ATM SWITCHINGATM SWITCHING

SWITCHINGSWITCHING A Switch is a network element that transfer packet from Input port to output A Switch is a network element that transfer packet from Input port to output

port.port. Switching is process of transfer of packets from Input or Inlet port to Outlet or Switching is process of transfer of packets from Input or Inlet port to Outlet or

output port. It is also known as Internal routing of packets.output port. It is also known as Internal routing of packets.

COMPONENTS OF SWITCH:-COMPONENTS OF SWITCH:-

Switching Fabric

I/P module

I/P module

O/P module

O/P module

Input module accepts packets arriving at I/P port & determine outgoing label & port of arrived packet.

Output Module linked to transmission media & underlying technology.

Switching fabric provide to route packets from I/P port to O/P port. It consist transmission link to carry packets & switching elements to perform function like Routing.

Performance Measures in Switch DesignPerformance Measures in Switch Design 6 parameters are classified either as QOS parameters:-6 parameters are classified either as QOS parameters:-

1.1. Packet loss Probability Packet loss Probability refers to the probability of packet being dropped refers to the probability of packet being dropped within switch before it can be successfully transferred to its destined within switch before it can be successfully transferred to its destined outlet. outlet.

2.2. Delay:- Delay:- It refers to time required to switch a packet , right from time it It refers to time required to switch a packet , right from time it enters I/P module to time it leaves O/P module. It include waiting time also enters I/P module to time it leaves O/P module. It include waiting time also of a packet in switch buffer.of a packet in switch buffer.

3.3. Throughput:-Throughput:- Ratio of O/P traffic rate and I/P ratio rate. Ratio of O/P traffic rate and I/P ratio rate.

4.4. Design Complexity:-Design Complexity:- A complex switch design invariably translates into A complex switch design invariably translates into higher costs or limiting switching capability. For this reason a simple design higher costs or limiting switching capability. For this reason a simple design with slightly lesser capability is preferred over complex design.with slightly lesser capability is preferred over complex design.

5.5. Scalability:-Scalability:- It refers to which a switch can be expanded. Two aspects with It refers to which a switch can be expanded. Two aspects with scalable switch design are:-scalable switch design are:-

11 Scalable switch capable of supporting additional inlet.Scalable switch capable of supporting additional inlet.

22 It also capable of supporting any increase in port speed of its inlets.It also capable of supporting any increase in port speed of its inlets.

66.. Cost:-Cost:- It is significant factors in deciding success or failure of switch design. It is significant factors in deciding success or failure of switch design.

Switching IssuesSwitching Issues Various issues in Switch design are:-Various issues in Switch design are:-

1 Conflict is phenomenon occurring in switching process by which switching is 1 Conflict is phenomenon occurring in switching process by which switching is blocked.blocked.

External conflictExternal conflict Internal conflictInternal conflict

2 Blocking and Buffering2 Blocking and Buffering Input bufferingInput buffering Output bufferingOutput buffering Shared bufferingShared buffering

3 Memory lookup:- Memory in switches used for to store connection related 3 Memory lookup:- Memory in switches used for to store connection related information (like outgoing labels) & to buffer the packets.information (like outgoing labels) & to buffer the packets.

4 Multicasting:- It is one to many transmission technique in which instead of 4 Multicasting:- It is one to many transmission technique in which instead of sending a copy of data to each recipient, data is send to a group of sending a copy of data to each recipient, data is send to a group of receivers using multicast address. For multicasting data, intermediate N/W receivers using multicast address. For multicasting data, intermediate N/W element (like switch) interpret multicast addresses and make multiple element (like switch) interpret multicast addresses and make multiple copies of data when require. Multicast is easier in those switch design that copies of data when require. Multicast is easier in those switch design that broadcast packets. broadcast packets.

Switching Architectures Switching Architectures OR OR Switching NetworkSwitching Network

1. 1. SHARED MEMORY Architecture:- SHARED MEMORY Architecture:- In shared memory switch, a common pool of In shared memory switch, a common pool of memory to buffer packets for all inlets and outlets. Shared memory is dual port memory to buffer packets for all inlets and outlets. Shared memory is dual port memory that perform simultaneously permit read & write operation. It occupies memory that perform simultaneously permit read & write operation. It occupies more than space than single port memory.more than space than single port memory.

Memory Controller

InputModule

InputModule

OutputModule

OutputModule

Central

Memory

M

U

X

DEMUX

1

N

1

N

…….. ………

…….. ………

Packets from all inlets are multiplexed into a single stream & written to dual port memory. At the output end , packets are read out sequentially & demultiplexed on the basis of their outgoing port identifier. Memory controller decide the order in which the packets are written and read out.

Aspects of shared memory architecture are:-Aspects of shared memory architecture are:-a)a) Buffering Buffering:- A shared buffering approach is used which has an advantage the significant cost reduction because :- A shared buffering approach is used which has an advantage the significant cost reduction because

lower memory required. Disadvantage is that complicated memory management is required.lower memory required. Disadvantage is that complicated memory management is required.

b) b) MulticastingMulticasting:-It is not suited for broadcast/ multicasting. Because of centralized approach. There exist techniques :-It is not suited for broadcast/ multicasting. Because of centralized approach. There exist techniques to multicasting in shared memory switch.to multicasting in shared memory switch.

