Connection-Oriented Networks – Wissam Fawaz1 Chapter 3: ATM Networks TOPICS –The ATM header...

Connection-Oriented Networks – Wissam Fawaz

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Chapter 3:ATM Networks

TOPICS– The ATM header – The ATM protocol stack– The physical layer– ATM switch architectures

– ATM adaptation layers– IP over ATM


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Asynchronous Transfer Mode (ATM)

• The word Asynchronous in ATM is in contrast to Synchronous Transfer Mode (STM) that was proposed earlier on, which was based on the SONET/SDH hierarchy.

• Transfer Mode refers to a telecommunication technique


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• ATM was standardized by ITU-T (old CCITT) in 1988 as the transfer mode

of B-ISDN • It can carry a variety of different types of traffic, such as– Voice– Video– Data

At speeds varying from fractional T1 to 2.4 Gbps


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• These different types of traffic have different Quality-of-Service (QoS) requirements, such as:– Packet loss– End-to-end delay

• ATM, unlike IP networks, can provide each traffic connection a different type of quality of service.


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Some features of ATM

• Connection-oriented packet-switched network

• Fixed cell (packet) size of 48+5 bytes

• No error protection on a link-by-link

• No flow control on a link-by-link• Delivers cells in the order in which they were transmitted

Header Payload

5 bytes 48 bytes


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The structure of the ATM cell

GFC VPI

VPI VCI

VCI

VCI

HEC

PTI CLP

Informationpayload

1 2 3 4 5 6 7 81

2

3

4

5

.

.

.

53

Byte

UNI cell format

VPI

VPI VCI

VCI

VCI

HEC

PTI CLP

Informationpayload

1 2 3 4 5 6 7 81

2

3

4

5

.

.

.

53

Byte

NNI cell format


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Fields in the ATM cell header

• GFC• Connection identifier: VPI/VCI,

• Payload type indicator (PTI)• Cell loss priority (CLP)• Head error control (HEC)


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ATM connections

• Identified by the combined fields– virtual path identification (VPI), and – virtual channel identification (VCI)

• VPI field: – 256 virtual paths at the UNI interface, and

– 4096 virtual paths at the NNI interface.

• VCI field: – a maximum of 65,536 VCIs.


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• VPI/VCI values have local significance. That is, they are only valid for a single hop.

• A connection over many hops, is associated with a different VPI/VCI value on each hop.

• Each switch maintains a switching table. For each connection, it keeps the incoming and outgoing VPI/VCI values and the input and output ports.


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Label swapping

VPI=10VCI=89

VPI=30VCI=53

VPI=100VCI=53

VPI=50VCI=77

VPI=30VCI=41

30 53 4 100 53 5

30 41 1 30 53 440 62 2 10 89 3

10 89 1 50 77 6

ATMswitch 1

ATMswitch 2

ATMswitch 3

VPI=40VCI=62

A

B

C

D2

1

3

44

1

5

6


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PVCs and SVCs• Depending how a connection is set-up, it may be– Permanent virtual circuit (PVC)– Switched Virtual circuit (SVC)

• PVCs are set-up administratively. They remain up for a long time.

• SVCs are set-up in real-time using ATM signalling. Their duration is arbitrary.


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Payload type Indicator

• PTI Meaning • 000 User data cell, congestion not experienced, SDU

type=0• 001 User data cell, congestion not experienced, SDU

type=1• 010 User data cell, congestion experienced, SDU type=0• 011 User data cell, congestion experienced, SDU type=1• 100 Segment OAM flow-related cell• 101 End-to-end OAM flow-related cell• 110 RM cell• 111 Reserved


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No error detected(No action)

Error detected(cell discard)

Head Error Control (HEC)

Correctionmode

DetectionmodeNo error detected

Single bit error detected(correction)

Multiple bit error detected(cell discarded


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The ATM protocol stack

ATM adaptation layer

ATM layer

Physical layer

voice Video Data


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The physical layer

• The physical layer transports ATM cells between two adjacent ATM layers.

