network technology cse3020 - 2006 1 network technology cse3020 week 10

Network Technology CSE3020 - 2006

1

Network Technology CSE3020

Week 10


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Asynchronous Transfer Mode (ATM)

• Initially, it was assumed that B-ISDN would continue the same fixed TDM approach of N-ISDN with channel speeds from 64k to 140Mbps. However, TDM is:

• inflexible in the face of the variety of speeds that needed to be switched.• inefficient in handling bursty traffic.• complicated since it needs to deal with switching of multiple rates.

ATM was proposed.


3

Asynchronous Transfer Mode (ATM)

A

C

B

t2 t1

Fixed Time sharing

Asynchronous Transfer Modetime

address

C2 B2 A2 C1 B1 A1

A2 C1 A1


4

ATM - Asynchronous Transfer Mode

• Developed as part of work on Broadband ISDN, but has found application in non-ISDN environments where high data rates are required

• Is a streamlined - minimal error and flow control capabilities - packet transfer interface using fixed size packets called cells of 53 octets

• Takes advantage of the reliability and fidelity of modern digital facilities to provide faster packet switching than X.25

• Provides both real-time and non-real-time services• Can support information transfer protocols not

based on ATM• Allows multiple logical connections over a single

physical interface


5

ATM Protocol Architecture

• Refer fig 11.1 Stallings 6E, fig 19.4, 19.20, 19.21 Forouzan 2E Illustrates the basic architecture for an interface between user and network

• The following layers relate to ATM functions:– ATM Layer, common to all services and provides packet

transfer capabilities:• fixed size cells• logical connections

– ATM Adaptation Layer (AAL), that is service dependent:• supports information transfer protocols not based on ATM, by

mapping the information into ATM cells


6

Why ATM?

• The shared media in LANs shows that the switching approach is the more efficient way to build a network.

• The use of Asynchronous (statistical) TDM to achieve a better handling of emerging multimedia traffic.

• A network that can cover a greater variety of services – voice, video, data (one for all).

• A network that is scalable (covers from LANs to WANs).


7

ATM Characteristics• Connection-oriented operation.• Minimal error and flow control capabilities.• Limited header functions.• Similarities between ATM and packet switching:

– Transfer of data in discrete chunks.– Multiple logical connections over single physical

interface.• Data rates (physical layer) 25.6Mbps to 622.08Mbps.• Small “packet” size for shorter delay and delay jitter.

• Because each packet is fixed and small, the term “cell” is used to describe a “packet” (header + payload) in ATM.


8

ATM Logical Connections

• Refer figs 11.2, 11.3 Stallings 6E, fig 19.6 Forouzan 2E

• A Virtual Channel Connection (VCC) is the basic unit of switching in an ATM network

• A VCC is set up between two end users through the network and supports the full duplex flow of data


9

ATM Logical Connections - Virtual Path Connection

• A VPC is a bundle of VCCs that have the same endpoints

• All the cells flowing in VCCs in a VPC are switched together

• Helps contain control cost, network management actions can be applied to a small number of groups of connections instead of a large number of individual connections


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virtual pathtransmission pathv

irtu

al ch

annels

SW1

SW2

SW3

UNI NNIUNIUser Network Interface Network-Network Interface


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Advantages of Virtual Path Connections (VPCs)

• Simplified network architecture:– Network transport functions can be separated into those

related to :• an individual logical connection (VCC) and • a group of logical connections (VPC)

• Network deals with fewer, aggregated connections• Performance - reduced processing and short VCC

connection setup time:– Much of work is done when VPC is set up– By reserving capacity on a VPC in anticipation of later call

arrivals, new VCCs can be established by executing simple control functions at the endpoints

– No call processing is required at transit nodes


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Call Establishment Using Virtual Paths

• Refer fig 11.3 Stallings 6E

• The process of setting up a VPC is decoupled from the process of setting up an individual VCC:– VP control mechanisms include calculating routes,

allocating capacity and storing connection state information

– To set up a VC, there must first be a VP to the required destination node with sufficient capacity to support the VC with appropriate Quality of Service

– A VC is set up by storing the required state information - VC/VP mapping


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Call Establishment Using VPs

Make connection

Make connection

Request for VCC Originates

Request for VCC Originates

VPC exists

?