1)Copy Networks1)Copy Networks

2) Address Buffering2) Address Buffering

Copy N/W makes multiple copies of multicast packet before packet enters to switch. After copies Copy N/W makes multiple copies of multicast packet before packet enters to switch. After copies made, each packet is treated as unicast packet. made, each packet is treated as unicast packet.

DisadvantageDisadvantage:- In copy N/W unnecessary wastage of buffer at internal capacity of switch.:- In copy N/W unnecessary wastage of buffer at internal capacity of switch.

Address buffering can removed the drawback of copy N/W by making multiple copies of packets Address buffering can removed the drawback of copy N/W by making multiple copies of packets addresses rather than copies of packets. When multicast packet arrive, a pointer entries is copied in each o f addresses rather than copies of packets. When multicast packet arrive, a pointer entries is copied in each o f logical queues of outlet for which multicast packet is destined.logical queues of outlet for which multicast packet is destined.

c) c) ScalabilityScalability:- Shared memory switches do not scale very well.:- Shared memory switches do not scale very well.

c) c) ScalabilityScalability:- Share memory switch do not scale very well.:- Share memory switch do not scale very well.

2. 2. SHARED - MEDIUM ArchitectureSHARED - MEDIUM Architecture:- A broadcast medium is used to switch :- A broadcast medium is used to switch packets to their destined outlets. Broadcasting is done through time division packets to their destined outlets. Broadcasting is done through time division multiplexing (using ring or bus topology to interconnect).multiplexing (using ring or bus topology to interconnect).

InputModule

InputModule

OutputModule

OutputModule

Switching

Fabric

1

N

1

N…….. ………

…….. ……… ……….

Filtering/Buffering

Filtering/Buffering

As packet arrives, , the I/P module determines the outgoing port of the packet, using packet virtual circuit number. The outgoing port number with other information like multicast identifier, packet priority is attached to packet in the form of routing tag. This tag is used for processing of packet. The packet is then broadcast to each outlet using broadcast medium. At output side, address filters decode outgoing port address of each packet. If packet is destined for outlet, the packet is copied to O/P buffer. Otherwise packet is discarded.

Aspects of Shared Medium architecture are:-Aspects of Shared Medium architecture are:-

a)a) Buffering Buffering:- Output buffering approach is used in shared medium :- Output buffering approach is used in shared medium architecture.architecture.

b) b) MulticastingMulticasting:-To broadcast/ multicasting a packet to all outlet, address :-To broadcast/ multicasting a packet to all outlet, address buffer is there for recognizing multicast address.buffer is there for recognizing multicast address.

c) c) ScalabilityScalability:- Scalability of switch is difficult due to speed up factor required :- Scalability of switch is difficult due to speed up factor required for address filters.for address filters.

3. 3. SPACE-DIVISION ARCHITECTURESPACE-DIVISION ARCHITECTURE:-:- To solve the problem of scalability, To solve the problem of scalability, Space division architecture consist two categories:-Space division architecture consist two categories:-

a)a) Crossbar switchingCrossbar switching

b) Banyan – based switchingb) Banyan – based switching

A)A) Crossbar SwitchingCrossbar Switching:- :-

It consist N*N matrix of cross points, each of which can be either enabled It consist N*N matrix of cross points, each of which can be either enabled or disabled. When a cross point enabled, a dedicated path is setup or disabled. When a cross point enabled, a dedicated path is setup between inlet & outlet.between inlet & outlet.

…………

………..

IM

IM

IM

OM

OM

OM

Control

i) Buffering:- Simplest option for buffering in cross points switching is to have a FIFO queue at each inlet (I/P buffering) but it leads to problem of Head of line (HOL) Blocking. The two approaches to solve this problem are:-

a) Centralized Controller

b) Distributed Controller

c) Cross point Buffering

In Centralized controller, find matrix configuration that maximizes number of packets being switched in single time slot. To solve the problem of external blocking, controller should have global knowledge of packets present in I/P queue & destined outlets of each packet. This requirement complicate design of controller.

In Distributed controlled, each O/P maintain its separate arbiter. Arbiter inspect packet coming from all I/P and choose one of them by algorithm.

In cross point buffering, packets are buffered at each cross point. Cross point buffering leads to severe scalability problem. Because N*N switch already has N2 cross points with cross point buffering, total number of buffer required N2B Enormous memory Requirement.

2. Multicasting:- It is difficult in crossbar switch.

B) Banyan Based Switching :- One major drawback of crossbar switching is that

number of cross point require for N*N switch is N2. Banyan based is an alternative that require fewer number of cross point. that N2. These switches derive their name because of tree topology.

Characteristics:-

1) Multistage Network2) Self Routing

Limitations on a Banyan Network:-

1) Internal & External Blocking

Blocking Solutions in Banyan Networks:-

1) Buffering:- Both I/P & O/P buffering is used. To overcome the problem of HOL, internal buffering is used. With internal buffering, one of two conflicting packets in 2*2 switch is allow to pass through, while second one is buffered. In the next time slot, for buffered packet of next stage should move forward.

2) Multicasting:- It is difficult in Banyan based due to connection oriented approach.