• It is subdivided into– transmission convergence (TC) sublayer– physical medium-dependent (PMD) sublayer.


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The transmission convergence (TC) sublayer

• HEC cell generation and verification– Implements the HEC state machine

• Decoupling of cell rate– Maintains a continuous bit stream by inserting idle cells

• Transmission frame generation and recovery– Such as SONET frames

• Cell delineation


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Cell delineation is the extraction of cells from the bit stream received from the PMD sublayer.

hunt

Presync

Sync

CorrectHEC

CorrectHEC for cells

IncorrectHEC for cells

IncorrectHEC


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• Physical medium dependent (PMD)– Timing function

• Used to synchronize the transmitting and receiving PMD sublayers.

– Encoding/decoding• PMD may operate on a bit-by-bit basis or using block coding such as 4B/5B and 8B/10B schemes.


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ATM physical layer interfaces

• SONET/SDH• Plesiochronous digital hierarchy (PDH)

• Nx64 Kbps• Inverse mulitplexing for ATM (IMA)• asymmetric digital subscriber line (ADSL)

• APON


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The ATM layer

• The ATM layer is concerned with the end-to-end transfer of information, i.e., from the transmitting end-device to the receiving end-device.

• Below, we summarize its main features.


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Connection-oriented packet switching

• The ATM layer is a connection-oriented point-to point packet-switched network with fixed-size packets (known as cells).

• A connection is identified by a series of VPI/VCI labels, as explained above, and it may be point-to-point or point-to-multipoint.

• Cells are delivered to the destination in the order in which they were transmitted.


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Cell switching in ATM networks

is carried out at the ATM level

End-device

Application Application

ATM switch End-deviceATM switch


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No error and flow control on each hop

• Low probability of a cell getting lost or delivered to the destination end-device in error.

• The recovery of the data carried by lost or corrupted cells is expected to be carried out by a higher-level protocol, such as TCP.

• When TCP/IP runs over ATM, the loss or corruption of the payload of a single cell results in the retransmission of an entire TCP PDU.


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Addressing

• Each ATM end-device and ATM switch has a unique ATM address.

• Private and public networks use different ATM addresses. Public networks use E.164 addresses and private networks use the OSI NSAP format.

• ATM addresses are different to IP addresses.


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Quality of service

• Each ATM connection is associated with a quality-of-service category.

• Each quality-of-service category is associated with a set of traffic parameters and a set of quality-of-service parameters.

• The ATM network guarantees the negotiated quality-of-service for each connection.


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Congestion control

• In ATM networks, congestion control permits the network operator to carry as much traffic as possible without affecting the quality of service requested by the users.

• It consists of call admission control and a policing mechanism.


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A generic ATM switch

Incoming links

. . .

. . .Outgoing

links

Input queues

Output queues

CPU

Switch fabric

1

N N

1

The ATM switch architecture


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The shared memory ATM switch architecture

. . .

. . .Shared memory

1

N N

1


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• If the speed of transmission on each incoming and outgoing link is V, then the switch can keep up at maximum arrival rate, if the memory's bandwidth is 2NV

• Total memory capacity is B cells• Each linked list i is associated with a minimum dedicated space and it is limited to an upper bound Bi, Bi<B so that Bi>B.


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Cell loss in a shared memory switch

• Cell loss occurs when a cell arrives at a time when the shared memory is full, that is it, contains B cells.

• Cell loss can also occur when a cell with destination output port i arrives at a time when the total number of cells queued for this output port is Bi

cells (even if the total number of cells in the shared memory is less than B.)


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Non-blocking output buffering switch

• In a non-blocking switch, the switching fabric does not give rise to internal or external blocking.

• An output buffering switch has buffers only at its output ports.

. .

.

. .

.

output ports

• A shared memory switch is non-blocking with output buffering


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Scheduling algorithms

• Let us consider a non-blocking switch with output buffering. Each output buffer holds cells that belong to different connections.

• Each of these connections is associated with a quality-of-service category.

• The cells belonging to these connections are grouped into queues, one per quality-of-service category, and these queues are served using a scheduling algorithm.