VPC exists

?

Establish a new VPC

Establish a new VPC

Block VCC or request

more capacity

Block VCC or request

more capacity

Reject VCC request

Reject VCC request

Can QoS be

satisfied?

Can QoS be

satisfied?

Request granted

?

Request granted

?

yes

no nono

yes yes


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Virtual Channel Connection Uses

• Between end users:

– End to end user data.

– Control signals.

– VPC provides overall capacity.

• VCC organization done by users.

• Between an end user and a network entity:

– Control signaling.

• Between two network entities:

– Network traffic management.

– Routing.


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Virtual Path/Channel Characteristics

• Quality of service.– Specified by cell loss ratio and cell delay variations.

• Switched and semi-permanent virtual channel connections.– Switched VCC is an on-demand connection.

– Semi-permanent VCC is of long duration and is set up by configuration.

• Call sequence integrity.

• Traffic parameter negotiation and usage monitoring.– Traffic parameters are average rate, peak rate, burstiness & peak duration.

– Network deals with congestion and manages existing and requested VCCs.

• Virtual channel Id restriction within a VPC:– Some VC Ids may not be available to the user of a VPC and be reserved for

network use eg VCCs for network management


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Control Signaling: VCC• Establishment and release of VPCs and VCCs.

• Takes place on separate connection.

• Four methods:– Semi-permanent VCC:

• User-to-user exchange and no control signaling.– Meta-signaling channel:

• Used as permanent control signal channel.– User-to-network signaling virtual channel:

• Used to set up VCCs to carry user data.– User-to-user signaling virtual channel:

• Within pre-established VPC• Used by two end users without network intervention to

establish and release user-to-user VCC to carry user data.


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Control Signaling: VPC• Semi-permanent:

– By prior agreement.

– No control signaling.

• Customer controlled:

– Customer uses a signaling VCC to request the VPC from the network.

• Network controlled:

– Network establishes a VPC for its own convenience.

– Path may be network-to-network or user-to-network or user-to-user.


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ATM Protocol Reference Model

Higher Layer

ATM Adaptation Layer (AAL)

ATM Layer

Physical Layer

Management PlaneControl Plane User Plane

Layer

Managem

ent

Pla

ne M

anagem

ent


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ATM Reference Model: Layers

• Physical Layer:

– Specification of transmission medium and signal encoding.

• ATM Layer:

– Provide packet transfer capabilities.

– Defines the transmission of data in fixed-size cells.

– Defines the use of logical connections.

• Adaptation Layer:

– Maps higher-layer information into ATM cells.

– Collects information from ATM cells for delivery to higher layers.


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ATM Reference Model: Planes

• User plane:

– Provides for user information transfer.

• Control plane:

– Call and connection control functions.

• Management plane:

– Plane management:

• Whole system functions and coordination between planes.

– Layer management:

• Relating to resources and parameters in protocol entities.


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ATM Plane-LayerLayer Name Functions Performed

Higher Layers Higher Layer Functions

Common Part (CP)ConvergenceSublayer (CS)

Service Specific (SS)

AA

L La

yer

SAR Sublayer Segmentation and Reassembly

ATM LayerGeneric Flow ControlCell Header Generation/ExtractionCell VCI/VPI TranslationCell Multiplexing/Demultiplexing

TransmissionConvergence(TC) Layer

Cell Rate DecouplingCell DelineationTransmission Frame AdaptationTransmission Frame generation/Recovery

Ph

ysic

al L

ayer

PhysicalMedium (PM)

Bit TimingPhysical Medium

Laye

r M

ana

gem

ent


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ATM Cells

Generic flow control Virtual path id

Virtual path id

Virtual channel id

Payload type CLP

Header error control

Information field (48 octets)

UNI format NNI format

Virtual path id

Virtual channel id

Payload type CLP

Header error control

Information field (48 octets)


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ATM Cells• Fixed size (53 octet).

