Transcript ATM Layers

ITEC4610
Network Switching and Routing
ดร. ประวิทย์ ชุมชู
หัวหน้าสาขาวิชาวิศวกรรมสารสนเทศและการสื่ อสาร(ICE)
MUT
Email: [email protected]
ห้องทางาน: F402
เบอร์โทรศัพท์ที่ทางาน: (02)9883655 ต่อ 220
เบอร์โทรศัพท์เคลื่อนที่: 065343850
MUT
Class II
IP over ATM
ดร. ประวิทย์ ชุมชู
หัวหน้าสาขาวิชาวิศวกรรมสารสนเทศและการสื่ อสาร(ICE)
MUT
Email: [email protected]
ห้องทางาน: F402
เบอร์โทรศัพท์ที่ทางาน: (02)9883655 ต่อ 220
เบอร์โทรศัพท์เคลื่อนที่: 065343850
MUT
Outlines
ATM Architectures
ATM Addressing
ATM Adaptation layers
The ATM Layer
The ATM Physical Layer
LAN Emulation (ELAN)
IP over ATM
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ATM
Data
Voice
Video
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IP over ATM
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ATM
• Asynchronous Transfer Mode (ATM) is a Broadband Integrated Services Digital
Networks B-ISDN designed by the ITU-T.
• Based on cell relay protocol designed by the “ATM Forum”
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ATM Cell
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ATM is packet switching technology
It uses short/fixed size packet, hence it is called cell technology
Long cells are better suited for data
Short cells are better suited for voice
As a compromise:
– ATM cell is 48 bytes + 5 bytes header = 53 bytes
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Protocol Architecture
• Similarities between ATM and packet switching
– Transfer of data in discrete chunks
– Multiple logical connections over single physical interface
• In ATM flow on each logical connection is in fixed sized packets
called cells
• Minimal error and flow control
– Reduced overhead
• Data rates (physical layer) 25.6Mbps to 622.08Mbps
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Protocol Architecture (diagram)
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Reference Model Planes
• User plane
– Provides for user information transfer
• Control plane
– Call and connection control
• Management plane
– Plane management
• whole system functions
– Layer management
• Resources and parameters in protocol entities
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Architecture of an ATM Network
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Architecture of an ATM Network
User end points are connected through user-to-network interface (UNI)
Switches are connected through network-to-network interface (NNI)
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Multiplexing Using Different Packet Sizes
Variable delays: small packet served after long packet will experience longer
delay as compared to a short packet
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Multiplexing Using Cells
Multiplexing fixed, small size cells results in constant short delay
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ATM Multiplexing
ATM is based on statistical (asynchronous) multiplexing
Asynchronous TDM (no empty slots) is more efficient than synchronous
TDM (possible empty slots if user has no data to send)
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TDM
Data rate of the transmission medium is greater than
sending and receiving devices. Link is divided in time
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4
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Synchronous TDM
Synchronous: multiplexer allocates exactly the same
time slot to each deviceb
Frame: consists of one complete cycle of time slots
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Asynchronous (Statistical) TDM
Each time slot can be assigned to any input device. The number of time
slots (m) in the frame is based on statistical analysis
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Frames and Addresses
Address bits are needed for each time slots More overhead.
No empty slots in the frames. Higher efficiency
a. Only three lines sending data
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Frames and Addresses
b. Only four lines sending data
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Frames and Addresses
c. All five lines sending data
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Outlines
ATM Architectures
ATM Addressing
ATM Adaptation layers
ATM Layer
The ATM Physical Layer
LAN Emulation (ELAN)
IP over ATM
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ATM Addressing
• The ITU-T standard is based on the use of E.164 addresses (similar to
telephone numbers) for public ATM (B-ISDN) networks.
• The ATM Forum extended ATM addressing to include private
networks. It decided on the subnetwork or overlay model of
addressing, in which the ATM layer is responsible for mapping
network layer addresses to ATM addresses.
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ATM Addressing
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ATM Address Fields
• AFI—Identifies the type and format of the address (E.164, ICD, or DCC).
• DCC—Identifies particular countries.
• High-Order Domain-Specific Part (HO-DSP)—Combines the routing domain (RD) and the
area identifier (AREA) of the NSAP addresses. The ATM Forum combined these fields to
support a flexible, multilevel addressing hierarchy for prefix-based routing protocols.
• End System Identifier (ESI)—Specifies the 48-bit MAC address, as administered by the
Institute of Electrical and Electronic Engineers (IEEE).
• Selector (SEL)—Is used for local multiplexing within end stations and has no network
significance.