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Static priorities

From switch fabric

CBR

RT-VBR

NRT-VBR

ABR

UBR

Output port

• Priorities among the queues

• Always serve high priority queue first, then second priority queue, etc.

• Aging• Purging


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Early deadline first (EDF) algorithm

• Each cell is assigned a deadline upon arrival at the buffer. The scheduler servers the cells according to their deadlines, so that the one with the earliest deadline gets served first.

• Cells belonging to delay-sensitive applications, such as voice or video, can be served first by assigning them deadlines closer to their arrival times.


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The round-robin scheduler

• Each output buffer is organized into a number of logical queues.

• The scheduler serves one cell from each queue in a round robin fashion

• Empty queues are skipped• Weighted round robin scheduling can be used to serve a different number of cells from each queue


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The ATM adaptation layer

• The purpose of AAL is to isolate higher layers from the specific characteristics of the ATM layer.

• AAL consists of the – convergence sublayer, and the – segmentation-and-reassembly sublayer.


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The AAL sublayers

SAP

Service Specific Convergence Sublayer (SSCS)

Common Part Sublayer (CPS)

Segmentation and Reassembly

Convergence Sublayer

SAP


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ATM Adaptation Layer 1 (AAL 1)

• This AAL can be used for applications such as: – circuit emulation services

•It emulates a point-to-point TDM circuit over ATM

– Constant-bit rate audio•Used to provide an interconnection between two PBXs over a private or public ATM network


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The SAR encapsulation for AAL 1

payloadSAR Header

47 bytesSN SNP

CRC-3

3 bits

Parity

1 bit3 bits1 bit

CSISequence. count


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The AAL 1 CS functions:

1. Handling of cell variation– Due to queueing delays, inter-arrival times

of cells vary (jitter).

– CS writes received data into a buffer, and then delivers the information to the application at constant bit rate.

ATM cloud

Inter-departure gaps

Sender

cell i-1

Inter-arrival gaps

Receiver

cell i

cell i+1

cell i-1

cell i

cell i+1

ti-1

ti si-1 si


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2. Processing of the sequence count– The sequence count values are processed by

CS in order to detect lost or misinserted cells. Detected misinserted cells are discarded. In order to maintain bit count integrity of the AAL user information, it may be necessary to compensate for lost cells by inserting dummy SAR-PDU payloads.

3. Forward error correction– For video and high quality audio forward

error correction may be performed in order to protect against bit errors. This may be combined with interleaving of AAL user bits to give a more secure protection against errors.


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4. Transfer of timing information a. Synchronous residual time stamp

(SRTS):CS conveys to the receiver in the CSI field the difference between a common clock derived from the network and the sender’s clock

b. Adaptive clock method:The receiver writes the received information into a buffer and reads out from the buffer. If its clock is fast/slow the occupancy in the buffer will be below/over the median


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5. Structured and unstructured data transfersTwo CS-PDU formats have been defined: a. CS-PDU non-P format: Constructed from 47 bytes of information supplied by an AAL user

b. CS-PDU P format: Constructed from a 1-byte header and 46 bytes of information supplied by an AAL use.The header consists of a 7-bit pointer (SDT pointer) and 1 even bit parity.


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Circuit Emulation Services

IWF A

IWF B

ATM network

UNI UNI

CBR User A

CBR User B

• The structured and unstructured data transfers are used in Circuit Emulation Services (CES), which emulate a T1/E1 link over ATM.

• CES is implemented in an interworking function (IWF).


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• Unstructured service– The entire DS1/E1 signal is transported by packing it bit by bit into the 47-byte payload of a CS-PDU non-P format, which is then carried by an ATM cell.

47 bytes -> 376 bits -> less than 2 DS-1 frames (193 bits/frame)


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• Structured transfers – It is used to carry fractional T1/E1, i.e. Nx64 Kbps

– Fractional T1/E1 generates blocks of N bytes every 125 sec. Such a block of data is referred to in the standards as a structured block.

– Blocks of N bytes are transported back-to-back over successive cells using both the CS-PDU non-P and P formats.


