ATM Switching

1. Course Code (MSc-TNS)
Asynchronous Transfer Mode (ATM)
Switching Report
Assignment 1
Author:
Abid Afsar
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2. Abstract: Asynchronous Transfer Mode (ATM) switching is a switching technique use for
Broadband Integrated Services Digital Network (B-ISDN). In this paper we will investigate
the ATM switching architecture, operation, functional components and feature such as,
switch fabrics, switch buffering, and so on. We will explore the ATM switching designs
types, and the trade-offs it involve. ATM switch sends information in the form of cell relay or
cell and use virtual channel for internal routing. The core duties of ATM switching involve
routing, cell forwarding, and management. Moreover, the ATM switching techniques were
not a part of ATM standards.
Keywords: ATM, Cell Relay, B-ISDN, virtual channel, switch architecture, ATM standards.
1. Introduction
Asynchronous Transfer Mode (ATM) is a standard network switching method formed by
International Telecommunication union telecommunication standardisation Sector (ITU-T). It
was developed in the early 1980’s for the purpose to unify telecommunication and computer
networks, and to get together the requirements of Broadband Integrated Services Digital
Network. The need of ATM is vital for an increase integration of every sort of application
such as data and voice, audio and video, and so on. The internet is evolving at a very high
speed and this shift in network paradigm brings a unique amalgamation of multimedia, high
speed, and real time services.
The integration of communication needs powerful switches to process, and to perform
routing with a high accuracy. ATM is a connection oriented switching technique, it use
virtual channel to connect network nodes. The virtual channel (VC) has a unique
identification and is refereed as virtual channel identification (VCI). ATM switching takes
different approach in regards to packets propagation, and size constraints as compare to
packet switching, frame really and Ethernet. It use cells of equal size, and propagates it by
using time division multiplexing mechanism. ATM taking services of data link layer and
physical layer of Open System Interconnection (OSI) model. ATM inherits the features of
circuit switching because it based on cell switching and multiplexing technology. The
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3. asynchronous nature of ATM makes it more efficient as compare to the synchronous
technique such as time-division multiplexing because it do not waste empty slots.
2. Switch Fabric Architecture:
ATM switch architecture is available in several flavours. The most know architectures which
are widely used are single bus, multiple bus and self routing. Self routing can be further
divide into two forms blocking and non-blocking switch architecture. The core purpose of
the ATM switch fabric is to transfer cell from one point to another. The switching use the
combination of software and hardware to pass the incoming data from network node to
input ports and from output ports to other network node, the process is referred as switching
fabrics or switching matrices. The switch designs can be hybrids and can accommodate one
larger switch fabrics to connect other switches or combinations of smaller switch fabrics
make one larger switch fabrics. We will discuss the switch architecture in terms components,
and attributes such as, input module (IM), output module (OU), admission connection control
(CAC), switch management (SM), scalability, blocking level, maximum overall speed, and
support of multicast. Diagrammatically the switch architecture can be represented as
Figure 1: A generic view of ATM switch
2.1. Single Bus:
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4. The simplest type of switch architecture is single bus. It consists on single bus which in
parallel connects multiple circuit board trace. The speed of single bus is between 1 and 10
Giga bit per second (Gbps). The complexity level involved with single bus is low. In terms of
scalability the single bus performance is poor because of the load on single bus. The blocking
level at each port is low due to bus controller call to arbiter to decide that who will get the bus
first and so on. And multicasting is easy in this sort of bus for the reason that all output
listening to the same bus. Diagrammatically it is represented as,
Bus
Figure 2: Single Bus
2.1. Multi Bus:
Multi bus switching architecture works on the concept of broadcasts and do broadcast on
each input port. It eliminates the requirement of bus arbitrations unlike single bus. As the
architecture is changed it adds extra requirements to output port. In multiple buses each bus
shared with multiple circuit board trace. The maximum speed range is between 1 to 20 Gbps.
In relation to output port, if it receives numbers of cells simultaneously then the input
buffering technique is used for arbitration purpose. And bus control entails blocking to
mange output ports. This architecture of bus is also called knockout switch architecture
because every outputs inlet receives only limited number of inputs simultaneously which
makes it favourable choice for scalability. And multicasting is entailed in the nature of this
sort of bus. Diagrammatically it is represented as,
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5. Multi- Buses
Figure 3: Multiple Buses
2.3. Self-Routing Bus:
Self routing is a complex switching architecture and use amalgamated switching attributes
and components. It is widely in use in Batcher Banyan networks; it can easily scale large
size of switching components, and is suitable for Very Large Scale Integration (VLSI). It still
needs in-depth research due to its complex nature of operations. The speed of self routing and
inputs are same, therefore it has a high probability of blocking. As compare to the
counterparts of switching architecture it run with high speeds. Diagrammatically it is
represented as
Self Routing
(Blocking)
Self Routing (Non-Blocking)
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6. Figure 4: Self Routing
3. Switching Operation:
ATM switch carry out a number of operations through input and output ports. Generally
switch involved a list of jobs such as routing, cell forwarding, and connection controls
managements with other switches and so on.