• ICD—Identifies particular international organizations.
• E.164—Indicates the BISDN E.164 address.
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Outlines
ATM Architectures
ATM Addressing
ATM Adaptation layers
ATM Layer
The ATM Physical Layer
LAN Emulation (ELAN)
IP over ATM
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ATM Layers
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ATM Layers
• ATM standard defines three layers
– Application adaptation layer AAL
– ATM layer
– Physical layer
• End points use all three layers
• Switches use only the bottom two layers
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ATM Layers in End-Point Devices and Switches
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AAL Types
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Application Adaptation Layer AAL
• Allows existing networks (such as packet networks) to connect to ATM
• AAL protocols accept transmissions from upper- layer services (e.g., packet data)
and map them into fixed-sized ATM cells
• Such transmission can be of any type (voice, data, audio, video) and can be of
variable or fixed rates
• At Rx, this process is reversed, e.g., segments are are reassembled into their
original format
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Data-Stream Types
• ATM deals with four types of data streams
– Constant-bit rate (CBR): real time application, such as real-time voice
(telephone calls), real-time video (TV) transmission delay must be minimal
– Variable-bit-rate (VBR): bit rate may vary from section to section of
transmission: compressed voice and video, data
– Connection-oriented packet data: X.25, TCP protocol
– Connectionless packet data: datagram applications: IP protocols
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Application Adaptation Layer : AAL1
• Supports applications at constant bit rate (CBR): voice, video, existing
digital telephone networks (E-1, T-1)
• AAL layer is divide into two sublayers:
– Convergence sublayer (CS)
– Segmentation and reassembly (SAR)
• Convergence sublayer (CS): divides the bit stream into 47-byte
segments and pass them to the SAR below
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AAL1
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Segmentation and reassembly (SAR)
• Adds one-byte header to each 47-byte segment (payload) received
from CS
• This header consists of four fields:
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– CS identifier (CSI): one-bit for signaling purposes
– Sequence count (SC): three-bit (modulo 8) sequence number for end-to-end
error and flow control
– CRC: three-bit cyclic redundancy check field calculated over the first four
bits. Beside single and multiple bit error detection they can correct a singlebit error
– Parity (P): one-bit parity calculated over the first seven bits (detects an odd
number of errors)
AAL2
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AAL2
• Supports variable bit-rate (VBR) applications
• SAR accepts 45-bit payload from the CS and adds one-byte header
and two-byte trailer, the result is 48-byte data unit passed to ATM
layer
• The overhead consists of three fields in the header and two fields in
the trailer
• Header:
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– CS identifier (CSI): one-bit for signaling purposes
– Sequence count (SC): three-bit (modulo 8) sequence number for end-to-end
error and flow control
– Information type (IT): 4 bits identifying the data segment as falling in the
beginning, middle or end of message
ALL-2 (Trailer)
• The trailer
– Length indicator (LI): The first six bits of the trailer are used with the final
segment of a message (when the IT in the header of the message indicates the
end of the message) to indicate the amount of padding in the final cell (LI
indicates where in the segment those bits start)
– CRC: last 10 bits are CRC of the entire data unit. Can correct single-bit error
in the data unit
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AAL3/4
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AAL3/4
• Initially, AAL3 was intended to support connection-oriented data
services and AAL4 to support connectionless services
• Combined later to a single format called AAL3/4
• Convergence sublayer:
– accepts data packet of no more than 65,535 (216-1) bytes from upper layer
service
– adds a header and trailer, which indicate the beginning and end of the
message and how much of the final frame is padding (Padding size is 0 - 43
bytes)
– Message (including header/trailer/padding) is passed 1n 44-byte segment to
the SAR layer
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AAL3/4
• CS header and trailer:
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Type (T): one byte T field set to zero
Begin tag (BT): one byte beginning flag (synchronization)
Buffer-allocation (BA): two bytes indicating the buffer size
Pad (PAD): indicates the three possible padding size:
• If # of data in the final segment is 40 bytes, no padding
• If # of data < 40 bytes, add padding to bring total to 40
• If # of data is between 41 and 44, add padding ) 43 to 40 to bring the total to 84.