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• The SDT pointer – The SDT pointer in the CS-PDU P format is used to help delineate the boundaries of these blocks.

– The actual rules as to when to use the SDT pointer in the P format are somewhat complex.


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An example: block size = 20 bytes

Seq, count 0 Seq, count 1 Seq, count 2 Seq, count 3



20 20 6 14 20 13 7 20 20 20 20 7

13 20 14 6 20 20 19 20 7 13 20 14

6 20 20 20 20 7 13 20 14 6 20 20 1

1


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ATM Adaptation Layer 2 (AAL 2)

• Defined for delay sensitive applications with a low bit rate, such as voice and voiceband traffic (facsimile, modem traffic)

• AAL 2 is used to interconnect two distant public or private telephone networks over an ATM network.


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• At the sender, AAL 2 multiplexes several streams onto the same ATM connection

• At the receiver, it de-multiplexes the date from the connection to the individual streams.


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IWF T1/E1

IWF

ATMnetwork

Centraloffice

PBX A

PBX B

IWF

IWF

PBX C

T1/E1

T1/E1

T1/E1

……

M U X

D E M U X

N input links

N output links

link

1

2

N

12

N

FTime slot 1

Time slot 2

Time slot 3

Time slot 24

. . .

M U X

D E M U X

N input links

N output links

link

1

2

N

12

N

FTime slot 1

Time slot 2

Time slot 3

Time slot 24

. . .


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The SSCS and CPS sublayers

• The AAL 2 services are provided by the convergence sublayer, which is subdivided into the– Service Specific Convergence Sublayer (SSCS)

– Common part sublayer (CPS).


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Functional model of AAL 2 (sender side)

• Each stream is served by a separate SSCS which is associated with a CID

SSCS

AAL-SAP

ATM-SAP

SSCSSSCS

CID=XCID=Y

CID=Z

CPS


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SSCS for AAL 2 trunking

• A specialized SSCS has been developed to support “ATM trunking using AAL 2 for narrowband services”.

• It is described in Chapter 12


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• A transmitting SSCS uses a timer to decide when to pass on the data to CPS.

• Data from an SSCS is packed into a CPS-packet

• CPS-packets from different SSCSs are packed into a CPS-PDU, which is exactly 48 bytes and it is carried in an ATM cell

CPS-packets and CPS-PDUs


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Packing CPS-packets into CPS-PDUs

CPS-PDUs

1 2 3 4 5

54321 3

ATM cells

CPS-packets


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The structure of the CPS-packet

and CPS-PDU

OSFP

CPS-PDU payload

SN

PAD

C I D

PPT LI

HEC UUI

CPS-packet payload

1

2

3

1

48

CPS-packet CPS-PDU

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8


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The CPS-packet fields

• Channel identifier (CID) - 8 bits:– Identifies a channel. Same value is used for both directions.– CIDs are allocated using the AAL negotiation procedures (ANP)

• Packet payload type (PPT) - 2 bits:– Indicates whether it carries voice or network

management data

• Length indicator (LI) - 6 bits:– Default maximum length of the CPS-packet payload is 45

bytes.

• Header error control (HEC) - 5 bits:– Pattern is: x5+x2+1.

• User-to-user-indication (UUI) - 3 bits:– Used to transfer information transparently between the

peers.


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CPS-PDU fields

Offset field (OSF) (6 bits)• Used to identify the beginning of a CPS-packet. It points to the first new CPS-packet in the CPS-PDU payload

• In the absence of a new CPS-packet, it points to the beginning of the pad

• The value of 47 indicates that there is no beginning of a CPS-packet in the CPS-PDU.


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An example

20 48

35

20

OSF =0 OSF=21 OSF=9

20 27 21 26 9 20 #1#2

#2 #3 #3 #4padding


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ATM adaptation layer 5 (AAL 5)

• Very popular AAL due to its simplicity• A user-PDU is encapsulated and then broken up to fragments, each carried by an ATM cell

• AAL 5 consists of – Convergence sublayer (CS)

•SSCS•CPS

– Segmentation and reassembly (SAR).


