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ATM-NETWORK MANAGEMENTARCHITECTURES

TRAN CONG HUNG , M.ScDeputy Head of Faculty of Information Technology II

Post & Telecommunication Institute of Technology of Viet NamEmail : [email protected]

Abstract The most important of Network Management

Architectures in ATM can brief definition of each element ofOperations, Administration, Maintenance, and Provisioning

(OAM&P)

OAM&P PHILOSOPHYA brief definition of each element of Operations,Administration, Maintenance, and Provisioning (OAM&P)

and how they interrelate is described below and depicted inthe flow diagram of Figure 1

Figure 1 OAM&P Process Flow

Operations involves the day to day, and often minute to minute, care and feeding of the data network in

order to ensure that it is fulfilling its designed purpose.

Administration involves the set of activities involved

with designing the network, processing orders, assigning

addresses, tracking usage, and accounting.

Maintenance involves the inevitable circumstances that

arise when everything does not work as planned, or it is

necessary to diagnose what went wrong and repair it.

Provisioning involves installing equipment, settingparameters, verifying that the service is operational, and

also deinstallation.

SNMP Based Network Management Systems(NMS)

The Simple Network Management Protocol (SNMP) defined

in IETF standards has five messages types : GET REQUEST

(or simple GET), GET NEXT REQUEST (or simply GETNEXT), SET REQUEST (or simply SET), RESPONSE, and

TRAP (which is like an alarm). The SET, GET, and GETNEXT messages are all replied to by the RESPONSE

message. The TRAP message is very important since it is the

notification of an unexpected event, such as a failure or asystem restart. SNMP normally operates over the User

Datagram Protocol (UDP), which then usually operates over

IP in the Internet Protocol (IP) stack, but may operate over

some other protocol.SNMP utilizes a subset of Abstract Syntax Notation 1

(ASN.1) to define a Management Information Base (MIB) asa data structure that can be referenced in SNMP messages.

The MIB defines objects in terms of primitives such as

strings, integers, and bit maps, and allows a simple form of

indexing. Each object has a name, a syntax, and an encoding.

The ATM Forum ILMI design provides essential interfacemanagement functions to early users of ATM until these

functions are standardized in ATM OAM cells. Figure 2

illustrates the configuration in which the ILMI operates. Each

ATM End System (ES), and every network that implements aPrivate Network UNI or Public Network UNI, has a UNI

Management Entity (UME) which is responsible formaintaining the information and responding to SNMP

commands received over the ATM UNI. The information in

the ILMI MIB can be actually contained on a separate private

or public Network Management System (NMS) or may beaccessed over another physical interface. NMSs may also be

connected to networks or end systems by other network

management interfaces. The Customer Network Management

User OAM&P Functions Network

Administration

Provisioning

Operations

Maintenance

Order

Charge

Schedule Report

Install

Verify

QueryTurn Up

Inquire

Respond

Monitor

Control

Dispatch Coordinate

Diagnose

Repair


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(CNM) capability is important for carrier based services.This should include at least physical port status, VPC/VCC

status, order parameters, and selected performance metrics.

Delivery of detailed performance counts will involve

additional complexity and cost.

OAM FLOW REFERENCE ARCHITECTURE

Currently OAM flows are only defined for point to point connections. A fundamental part of theinfrastructure for network management is that of

Operations, Administration, and Maintenance (OAM)information. Figure 3 shows the reference architecturethat describes how ATM OAM flows relate toSONET/SDH management flows . The F1 flows are forthe regenerator section level (called the Section level inSONET), F2 flows are for the digital section level(called the Line level in SONET), and F3 flows are forthe transmission path (call the Path level in SONET).ATM adds F4 flows for Virtual Paths (VPs) and F5flows for Virtual Channels. (VCs), where multiple VCsare completely contained within a

single VP. Each flow can be either connected ofterminated at an endpoint. Each of the F4/F5 flows maybe either end to end or segment oriented. End to end flows are received only by the device thatterminates the ATM connection. Only network nodesreceive segment OAM flows. Indeed, network nodes

must remove segment flows before they ever reachdevices that terminate an ATM (VP or VC) connection.OAM flows maybe either segment or end to end. Anend to end flow is from one endpoint at the samelevel to the other endpoint. A segment flow is from oneconnection point to another connection point.