• 5 octet header and 48 octet information field.

• Small cells reduce queuing delay for high priority cells.

• Small cells can be switched more efficiently.

• Easier to implement switching mechanism in hardware.


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ATM Cells: Header Format• Generic flow control (GFC):

– Only at user to network interface.– Controls flow only at the local user-network interface.

• Virtual path identifier (VPI):– Constitutes a routing field for the network.– 8 bits at the UNI and 12 bits at the NNI.

• Virtual channel identifier (VCI):– Used for routing to and from the end user.

• Payload type (PT):– e.g. user info or network management

• Cell loss priority (CLP):– Provide guidance to the network in the event of congestion.

• Header error control:– Error control and synchronization.


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Generic Flow Control (GFC)• Control traffic flow at user to network interface (UNI) to

alleviate short term overload.

• Two sets of procedures:– Uncontrolled transmission.– Controlled transmission.

• Every connection is either subject to flow control or not.

• Subject to flow control:– May be one group (A) default– May be two groups (A and B)

• Flow control is from subscriber to network.– Controlled by network side.


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Header Error Control• figs 11.5, 11.6 Stallings 6E

• ATM cell includes an 8 bit HEC that is calculated based on the remaining 32 bits of the header

• The generator polynomial is x8 + x2 + x + 1

• Single bit errors can be corrected


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Header Error Control


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HEC Operation at Receiver

• refer fig 11.5, 11.6, 11.7 Stallings 6E– Receiver on initialization --> correction mode

– In Correction Mode• when a cell is received --> HEC calculation --> comparison

is performed• If no errors are detected --> no action --> correction mode• If an error is detected:

– single bit --> correct --> detection mode– multi bit --> cell discarded --> detection mode

– In detection mode• If Error detected --> cell discarded --> detection mode• No attempt is made to correct errors• If no errors are detected --> no action --> correction mode


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Transmission of ATM Cells• ATM cells may be transmitted at:

- 622.08Mbps - 155.52Mbps - 51.84Mbps - 25.6Mbps

• Two transmission structure:– Cell Based physical layer:

• No framing imposed.• Continuous stream of 53 octet cells.• Synchronization based on header error control field.

– SDH based physical layer:• Imposes structure on ATM cell stream.• Can carry ATM or STM payloads.• Specific connections can be circuit switched using SDH

channel.• SDH multiplexing techniques can combine several ATM

streams.


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ATM QoS – User Related Attributes

• Sustainable Cell Rate (SCR): the average cell rate calculated over the duration of the connection.

• Peak Cell Rate (PCR): defines the sender’s maximum cell rate on the connection

• Minimum Cell Rate (MCR): defines the minimum cell rate acceptable to the sender. eg 50,000 cells per sec

• Cell Variation Delay Tolerance (CVDT): is a measure of the variation in cell transmission time. eg if 5ns, this means the difference between minimum and maximum delays should no exceed 5ns.


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ATM QoS – Network Related Attributes

• Cell Transfer Delay (CTD): the average time needed for a cell to travel from source to destination. max CTD and min CTD.

• Cell Delay Variation (CDV): Difference between max CTD and min CTD .Voice traffic is sensitive to this.

• Cell Loss Ratio (CLR): defines the fraction of cells lost during transmission. Data traffic is sensitive to this.

• Cell Error Ratio (CER): defines the fraction of cells delivered in error


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ATM Service Categories

• Real time service:– Applications concern with the amount of delay and the variability

of delay (jitter).– Constant bit rate (CBR).– Real time variable bit rate (rt-VBR).

• Non-real time service:– Bursty traffic and do not have tight constraints on delay and delay

variations. – Non-real time variable bit rate (nrt-VBR).– Available bit rate (ABR).– Unspecified bit rate (UBR).