3.1. User platform: The job of switch is to route input cells to designated output ports. The
cells are encapsulated in a header with payload bytes. When these cells reach to input
ports a virtual connection is established which are uniquely identified by virtual path
identifier and virtual circuit identifier (VPI/VCI) and drive these cells to expected end
user node. In user plan the core functions are mainly performed by input module, switch
matrix, and output modules.
3.2. Control Platform: The control platform is responsible for connection establishing, and
controls of virtual path and virtual circuit (VP/VC). The information in the payload in this
case in not visible to the network. In this platform the switch tagging signal and even
produce some signal by itself. The Connection Admission Control (CAC) doing the job
of signalling and which is indispensable. The switch fabric may or may not receives the
signalling or may be pass by Signalling System No. 7 (SS7) network signalling protocol
standard.
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7. 3.3. Management Platform: This platform is to control and maintain the operation of
network, and to guarantee that everything is working at optimum rate. The management
operations are further categorised as, traffic management operation, accounting
management operation, performance management operation, fault management operation,
security management operation and so on. These operations are accomplished by switch
management module; it is also responsible for ATM Operation and Management layer
(OAM) functions. Furthermore, it has to determine OAM signalling and may produce
OAM signal. The switch management module also reinforce UNI interface that is called
Interim Local Management Interface (ILMI).
3.4. Traffic Control Operations: The operations such as connection admission control
(CAC), congestion control, and network parameter UPC/NPC are also reinforcing by
switching technique. The operation to control congestion is the responsibility of switch
management module, and the operations to maintain network parameter UPC/NPC is the
responsibility of input module.
4. Switch Interfaces
4.1 Input Module: The function of input module (IM) is to manage table lookup and VCI/VPI
translation. When a cell arrives at input module in first attempt it is VPI value is extracted
in cell header because VPI value is used in the address table for link identification. The
VPI/VCI are not unique on every link and chances of duplication can occur, to resolve
this issue ATM switch use separate address table for each link and also add link identifier
with VPI/VCI. A number of methods can be used for table lookups such as hashing,
search trees or content addressable memories and so on. It can be determine from the
table lookups that the particular VPI value may matches VPC or VCC. If the VPI value
corresponds to VCC then the concerned information can be fetched from translation
lookup table, such as new VPI value, outgoing interface identifier, cell priority, jitter, and
delay. In addition the input module convert SONET optical signal to electrical signal,
doing HEC error check for every incoming cell and discard faulty of damage cells, it
differentiate between OAM cells and signalling cells.
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8. 4.2 .Output Module: The function of the output module is the converse of input module. It
prepares the ATM cells for physical transmission medium. It converts the electrical
signals into optical signals, create new HEC field and encapsulate it in header,
multiplex user cells, OAM cells, and signalling cells; doing cell rate decoupling.
5. Cell Switch fabric:
The main purpose of the cell switch fabric module is to transfer packets from input ports
to out ports. A typical switch fabric is a combination of switch elements and transmission
links. Switch fabric is also called interconnection structure. The transmission link only
carries packets and has no power to make intelligent decisions. On the other hand switch
elements are performing intelligent decisions such as internal routing, management and so
on. In addition, it also accomplishing activities such as cell buffering, multitasking and
broadcasting, cell priority and delay scheduling, provide redundancy in relation to fault
tolerance, monitor congestion, traffic concentration and multiplexing.
5.1. Connection Admission Control (CAC): The function of CAC is to establish,
terminate, modifies VP/VC connections. The CAC core duties cover the followings issues
such as to work with higher layer signalling protocol, VPCs/VCCs switch resource
distribution and section of route, parameters generation of UPC/NPC, negotiations in
regards to user traffic contracts, and modification of VPCs/VCCs, provide signalling
reinforcement to ATM Adaptation layer (AAL) in terms of interception or generation of
signalling cells, and making decisions on VPCs/VCCs request acceptance or refusal. In
case of centralised CAC the single processing unit receives the signalling cells from input
module. And further CAC performed interpretation of incoming signalling cells and
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9. making all decisions in relation to admissions control, and switch resource allocation for
different connection.
5.2. Switched Management: The function of the switched management is to manage the
internal operations and components, elements of the switch which are listed as support
of network management, configuration management, customer network management,
switch and their security control, traffic management and performance management. In
relation to switch management area and research. The area is under development and
have no particular standards are established until today. The switch management is a wide
and complex area, entails a number of management operations and the level of
management can vary from nominal to complex. The initial jobs of switch management is
to gather information and their management, close contacts with network manger and
user, and supervisions and coordination of all operations performing in relation to switch.