PrivateNMS

Agent

PublicNMS

Agent

Other Network Management

Interfaces

ATM End

System

Agent

UME

Private ATM

Network

Agent

UME UME

Private

ATM

UNI

ILMI

(SNMP/AAL)

Public ATM

Network

Agent

UME UME

ATM End

System

Agent

UME

PublicATM

UNI

ILMI

(SNMP/AAL)

PublicATM

UNI

ILMI

(SNMP/AAL)

Figure 2 ATM Forum ILMI Configuration


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OAM CELL FORMATS

Figure 4 shows the ATM OAM cell format : There are VirtualPath (VP) flows (F4) and Virtual Channel (VC) flows (F5)

between connection endpoints that are defined as end to end OAM flows. There are also F4 and F5 OAM flows that

occur across one or more interconnected VC or VP links that

are called segment OAM Flows. VP flows (F4) utilize

different VCI to identify whether the flow in either end toend (VCI = 3) or segment (VCI = 4). Recall that the first 16

VCIs are reserved for future tandardization. For a VC flow(F5), a specific VCI cannot be used because all VCIs are

available to users in the service. Therefore, the Payload Type

(PT) differentiates between the end to end (PT=100) and

Segment (PT=101) flows in a VCC.

Table 1 summarizes the OAM type and function type fields in

the OAM cells from Figure 4. The three OAM types are faultmanagement, performance management, and activation /deactivation. Each OAM type has further function types with

codepoints as identified in Table 1. For the fault management

OAM type there are Alarm Indication Signal (AIS), Remote

Defect Indication (RDI) (also called Far End ReportingFailure (FERF), and continuity check function types. For the

performance management OAM type there are forwardmonitoring and backward reporting types, or a third type that

is a combination of these two, called monitoring and

reporting. The third OAM type defines activation and

deactivation of the other OAM types. Currently, there are

activation and deactivation function types for performance

management and the continuity check.

Virtual Channel Connection (VCC)

VC Link VC Link

VP Link VP Link

Transmission Path

Digital Section (Line)

Regenerator Section

ATMLayer

Physical

Layer

F5

F4

F3

F2

F1

Virtual Path Connection VPC

Figure 3 : ATM Management Plane Reference Architecture

G C

F VPI VCI PT L HEC OAM Function Function specific Reserved CRC-10

C P Type Type Fields

5 bytes 4 bits 4 bits 45 bytes 6 bits 10 bits

Same as

User Cell

VCI=3 (Segment)

VCI=4 (End to end)

F4 (VPC) OAM Cell Format

G C

F VPI VCI PT L HEC OAM Function Function speci fic Reserved CRC-10

C P Type Type Fields

5 bytes 4 bits 4 bits 45 bytes 6 bits 10 bits

Same as

User Cell

PT=100 (Segment)

PT=101 (End to end)

F5 (VCC) OAM Cell Format

Figure 4 : ATM OAM Cell Types and Format


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OAM Type Function Type0001 AIS 0000

0001 RDI/FERF 0001

0001 Continuity Check 0100

Fault Management

0001 Loopback 1000Performance management 0010 Forward Monitoring 0000

0010 Backward Reporting 0001

0010 Monitoring& Reporting 0010

1000 Performance Monitoring 0000Activation/Deactivation

1000 Continuity Check 0001

Table 1: OAM Types & OAM Function Types

Note that there are a significant number of unassignedcodepoints in the OAM and function types. The definition of

OAM cellformats, functions, and protocols is ongoing andevolving in standards and specification development . For this

reason , the ATM Forum UNI specification recommends that

these OAM functions be implemented in software.

FAULT MANAGEMENT

+ AIS and RDI/FERF Theory and OperationFigure 5 illustrates the ATM OAM cell AIS and RDI/FERF function

specific fields. The meaning of each field is described below.

Failure Type is an indication of what type offailure has occurred. Currently no specificvalues are standardized.

