Buffering Pros Cons

• Input buffering
• Head of line blocking
• Change FIFO buffer to FIRO (First-in-random-out)

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Buffering Pros Cons

• Output buffering
• Optimal in terms of throughput and delay
• Can add multiple buffers per output to enhance
  delivery of multiple cells per cycle
• Internal buffering
• Can create internal head of line buffers
• Introduces varying delay in cell delivery
• Recirculating buffers
• Allows cells to re-enter the internal space
  division network
• Issues larger network for recirculation
  control mechanism to maintain cell sequence

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Buffer Sharing

• Lowers the total amount of buffer space
  requirement
• Uses statistical sharing
• Useful in handling traffic bursts
• If Im getting blown out of the water by a large
  file transfer or video file, I can borrow someone
  elses buffer space to handle the burst
• Prevents cell loss

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Multicasting / Broadcasting

• Broadcast an input to a selected number of
  outputs or to all outputs
• Video is one such application
• Shared medium and fully interconnected output
  buffered fabrics
• Already broadcast ready, so design address
  filters to filter by multicast address as well as
  port address
• Shared memory fabric
• Requires additional hardware
• Can either make multiple copies of cell 2 B
  multicast or can read memory repeatedly
• Former requires more memory latter requires
  control circuitry

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Space Division Crossbar Switch
NxN Square Space Switch w/N2 Crosspoints
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Advantages/Disadvantages

• Need n(n-1)/2 crosspoints
• For n 1000, need 500,000 crosspoints
• Splitting crossbar switch into smaller pieces
• Connect smaller pieces together to form
  multistage switches as shown on next slide
• Reduces the number of crosspoints
• Generates a space division switch

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Space Division - Banyan
4x4 8x8 Banyan Networks
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Advantages/Disadvantages

• Switching performed by simple elements
• Cells routed in parallel
• Modular, can scale recursively
• Implement in hardware
• Internal blocking (Sorter network)
• Input buffer
• Internal buffer
• Requires routing function
• Self routing
• Depending on input, it can route or block (153)

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Other MINs

• Batcher-Banyan
• Benes Network
• Clos Network
• Cascaded Banyan
• Omega Network
• Delta Network
• Goals of these networks
• Minimize cross points to scale the network

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Time Division - Shared Memory
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Advantages

• Output buffers are shared between all ports
• 100 throughput under heavy load
• Minimizes the number of buffers for a particular
  cell loss ratio
• Scalability price
• Input S/P? Reduce internal speed and to write to
  memory
• Non-blocking? Multi-stage?

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Disadvantages

• Shared memory must operate N times faster
• Memory access time limited
• Need fast controller to process header info as
  fast as shared memory rate
• Issues w/Multi- and Broadcasting

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Multicast Support 4 Shared-Memory

• Multiple-write, multiple-read (MWMR)
• Advantages
• Cells treated as unicast cells after replication
• Fair scheme multicast likely to be dropped 1st
• Disadvantages
• Cell loss due to worst case scenario
• Requires reading N2 cells
• N2 cells to write limited memory cycle time

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Multicast Support 4 Shared-Memory

• Single-write, multiple-read (SWMR)
• Cell stored directly in a separate queue
• Multiple reads operations performed to output
• Advantages
• Prevents writing N2 cells to memory each cycle
• No additional hardware switch size and fanout
  (N) small
• Disadvantages
• Replication process speeds up by factor of N
• Memory bandwidth limitation if memory size grows
  too big

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Multicast Support 4 Shared-Memory

• Single-write, single-read (SWSR)
• No replication at input
• Single read for output
• Replicated at output
• Dedicated multicast queue and output port (FIFO)
• Implement w/ Output buffers or Output masks
• Advantages
• Efficient (memory space)
• Efficient (memory access time)
• Disadvantages
• Additional hardware for replication (fanout bus)
• Additional buffer space

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Multistage Shared Memory Switch Designs

• Single stage common memory switch provides the
  lowest delay and the highest link utilization
• Size of single-stage switch limited by
• Input/output pin limitations
• Power dissipation
• Memory control logic
• Handle N incoming and outgoing packets in ea.
  time slot
• Memory bandwidth at least the sum of the input
  and output bandwidths
• Cycle Time cell length (b) / 2 N link speed
  (b/s)

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Linked-List-Based Shared Memory Switch
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Hybrid Shared Dedicated Output Buffer Switch
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Time Division - Shared Medium
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Advantages/Disadvantages

• Output modularity
• Easy implementation
• Larger buffer requirement for a particular cell
  loss rate
• Speed has to be N times faster than input ports
• Input buffering (bus speed has to NxV)

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Combination - Fully Interconnected
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Advantages/Disadvantages

• Queueing at output ports
• output buffering is desirable (performance point
  of view)
• hardware requirements on this method are much
  larger than on input buffering
• Natural multicasting and broadcasting
• Address filters and buffers
• simple to implement (not aggregating) and modular
• speed port speed
• Scalable to any size and speed
• Buffer growth is a limiting factor
• Example Knockout switch

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• THE END!

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Questions Answers

• Hold your fire!

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BIBLIOGRAPHY
1 Raed Y. Awdeh and H.T. Mofutah, Survey of
ATM Switch Architectures, Computer Networks and
ISDN Systems, 1995. 2 Fouad A. Tobagi, Fast
Packet Switch Architectures for Broadband
Integrated Services Digital Network, Proceedings
of the IEEE, Jan 1990. 3 Joan Garcia-Haro and
Andrzej Jajszczyk, ATM Shared-Memory Switching
Architectures, IEEE Network, July/Aug 1994. 4
Sanjeev Kumar and Dharma P. Agrawal, On
Multicast Support for Shared-Memory Based ATM
Switch Architecture, IEEE Network, Jan/Feb
1996. 5 Sonia Fahny, A Survey of ATM
Switching Techniques, Department of Computer and
Information Science, The Ohio State University.
http//www.cs.purdue.edu/homes/fahmy/cis788.08Q/at
mswitch.htmlintroAsdf 6 Andrew S. Tanenbaum,
Computer Networks. New Jersey, Prentice Hall
PTR, 1996. 7 Carey Williamson, ATM Switch
Architectures, Department of Computer Science,
University of Saskatchewan. http//www.netera.ca/t
raining_res/3.120Switch20Architectures.ppt
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Bibliography Contd

• 8 Daniel Sobirk and Johan M. Karlsson, ATM
  Switching Structures A Performance Comparison,
  Department of Communication Systems, Lund
  Institute of Technology. http//www.tts.lth.se/Pe
  rsonal/daniels/papers/nts12.pdf
• 9 Barry Hill, ATM Asynchronous Transfer
  Mode, Machine Vision Laboratory, Worcester
  Polytechnic Institute. http//www.ece.wpi.edu/cour
  ses/ee535/hwk11cd95/bkh/bisdn.html
• 10 Gail C. Hudek, Blocking in Digital
  Cross-Connect Systems, IEEE Network, Jan 1993.