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• Provides a non-assured transfer operation.

• User-PDUs of a length up to 65,535 bytes can be transferred.

• Erroneous CPS-PDUs can be detected at the receiver’s side. No recovery of an erroneous CS-PDU takes place. Instead, an indication is sent to the the higher-level application.

CPS


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• Pad: from 0 to 47 bytes, so that the entire CPS-PDU becomes an integer multiple of 48 bytes. The User-PDU can be up to 65,535 bytes

• CPS User-to-user indication (CPS-UU): 1-byte field• Common part indicator (CPI): 1-byte field for future use

• Length: 2-byte field gives length of User-PDU.• CRC pattern: 4-byte field contains the FCS calculated using the pattern x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1.

User-PDU Pad CRC-32LengthCPS-UU

CPS encapsulation


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SAR sublayer - transmitter

• SAR segments a CPS-PDU into a sequence of 48-byte segments.

• No additional encapsulation• Each segment is carried in the payload of an ATM cell

• Last cell has its PTI marked with SDU=1.


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SAR sublayer - Receiver

• SAR appends payloads of the ATM cell into a buffer until1.It encounters an SDU=1 in PTI field

• It checks the CRC and then passes the PDU to the application above with an indication as to whether it is correct or not.

2.Buffer is exceeded• It passed the PDU to the application

above with an indication that buffer was exceeded.


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Classical IP and ARP over ATM

• This is a technique proposed by IETF for supporting IP over ATM in a single logical IP subnet (LIS).

• A LIS is a group of IP hosts that share a common IP network address and subnet mask, and they all communicate with each other directly over ATM connections.


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Group of host with the same network addressand same subnet maskIP address: 193.14.0.0

We now replace the transport network with ATM switches

A logical IP subnet (LIS)


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PHY

ATM

AAL

CIP

IP

TCP

PHY

ATM

AAL

CIP

IP

TCP

IP packet

CS-PDU

SAR

ATM

IP packet

CS-PDU

SAR

ATM

Computer Computer

Classical IP

ATM switch


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ATMARP and InATMARP

• A LIS member has both an IP and an ATM address.

• IP addresses are resolved to ATM addresses using the ATMARP protocol within the LIS (based on ARP).

• The inverse ATMARP (InATMARP) protocol is used to resolve an ATM address to an IP address. (It is similar to the inverseARP).


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The ATMARP server

• It maintains a table or a cache of IP and ATM address mappings

• It learns about the IP and ATM addresses of the LIS members (IP clients) through the messages exchanged between ATMARP and the LIS hosts.

• It typically resides on an ATM switch. (An ATM switch load commonly contains the ATMARP server as well).


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ATMARP messages

• ATMARP_request: Used to request the ATMARP server the ATM address of a destination IP client.

• ATMARP_reply: Used by the ATMARP server to respond to an IP client with the destination ATM address.

• InATMARP_request: Sent from the ATMARP server to an IP client to obtain its IP address.

• InATMARP_reply: This is the response from an IP client with its IP address.

• ATMARP_NAK: Negative response issued by the ATMARP server to a requesting IP client.


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Registration

• An IP client must first register its IP and ATM addresses with the ATMARP server. – The client establishes a connection to the

ATMARP server (it knows the ATMARP server’s ATM address).

– It then transmits an ATMARP_request, asking its own ATM address.

– The ATMARP server checks against duplicate entries in its table, time stamps the entry, adds it to its table, and sends an ATMARP_reply. The entry is valid for a minimum of 20 minutes and has to be refreshed.


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Address resolution

• An ATMARP client 1 can communicate with an ATMARP client 2 immediately if it knows its ATM address.

• If the destination ATM address is not known, client 1 invokes the ATMARP process. – It sends an ATMARP_request to the ATMARP

server. – If the server has the requested address in

its table, it returns an ATMARP_reply. – Otherwise, it returns an ATMARP_NAK.

Connection-Oriented Networks – Wissam Fawaz1 Chapter 3: ATM Networks TOPICS –The ATM header...

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