Failure Location is an indication of where thefailure occurred. Currently no specific values

are standardized.Failure Type * Failure Location* Unus ed

1 byte 9 byte s 35 bytes

* Default Coding = 6A Hex for all octetsFigure 5 : Funct ion Specific Fields for AIS and RDI/FERF

Figure 6 illustrates the operation and theory of the Alarm

indication Signal (AIS) and Far End Reporting Failure(FERF) {for equivalently Remote Defect Indication (RDI)}

ATM OAM cell function types. We cover two examples, (a)

where a failure occurs in both directions simultaneously, and

(b) where a failure occurs in only one direction. In bothexamples there is a VP (or VC) connection between node 1

and node 4.Part (a) illustrates the typical failure of both directions of the

physical layer between nodes 2 and 3 that causes the

underlying VPs and VCs to simultaneously fail. The failures

in each direction are indicated as Failure A and Failure B in the figure so that the resulting AIS and RDI/FERF cells

can be traced to the failure location. A node adjacent to thefailure generates an AIS signal in the downstream direction to

indicate that an upstream failure has occurred, as indicated in

the figure. As can be seen from example (a), both ends of theconnection (nodes 1 and 4) are aware of the failure because of

AIS alarm that they receive. However, by convention, each

generates a RDI/FERF signal.

Example (b) illustrates the purpose of the FERF (or RDI)signal. In most communications applications the connection

should be considered failed, even if it fails in only one

direction. This is especially true in data communication.

Example (b) illustrates the case of a failure that affects only

one direction of a full duplex connection between nodes 2 and3. Node 3, which is downstream from the failure, generates anAIS alarm, which propagates to the connection end (node 4),

which in turn generates the RDI/FERF signal. The RDI/FERF

signal propagates to the other connection end (node 1), which

is now aware that the connection has failed. Without theRDI/FERF signal, node 1 would not be aware that there was a

failure in the connection between nodes 2 and 3. This methodwill also detect any combination of single direction failures.

Note that the node(s) that generate the AIS signals know

exactly where the failure is, and could report this to a

centralized network management system, or take a distributedrerouting response.

a) Failure in Both Directions

b)Failure in One Direction

Figure 6: Illustration of AIS and RDI/FERF theory and Operation

+ Loopback Operation and Diagnostic UsageFigure 7 illustrates the ATM OAM cell Loopback function-

specific fields.

A summary of the ATM OAM cell Loopback function-specific fields is:

Loopback Indication is a field that contains 01when originated, and is decremented by the receiver. It should

be extracted by the sender when it is received with a value of

00. This prevents the cell fromlooping around the networkindefinitely.

Correlation Tag is a field defined for use by theOAM cell originator since there may be multiple OAM cells

in transit on a particular VPC/VCC, and this allows the sender

to identify which one of these has been received.

1 432

Upstream from ADownstream from A

RDI/FERF-B AIS-A

Failure A

Failure B

FERF-AAIS-BDownstream from B

Upstream from B

Downstream from A

1 432

Upstream from A

AIS-A

Failure A

RDI-FERF-A


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Loopback Location ID is a field provided to thesender and receiver for use in segment loopbacks to identify

where the loopback should occur. The default value of all 1sindicates that the loopback should occur at the end point.

Source ID is a field provided so that theloopbacksource can be identified in the cell. This can be used by nodes

to extract OAM cells that they have inserted for extractionafter they have loopback to the source.

Loopback

Indication

Correlation

Ta g

Loopback

Location ID

Source

ID

Unused

1 byte 9 bytes 12 bytes 12 bytes 16 bytes

Figure 7 ATM OAM Loopback Function- Specif ic Fields.

As seen in the preceding section, AIS and RDI/FERF are

most useful in detecting and identifying to the connection

endpoints that a failure has occurred.

Figure 8 illustrates how these loopback primitives can beused to diagnose a failure that would not be detected by AIS

and RDI/FERF at any node. An example of such a failurewould be a misconfigured VP or VC cross-connect. The

example shows two endpoints and two intervening networks,

each with three nodes. Part (a) shows the verification of end

to-end continuity via an end-to-end loopback to endpoint 1. Ifthis were to fail, then network 2 could diagnose the problem

as follows. Part (b) shows verification of connectivitybetween a node in network 2 to endpoint 2 via an end-to-end

loopback. If this fails, then the problem is between network 2

and endpoint 2. Part (c) shows verification of connectivity to

endpoint 1 via an end-to-end loopback. If this fails, there is aproblem in the link between endpoint 1 and network 1 , a

problem in network 1 , or a problem in the link betweennetwork 1 and 2. Par (d) shows verification of connectivity

across networks 1 and 2 via a segment loopback. If this

succeeds, then the problem is the access line from endpoint1

to network 1. Part (e) shows verification of connectivity fromentry to exit in network 1. If this succeeds, then the problem

is in network 1. Verification within any of the networks couldalso be done using the segment loopback.