The first 44 bytes make a complete segment, the next 40 bytes and the trailer make
the last segment
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AAL3/4
• CS header and trailer cont’d
– Alignment (AL): one byte AL field to make the trailre 4 bytes long
– Ending Tag (ET): one byte ending flag (synchronization)
– Length (L): two-byte field indicates length of data
• Segmentation and reassembly: accepts 44-byte pay load from CS and adds 2-byte
header and 2-byte trailer. The resulting 48-byte data unit is passed to ATM for
inclusion in the cell
• SAR Header and trailer:
– Segment type (ST): 2-bit ST indicates whether the segment belongs to the beginning,
middle, or the end of a message, or is a single-segment message
– Convergence sublayer identifier (CSI): one-bit for signaling purposes
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AAL3/4
• SAR Header and trailer cont’d
– Sequence count (CS): three-bit (modulo 8) sequence number for end-to-end
error and flow control
– Multiplex identification (MID): 10-bit identifies cells coming from different
data flows and multiplexed on the same virtual connection
– Length indicator (LI): first 6-bit of the trailer indicate the amount of
padding/message in the last segment.
– CRC: last 10 bits of the trailer are the CRC of the entire data unit
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AAL5
• Called “simple and efficient application layer (SEAL)
• Assumes that all cells travel sequentially and rest of functions provided by CS
and SAR are already included in upper layers, hence no provision for
addressing, sequencing or other CS and SAR heading information
• Only padding and four-field trailer are added at CS
• Padding and trailer are added to the end of the entire message of 65,536 bytes or
less
• Segments consist of 48 bytes of data, last segment has 40 bytes of data and 8
bytes overhead (trailer)
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AAL5
• Trailer:
–
–
–
–
–
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Pad (PAD): between 0 and 47 bytes, making the message divisible by 48
User-to-user ID (UU): one byte UU field (user defined)
Type (T); one-byte T field (not defined)
Length (L): two-byte L fields indicates amount of padding/data
CRC: 4-byte error check for the entire data unit
AAL5
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Outlines
ATM Architectures
ATM Addressing
ATM Adaptation layers
ATM Layer
The ATM Physical Layer
LAN Emulation (ELAN)
IP over ATM
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ATM Layer
• Provides: routing, traffic management, switching, and multiplexing
services
• Accepts 48-byte segments from AAL sublayers and adds to them 5-bytes
headers
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ATM Header
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ATM Header Format
• Two formats, one for user-to-user interface (UNI) and one for networkto-network interface (NNI)
– Generic flow control (GFC): 4-bit field flow control at UNI level (flow
control is not necessary for NNI, hence bits are added to VPI in the NNI
header)
– Virtual path identifier (VPI): 8-bit field for the UNI and 12-bit for the NNI
– Virtual channel identifier (VCI): 16-bit field in both frames
– Payload type (PT): 3-bit PT field, defines the payload as user data or
managerial information
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PT Fields
The interpretation of the last two bits depends on the first bit
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ATM Header cont’d
• Cell loss priority (CLP): one-bit CLP field for congestion control
– Cells with CLP bit = 0 are cell of higher priority, they should not be discarded as long as
there are cells with a CLP set to 1
– Users who violate the rate assigned to them by sending at higher rate, the network will
set the CLP to 1 to indicate that these cells must be dropped if the link becomes
overloaded
• Header error correction (HEC): one-byte field used for multiple bit error
detection and a single-bit error correction over the first four bytes of the header
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Header Error Control
• 8 bit error control field
• Calculated on remaining 32 bits of header
• Allows some error correction
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Effect of Error in
Cell Header
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Outlines
ATM Architectures
ATM Addressing
ATM Adaptation layers
ATM Layer
The ATM Physical Layer
LAN Emulation (ELAN)
IP over ATM
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The ATM Physical Layer
•
The ATM physical layer has four functions:
– Cells are converted into a bitstream
– the transmission and receipt of bits on the physical medium are controlled,
– ATM cell boundaries are tracked
– cells are packaged into the appropriate types of frames for the physical medium.
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The ATM physical layer is divided into two parts: the physical medium-dependent (PMD) sublayer and the
transmission convergence (TC) sublayer.
The PMD sublayer provides two key functions.
– synchronizes transmission and reception by sending and receiving a continuous flow of bits with associated timing
information.
– specifies the physical media for the physical medium used, including connector types and cable.
– Examples of physical medium standards for ATM include Synchronous Digital Hierarchy/Synchronous Optical
Network (SDH/SONET), DS-3/E3, 155 Mbps over multimode fiber (MMF) using the 8B/10B encoding scheme, and
155 Mbps 8B/10B over shielded twisted-pair (STP) cabling.
•
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The TC sublayer has four functions: cell delineation, header error control (HEC) sequence generation and
verification, cell-rate decoupling, and transmission frame adaptation.
