university of arizona ece 478/578 309 packet relay u relaying: switching packets asynchronously u...

University of Arizona ECE 478/578

1

Packet RelayPacket Relay

Relaying: Switching packets asynchronously Types of packet relay:

1. Cell relay: Fixed-size packets Used in ATM and SMDS

2. Frame relay: Variable-length packets


2

Advantages of Fixed-Size PacketsAdvantages of Fixed-Size Packets

Simple switch-hardware design Hardware store-and-forward is easier Dynamic storage allocation is easier (no memory

fragmentation)

More deterministic scheduling (for performance guarantees)

High degree of parallelism in large switches Synchronized multiprocessors Multiple levels of buffering can be easily clocked


3

Disadvantages of Fixed-Size PacketsDisadvantages of Fixed-Size Packets

Segmentation and reassembly (SAR) Overhead in the case of small-size cells


4

Integrated Services Digital Network (ISDN)Integrated Services Digital Network (ISDN)

Evolutionary technology from digital telephony Intended as a digital interface for voice and data More popular in Europe ISDN terminology:

Functional grouping: A set of capabilities in an ISDN user interface (similar to layer functions)

Reference points: Logical interfaces between functional groupings (similar to SAPs)


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ISDN SpecificationISDN Specification

Types of functional groupings: Terminal Type 1 and 2 (TE1 and TE2) Network Termination 1 and 2 (NT1 and NT2)

Four reference points: R, S, T, and U


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Typical ISDN TopologyTypical ISDN Topology

TE 1 NT 2 NT 1 LT/ET

S T U

Network

TA NT 2 NT 1 LT/ET

S T U

TE 2

R


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Typical ISDN Topology (Cont.)Typical ISDN Topology (Cont.)

TE1 = end-user ISDN terminal TE2 = non-ISDN terminal TA = terminal adaptor NT1 = device that connects 4-wire subscriber

wiring to 2-wire local loop. Responsible for physical layer functions

NT2 = more complex than NT1. Contains layer 2 and 3 functions. Performs concentration


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ISDN Access RatesISDN Access Rates Basic Rate Interface (BRI)

Two 64-kbps B channels for data One 16-kbps D channel for control (out of band) Designated as 2B+D Up to eight TE1s can be multiplexed onto a BRI

Primary Rate Interface (PRI) 23 64-kbps B channels for data (total of 1.544 Mbps) One 64-kbps D channel for control Designated as 23B+D In Europe the PRI consists of 31B+D (E1 line)


9

Broadband ISDN (B-ISDN)Broadband ISDN (B-ISDN)

Set of protocols that is standardized by ITU-T Started as an extension of ISDN. However, ISDN

and ISDN interfaces are NOT compatible ATM is the transport protocol for B-ISDN


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Driving Forces Behind B-ISDNDriving Forces Behind B-ISDN

Emergence of bandwidth-intensive applications Desire to integrate data, voice, and video over a

single channel (why?) Need to provide performance guarantees for real-

time applications


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Synchronous Transfer Mode (STM)Synchronous Transfer Mode (STM)

Based on time-division multiplexing (TDM) Each connection is reserved a time slot Bandwidth is wasted if user is idle

Time DivisionMultiplexer

Stream #1

Stream #1

Stream #1

frame frame frameframe

wasted bandwidth


12

Asynchronous Transfer Mode (ATM)Asynchronous Transfer Mode (ATM)

Based on statistical (i.e., asynchronous) multiplexing Bandwidth is allocated on demand Each packet (cell) carries its connection ID

StatisticalMultiplexer

1

2

3

Stream #1

Stream #1

Stream #1

1 1 1

2

3

1 2 3 1 1 2 1 3


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So What is ATM?So What is ATM?

Transport technology for B-ISDN Based on fixed-length packets (cells) A cell consists of 53 bytes:

User payload: 48 bytes Cell header: 5 bytes

Hardwired store-and-forward architecture Connection-oriented fast packet switching!


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What Does ATM Provide?What Does ATM Provide?