A bottleneck situation arises when switch is overloaded in centralized switch
management. This can negatively affects the overall performance of the switch
management. Therefore, it is vital that a switched management administer and supervise
all incoming inputs to input module, and similarly for output module.
5.3. Concentration: In ATM switch the concentration work is to resolve all the traffic flow
at the input module before recuing the switch fibre module. Concentrator workout to
combine lower variable bit rate to higher variable bit rate for the sake to standardize the
flow or speed of traffic at input interface.
5.4. Routing and Buffering: Routing and buffering is a core function of switch fabric.
Although ATM switch is not oblige to use routing protocol. Two types of routing are
normally which are dynamic and static, if static routing protocol is in implementation
then dynamic routing is not required. In ATM switch the concept of virtual path was
purposely added to make things easier for routing. The incoming cell arrives at input
ports at switch fabric are simply route to the anticipated output ports. The role of the
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10. virtual path is working as similar as pipes with a specified bandwidth assigned to it. To
implement dynamic routing in ATM switch it use a link-state routing protocol is referred
as Public Network Network Interface (PNNI). Buffers are required when an input or
output module receives a number of cells simultaneous for same input or output.
5.5. Shared Memory Approach: in this approach the incoming cells are in first attempt
converted from parallel to serial structure and stored in a random access memory
sequentially. The controller job is to make decisions in regards memory reading, writing,
and sequence. It can also inspect cell header, and switch fabric internal routing tags.
6. Switched Designs Principal
6.1. Buffering Approaches: The use of buffer has a positive effect on switch performance
and efficiency. Three approaches of buffering are applied in switch which are input
buffering, internal buffering and out buffering. The input buffer is placed before the
input module to control the simultaneous traffic for the input ports but this is suffer from
head of line problem and it is still subject for further study. Out buffer is implementing at
the exit point of out module to manage the flow of cells to the next network node and
improve the level of traffic which are passing through. It improves the overall delay time
and throughput but it entails speed factor which negatively impact the level of scalability
for large networks. Internal Buffering
6.2. Internal Blocking: The internal blocking is a one of the operating mode of ATM
switches. It arises when N numbers of cells are addressed to N number of multiple
outputs which result conflict or collide with each other at output ports level. In general
unblocking switches are preferable to blocking switches but un-blocking fabric switch do
not use buffer which is known cause of congestion and collision. It negatively affects the
throughput, performance of the fabric module. It also affects the scalability of the fabric
switch.
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11. 7. Conclusion
In this brief report our central focus was on ATM switching. ATM switching was
elaborated in a detail which involves switching architecture approaches that entails single
bus, multi bus, and self-routing. In this paper ATM switching system is consist on
interfaces which is composed of input and output module, the switched management
performing duties such as connection, OAM layer functions, and cell fabric switch
module were briefly studied. Furthermore, the cell switch fabric is also refereed as switch
matrix that constitutes concentration, buffer management, and shared memory were
discussed.
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12. 8. Glossary
ATM - Asynchronous Transfer Mode
B-ISDN - broadband integrated services digital network.
CAC - connection admission control
NNI - network to node interface
PNNI - private network to network interface
PVC – permanent virtual circuit
VC - virtual circuit
VCI - virtual circuit Identifier
VCC - virtual channel connection
HEC- header error check
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13. Bibliography
[1]. McDysan, David E., and Darren L. Spohn. Hands-on ATM. New York: McGraw-Hill,
1998. Print
[2]. Kasera, Sumit. . Atm Networks Concepts And Protocols. Second ed. New Delhi: Tata
McGraw-Hill Professional, 2008. Networking Ser. Google. Tata McGraw-Hill Publishing
Company Limited, 01 Jan. 2008. Web. 12 Nov. 2011. <http://books.google.ie>.
[3]. Perros, Harry G. "An Introduction to ATM Networks - Harry G. Perros." Google Books.
N.p., n.d. Web. 14 Nov. 2011. <http://books.google.ie/books?id=ghy9BOw6svMC>.
[4]. "Asynchronous Transfer Mode (ATM) Switching." CCS Voice, Data, Video, Fiber Optic
Cabling Los Angeles, California. N.p., n.d. Web. 14 Nov. 2011. <http://www.ccs-
cabling.com/>.
Reading List
[1]. "ATLAS I Overview Transparencies (ICS-FORTH, ATM Switch)." CARV-ICS-
FORTH, Heraklion, Crete, Greece. N.p., n.d. Web. 14 Nov. 2011.
<http://archvlsi.ics.forth.gr/atlasI/gen_talk1.html>.
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