Available bandwidt

h CBRVBR

ABR or UBR


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ATM Services: Real time• Constant bit rate (CBR):

– Fixed data rate continuously available, with tight upper bound on delay.

– Used for uncompressed audio and video (video conferencing, interactive audio, audio/video distribution and retrieval).

• Real time variable bit rate (rt-VBR):– Time sensitive applications, requiring tightly constrained

delay and delay variation.– rt-VBR applications transmit at a rate that varies with time

(e.g. compressed video) – Produces varying sized image frames, so compressed data

rate varies.– Can statistically multiplex connections.


34

ATM Services: Non-Real time

• Non-real time variable bit rate (nrt-VBR):– Improved QoS in the areas of loss and delay.– End system specifies peak cell rate, average rate and burstiness. – Network allocates resources to provide low delay and cell loss.– Used for data transfer with critical response time requirements.

• e.g. Airline reservations, banking transactions & process monitoring.

• Available bit rate (ABR).– Application specifies peak cell rate (PCR) and minimum cell rate

(MCR).– Resources allocated to give at least MCR.– Spare capacity shared among all ABR sources.

• e.g. LAN interconnection.


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ATM Services: Non-Real time

• Unspecified bit rate (UBR).– Additional capacity is available due to:

• Not all resources dedicated to CBR and VBR traffic.

• Bursty nature of VBR.

– These unused capacity is used for UBR services.

– For application that can tolerate some cell loss or variable delays.

• e.g. TCP based traffic.

– Cells forwarded on FIFO basis.

– Both delay and variable losses are possible.

– Best effort service.


36

ATM Adaptation Layer (AAL)

• Support for information transfer protocol not based on ATM.– PCM voice:

• Assemble bits into cells.

• Re-assemble into constant flow.

– IP:

• Map IP packets onto ATM cells.

• Fragment IP packets into a number of cells.

• Use of IP over ATM allows to retain all IP infrastructure for an ATM network.


37

AAL Services• Handling of transmission errors.

• Segmentation and re-assembly.

• Handling of lost and misinserted cell conditions.

• Flow control and timing control.AAL Supported Application

types• Circuit emulation (e.g. T-1 over an ATM).

• VBR voice and video.

• General data service.

• IP over ATM.

• Multiprotocol encapsulation over ATM (MPOA) (e.g. IPX, AppleTalk, DECNET).

• LAN emulation.


38

AAL Protocols• Organized in two logical sublayers:

– Convergence sublayer (CS):• Support for specific applications.• AAL user attaches to AAL at Service Access Point (SAP).

– Segmentation and re-assembly sublayer (SAR):• Packaging information received from CS into cells.• Unpacking the information at the other end.

• Four protocol types:– Type 1– Type 2– Type 3/4– Type 5


39

AAL Protocols


40

AAL Protocols• AAL Type 1:

– CBR source.– SAR packs and unpacks bits.– Block accompanied by sequence number.

• AAL Type 2:– VBR information.– Analog applications (video and audio).

• AAL Type 3/4:– Connectionless or connection oriented.– Message mode or stream mode.

• AAL Type 5:– Streamlined transport for connection oriented higher layer

protocols.


41

AAL ProtocolsAAL: Segmentation And Reassembly Protocol Data Unit

48 octets

22 octets 44 octets

1 octet 47 octets

SN SNP SAR-PDU payload

ST SN MID SAR-PDU payload LI CRC

SAR-PDU payload

AAL 1

AAL 3/4

AAL 5

SN = Sequence number (4 bits)SNP = Sequence number protection (4 bits)MID = Multiplexing identifier (10 bits)LI = Length indication (6 bits)ST = Segment TypeCRC = Cyclic Redundancy Check (10 bits)


42

AAL Protocols and Services

CBR rt-VBR nrt-VBR ABR UBR

AAL 1 Circuit Emulation, ISDN, Voice over ATM

AAL 2 VBR voice and video

AAL 3/4 General data services

AAL 5 LAN Emulation

Voice on demand, LAN emulation

Frame relay, LAN emulation

LAN emulation

IP over ATM


43

Required Reading

• W. Stallings, Data and Computer Communications Prentice-Hall.