Efficient bandwidth utilization (via statistical multiplexing)

Quality of service (QoS) Maximum cell transfer delay Cell delay variation (jitter) Cell loss rate

Cell sequencing (important for real-time apps) Unified transport solution for diverse traffic types Scalability


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Cell Size ConsiderationsCell Size Considerations

Transmission efficiency

PL no. of payload bytes HD no. of header bytes

Impact of cell loss on voice quality Loss of 32-byte cell 4 ms interruption > 32 ms interruption is quite disruptive

Echo cancellation

/( )E PL PL HD


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Physical Layer

ATM Layer

ATM Adaptation Layer

Higher Layers

Control Plane User Plane

Layer Management

Plane Management

B-ISDN Protocol StackB-ISDN Protocol Stack

Three “planes”: User plane Control plane Management plane

Plane management Layer management


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B-ISDN Physical LayerB-ISDN Physical Layer

Consists of two sublayers: Transmission Convergence (TC) sublayer Physical Medium (PM) sublayer

Functions of the TC sublayer: Generation/recovery of transmission frames Transmission frame adaptation Cell delineation Cell header processing (HEC generation) Cell rate decoupling


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B-ISDN Physical Layer (Cont.)B-ISDN Physical Layer (Cont.)

Functions of the PM sublayer: Bit timing Line encoding Other medium-dependent functions


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Common Physical Layer InterfacesCommon Physical Layer Interfaces

Multimode Fiber: 155 Mbps SONET STS-3c (SDH) 100 Mbps 4B/5B coding

Single-Mode Fiber at 100 Mbps 4B/5B coding Coax cable at 45 Mbps DS3 rate Subrates (for ATM over unshielded twisted pair)

51.84 Mbps 25.92 Mbps 12.96 Mbps


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SONET HierarchySONET Hierarchy

Rate (Mbps) Optical Level Electrical Specs SDH

51.84 OC-1 STS-1 ---

155.52 OC-3 STS-3 STM-1

622.08 OC-12 STS-12 STM-4

1244.16 OC-24 STS-24 STM-8

2488.32 OC-48 STS-48 STM-16


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SONET STC-3c Physical LayerSONET STC-3c Physical Layer

TransmissionConvergence Sublayer

PhysicalMediaDependentSublayer

- HEC generation/verification- Cell scrambling/descrambling- Cell delineation- Path signal identification- Frequency justification/Pointer processing- Multiplexing- Scrambling/descrambling- Transmission frame generation/recovery

- Bit timing, Line coding- Physical medium

B-ISDNSpecificFunctions

B-ISDNIndependentFunctions


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ATM Layer FunctionsATM Layer Functions

Cell multiplexing and demultiplexing

VPI/VCI translation Traffic management (e.g., shaping, policing) Cell header processing (except for the HEC field) Cell rate decoupling (for SONET and DS3) OAM functions

54 5 5 4 4 3 5 5 5 3

4 4 1 1 2 1 41 2 3 1 1 2 1 3

1 2

Switch


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ATM Cell FormatATM Cell Format

8 7 6 5 4 3 2 1

1

2

3

4

5

6..53

OC

TE

T

GFC VPI

VPI VCI

VCI

VCI PT CLP

HEC

Cell Payload(48 octets)

BIT


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RemarksRemarks

The previous cell format is for the User-to-Network Interface (UNI)

Between an end-system and an ATM switch An end system could be, an IP router with an ATM

interface, a PC/workstation, or a LAN switch

In the Network-to-Network Interface (NNI), the GFC field is used as part of the VPI field

NNI is typically between two ATM switches Two flavors of NNI are used (Private and Public)


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ATM SwitchingATM Switching

UNI = User Network Interface

PNNI = Private Network Node Interface

AAL

ATM

Physical

AAL

ATM

Physical

ATMNetwork

UNI UNIPNNI PNNI

User A User B


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Generic Flow Control (GFC)Generic Flow Control (GFC)

Four bits in the cell header Only in cells at UNI (intermediate switches

overwrite it) Intended for link-by-link flow control Typically, GFC is not used

GFC VPI

VPI VCI

VCI

VCI PT CLP

HEC



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Connection IdentifiersConnection Identifiers

ATM uses a 2-level connection hierarchy: Virtual channel connection (VCC or VC) Virtual path connection (VPC or VP)

A VP is a bundle of VCs Each connection has a VP identifier (VPI) and a

VC identifier (VCI) Cell switching is performed based on:

VPI alone (VP switching), or Both VCI and VPI (VP/VC switching)

GFC VPI

VPI VCI

VCI

VCI PT CLP

HEC



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Connection Identifiers (Cont.)Connection Identifiers (Cont.)