ATM Logical Connections
• Virtual channel connections (VCC)
– Analogous to virtual circuit in X.25
• Basic unit of switching
• Between two end users
• Full duplex
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ATM Logical Connections Cont’d
• Fixed size cells
• Data, user-network exchange (control) and network-network exchange
(network management and routing)
• Virtual path connection (VPC)
– Bundle of VCC with same end points
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ATM Logical Connections
• Virtual channel connections (VCC)
– Analogous to virtual circuit in X.25
• Virtual path connection (VPC)
– Bundle of VCC with same end points
• Advantages of Virtual Paths:
–
–
–
–
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Simplified network architecture
Increased network performance and reliability
Reduced processing
Short connection setup time
Example of VPs and VCs
• 8 endpoints are communicating using 4 VCs
• 1st two VCs share the same virtual path from switch I to switch III, hence
these two VCs are bundled to form one VP
• Other two VCs share same path from I to IV and hence combined by another
VP
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Call Establishment
Using VPs
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Connection Identifiers
A virtual connection is identified by a pair of numbers:
the VPI and VCI
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Virtual Connection Identifiers
in UNIs and NNIs
256 x 65536 =
16,777,216 VCs
4096 x 65536 =
268,435,456 VCs
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An ATM Cell
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SVC Setup
-PVC
- (permanent virtual circuited)
-SVC
-( swithed virtual circuits)
- Connection less
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Routing with a VP Switch
• VP switch routes cells using only VPI
•Switch routing decision is based on four piece of information stored in
switching table: (1) arrival interface number, (2) VPI, (3) corresponding o/g
interface number and (4) the new VPI
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A Conceptual View of a VP Switch
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Routing with a VPC Switch
• VPC switch routes cells using both VPI and VCI
•Switch routing decision is based on six piece of information stored in
switching table: (1) arrival interface number, (2) i/c VPI, (3) i/c VCI (4)
corresponding o/g interface number, (5) o/g VPI and (6) o/g VCI
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A Conceptual View of a VPC Switch
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Outlines
ATM Architectures
ATM Addressing
ATM Adaptation layers
ATM Layer
The ATM Physical Layer
LAN Emulation (ELAN)
IP over ATM
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LAN Emulation (ELAN)
Stations attached via ATM the same capabilities
that they normally obtain from legacy LANs, such
as Ethernet and Token Ring
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LAN Emulation (ELAN)
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Outlines
ATM Architectures
ATM Addressing
ATM Adaptation layers
ATM Layer
The ATM Physical Layer
LAN Emulation (ELAN)
IP over ATM
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IP Over ATM
Objectives
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Upon completion you will be able to:
• Review the features of an ATM WAN
• Understand how an a datagram can pass through an ATM
WAN
• Understand how an IP packet is encapsulated in cells
• Understand how cells are routed in an ATM network
• Understand the function of ATMARP
IP over ATM and LAN Emulation
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An ATM WAN in the Internet
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ATM layers in routers and switches
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Note:
End devices such as routers use all
three layers, while switches use only
the bottom two layers.
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AAL5
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Note:
The AAL layer used by the IP protocol
is AAL5.
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ATM layer
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ATM headers
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Fragmentation
Tr=000, not last cell
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Tr=001,last cell
Note:
Only the last cell carries the 8-byte
trailer added to the IP datagram.
Padding can be added only to the last
cell or the last two cells.
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Note:
The value of the PT field is 000 in all
cells carrying an IP datagram
fragment except for the last cell;
the value is 001 in the last cell.
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ATM cells
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Entering-point and exiting-point routers
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ATMARP
ATMARP finds (maps) the physical address of the exiting-point router
given the IP address of the exiting-point router. No broadcasting is
involved.
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ATMARP packet
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Table 23.1 OPER field
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Note:
The inverse request and inverse reply
messages can bind the physical
address to an IP address in a PVC
situation.
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Binding with PVC
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Binding with ATMARP
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Note:
The request and reply message can be
used to bind a physical address to an
IP address in an SVC situation.
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Note:
The inverse request and inverse reply
can also be used to build the server’s
mapping table.
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Building a table
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LOGICAL IP SUBNET (LIS)
An ATM network can be divided into logical (not physical) subnetworks.
This facilitates the operation of ATMARP and other protocols (such as
IGMP) that need to simulate broadcasting on an ATM network.
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LIS
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Note:
LIS allows an ATM network to be
divided into several logical subnets. To
use ATMARP, we need a separate
server for each subnet.
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Summary
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ATM
IP over ATM
IP over Ethernet
IP over Pla. Pla.