>> Chapter 11 6E, 7E


44

Following slides are for interest only


45

ATM LANs

• An ATM network can be used as device to connect several local computers allowing them to exchange information. It acts as a LAN in this case.

• An ATM network can also be used as device to connect several LANs. It acts as a bridge or a backbone network.

• PROBLEM: It is expected that ATM should provide connectionless MAC service like IEEE 802 MAC sublayer. However, ATM switches are connection-oriented.


46

LAN Emulation (LANE)

• The ATM MAC sublayer emulates the LAN service by creating the appearance of a virtual shared medium from a point-to-point network.

• Creating new servers to simulate a broadcast services.

• The concept is: A LAN Emulation Client (LEC) performs a transmission to a server, the server then send the received information to all LECs. A LEC is an ATM end system.


47

Frame Relay• Designed to be more efficient than X.25.

• X.25:

– In band signaling used.

– Multiplexing of virtual circuits at layer 3.

– Layer 2 and 3 include flow and error control.

– Considerable overhead.

– Not appropriate for modern digital systems with high reliability.

• Developed before ATM and larger installed base than ATM.

• ATM now of more interest on high speed networks.


48

Frame Relay• Call control carried in separate logical connection.

• Multiplexing and switching at layer 2.

– Eliminates one layer of processing.

• No hop-by-hop error or flow control.

– End-to-end flow and error control (if used) are done by higher layer.

– Single user data frame sent from source to destination and ACK (from higher layer) sent back.


49

Frame Relay: Advantages & Disadvantages

• Lost link-by-link error and flow control.– Increased reliability makes this less of a problem.

• Streamlined communications process.– Lower delay.– Higher throughput.

• Access speed up to 2Mbps.


50

Frame Relay: Protocol Architecture


51

Frame Relay: Control Plane• Between subscriber and network.

• Separate logical channel used.– Similar to common channel signaling for circuit switching

services.

• At data link layer, LAPD (Link Access Procedure for Data services) protocol is used for:– Provide reliable data link control.– Error and flow control between user and network.


52

Frame Relay: User Plane• Actual transfer of information between end users.

• LAPF (Link Access Procedure for Frame Mode Bearer Services) protocol is used for:

– Frame delimiting, alignment and transparency.

– Frame multiplexing/demultiplexing using addressing field.

– Ensure frame is integral number of octets.

– Ensure frame is neither too long nor short.

– Detection of transmission errors.

– Congestion control functions.


53

Frame Relay: Frame Formats


54

Frame Relay: User Data Transfer

• One frame type used for carrying user data.

– No control frame.

• No inband signaling.

• No sequence numbers.

– No flow control and no error control.


55

ATM Logical Connections• Referred to as Virtual channel connections (VCC).

• Analogous to virtual circuit in X.25.

• Basic unit of switching.

• VCC is set up between two end users through the network.

• Variable-rate, full-duplex flow of fixed size cells.

• VCCs are used for data, user-network exchange (control) and network-network exchange (network management and routing).

• Virtual path connection (VPC).

– Bundle of VCC with same end points.


56


Virtual Channel ConnectionVirtual Path Connection

Transmission Path

ATM cells

Location A

Location C

Location B


57

Advantages of Virtual Paths

• Simplified network architecture.– Individual logical connection (virtual channel) and a group of logical

connections (virtual paths).

• Increased network performance and reliability.– Network deals with fewer aggregated entities.

• Reduced processing and short connection setup time.– Much of the work with virtual path set up.– Reserving capacity on a virtual path for new virtual channels.– Minimal processing for a new virtual channel to an existing virtual path.

• Enhanced network services.– User may define closed user groups or closed network of virtual channel

bundles.

network technology cse3020 - 2006 1 network technology cse3020 week 10

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