Some VPI and VCI values are reserved for signaling and control functions:

Connection requests: VPI=0, VCI=5 PNNI topology state packets: VPI=0, VCI=18 Resource Management (RM) cells: VCI=6 VCI values < 32 are reserved for control functions

VCIs and VPIs have local scope


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VP and VP/VC SwitchingVP and VP/VC Switching

VCI 1VCI 2

VPI 1

VPI 3

VCI 1VCI 2

VP Switching

VCI 1VCI 2

VPI 1

VPI 3

VCI 5

VCI 3

VP/VC Switching

VPI 4


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Payload Type Meaning

Payload Type (PT) FieldPayload Type (PT) Field

000 user cell, no congestion, cell type 0001 user cell, no congestion, cell type 1010 user cell, congestion indication, cell type 0011 user cell, congestion indication, cell type 1100 OAM cell (link-by-link)101 OAM cell (end-to-end)110 RM cell (used in ABR service)111 reserved for future use

GFC VPI

VPI VCI

VCI

VCI PT CLP

HEC



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Cell Loss Priority (CLP)Cell Loss Priority (CLP)

CLP = 1 for low priority CLP = 0 for high priority CLP is used in selective cell discarding to:

penalize greedy users (traffic policing) request differential QoS (e.g., coded video)

CLP is a key parameter in traffic management

GFC VPI

VPI VCI

VCI

VCI PT CLP

HEC



32

Header Error Control (HEC)Header Error Control (HEC)

Checksum over cell header Performed by the physical layer Corrects all single-bit errors Detects about 84% of multiple-bit errors

Cells with multiple errors are discarded

GFC VPI

VPI VCI

VCI

VCI PT CLP

HEC



33

ATM Layer in the OSI ModelATM Layer in the OSI Model

Different opinions: Network layer (since it performs routing) Data-link layer (in IP over ATM and in MPOA) Physical layer (in LAN emulation)

Conclusion: There is no 1-to-1 correspondence between

B-ISDN and OSI layered models


34

ATM Adaptation Layer (AAL)ATM Adaptation Layer (AAL)

Purpose: Adapt upper “applications” to ATM layer Different applications have different needs

Four AALs are used AALs were originally classified according to:

Real-time versus non-real-time Connection oriented versus connectionless Constant bit rate (CBR) versus variable bit rate (VBR)


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AAL FunctionsAAL Functions

Segmentation and reassembly of upper-layer PDUs Delay variation recovery Cell losses recovery Circuit emulation (e.g., voice over ATM) Connectionless service over ATM Clock synchronization And others ...


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AAL StructureAAL Structure

Convergence Sublayer (service specific part)

Convergence Sublayer (common part)

Segmentation & Reassembly Sublayer

Note: In some AALs, the convergence sublayer consists of onepart only


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Types of AALTypes of AAL

1. AAL1 Intended for TDM-like circuit emulation Supports clock synchronization and timing recovery Provides sequence numbers

2. AAL2 Optimized for the transport of VBR video traffic Provides timing information and sequence numbers Not quite popular


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Types of AAL (Cont.)Types of AAL (Cont.)

3. AAL3/4 Provides both connectionless and connection-oriented services

over ATM Supports the multiplexing of messages from multiple users

over the same VC Not popular either

4. AAL5 Intended for data applications (e.g., TCP over AAL5) Provides minimal functionality Most popular AAL


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Barriers to the Deployment of ATMBarriers to the Deployment of ATM

Lack of “killer applications” Cost of new infrastructure Other competitive technologies for LANs Uncertainty about the new technology Incomplete standards

ATM is mainly being deployed in the Internet backbone and

within specialized networks

university of arizona ece 478/578 309 packet relay u relaying: switching packets asynchronously u...

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