+ Continuity Check

The continuity check can detect failures that AIS cannot,such as an erroneous VP aross-connect change, as illustrated

in Figure 9 .Part (a) shows an VP connection traversing three

VP cross-connect nodes with VPI mappings shown in thefigure carrying only Continuity Check (CC) cell traffic. In

part (b) an erroneous cross-connect is made at node 2,

interrupting the flow of CC cells. In part (c) node 3 detects

this continuity failure and generates a VP-RDI/FERF OAMcell in the opposite (upstream) direction.

a) Intial Virtual Path Connection

b) Erroneous Change

c) Fault Notification

Figure 9: Illustration of Continuity Check (CC) OAM Cell Usage

+ RESTORATIONThe standard currently do not specify what can be done in

respnse to a fault at the ATM layer. There are SONET andSDH standards, however, that define physical layer protection

switching on a point-to-point, 1:N redundant basis or a ring

configuration. There are also restoration stagtefies for partialmesh networks. These same concepts could also be applied to

restore ATM connections. Resoring Virtual Paths (VPs) that

carry a large number of Virtual Channels (VCs) would be anefficient way to perform ATM-level restoration. We briefly

discuss these three restoration methods with reference toFigure 10.

The term 1:N (read as one for N) redundancy means that

there is one bidirectional protection channel for up to N

working bidirectional channels, as illustrated in Figure 10 (a).If a working channel fails, its endpoints are switched to the

protection channel. If a failure occurs and the protectionchannel is already in use or unavailable, then the failed

working channel cannot be restored.

Unused

0000000

0/1

7 bit 1 bit

EndPoint

1

EndPoint

2

Node

1

Node

3

Node

2

End

Point

1

End

Point

2

Node1

Node

3

Node

2

End

Point

1

End

Point

2

Node1

Node

3

Node

2

VP=1 VPI=17 VPI=31 VPI=1



CC

CC

CC

End Point 1 Network 1 Network 2 End Point 2

a.

b.

c.

d.e.

Figure 8 : Usage of Loopback in Verif ication/Problem Diagnostic


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Figure 10 (b) illustrates a bidirectional ring. Traffic from node1 to node 3 is sent in both directions around the ring. At each

node signals may be added as shown by the plus sign inside

the circle, or dropped as shown by the minus sign inside the

circle. At the receiver (node3) only one of the signals is

selected for output. Upon detection of a failure the receiverwill switch to the other, redundant signal, as shown in theexample for a failure between nodes 2 and 3.

a)Point to Point

b)Bi-directional Ring

c) Parital mesh

Figure 10 (C) illustrates a partial mesh network. In the

example, traffic between nodes 3 and 4 initially follows theroute shown by the solid line. A failure between nodes 1 and

2 impacts the node 3 to 4 traffic on its current route. This can

be detected in a centralized, distributed, or hybrid network

management system to find a new route shown by he dashedline in the example. Longer distance networks tend to have

the type of lattice structure shown in the example and tend touse this type of restoration . In the example of Figure 10(c),

1:2 redundancy is provided for some routes.

The mirror image of this capability is in place for traffic betweennodes 3 and 1. Note that the ring architecture achieves 1:1redundancy, or in other words, only half of the transmissionbandwidth is available to traffic. These types of ring architectures are

economically attractive for metropolitan areas and can be furtheroptimized for ATM using Virtual Path level Add Drop Multiplexing

(ADM) to improve multiplexing efficiency

REFERENCES[1] David E. MC DySan and Darren L. Spohn , "ATM theory andApplication", McGraw-Hill series on Computer Communication,International Editions 1995, Printed in Taiwan.[2] Othmar Kyas, "ATM Networks", International Thomson Computer Press,

ITP An International,Thomson Publishing, First printed 1995, Reprinted1995,Printed in the UK by the Alden Press, Oxford.[3] Document on Internet :

http://www.tmo.hp.com/tmo/pia/VXIbus/PIAProd/datasheets/English/HPQ0SS1.html

+ Test & Measurement , Product Information HEWLETT PACKARD[4]Dr.Kon MIYAKE, Standardization ^ Technology on B-ISDN & ATM

Network, Nippon Telegraph and Telephone Corporation(NTT), Technical

Seminar on ITU-T Standardization(Tss-96) Nov,1996.

FailureWorking

Protection

Working

1

N

1

N

N+1

Failure

1 2 3

45

6

Restored

RouteInitial Route

1

2

3Initial Route

Restored Route

Figure 10 : Basic Restoration Method Examples


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