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ATM-1Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
Asynchronous Transfer Mode (ATM) Networks
! All source media is first broken down into a stream of fixed sized unitsknown as cells.
! Cell streams relating to different media types are multiplexed togetheron a statistical basis for transmission and switching.
! The resulting networks are known as Cell-Based Networks orAsynchronous Transfer Mode (ATM) Networks.
! Transfer Mode:- Packet oriented transfer mode using ATDM- No link-by-link error control- No link-by-link flow control- End-to-End error control if needed- Use of internal virtual circuits- Switching based on table look-up- Fixed size blocks (cells)
ATM-2Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
ATM Protocol Architecture
! ATM Layer:- Common to all services and provides cell transfer capabilities.- Provides logical connections upon the physical layer:
! Virtual Channels! Virtual Paths
! ATM Adaptation Layer (AAL):- Provides a range of alternative service types known as service
classes.- Converts the source information into streams of 48-octet
segments.
ATM-3Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
ATM Connection Relationships
! Logical connections in ATM are referrred to as Virtual ChannelConnections (VCCs).
! VCCs are a concatenation of Virtual Channels (VCs).
! A Virtual Path is a group of VCs going the same way.
ATM-4Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
ATM Logical Connections
! Logical connections in ATM are referrred to as Virtual ChannelConnections (VCCs).
! VCCs are used for:- user-to-user exchange of variable rate, full duplex, fixed-size cells
carrying user data.- user-to-network exchange of control signaling information.- network-to-network exchange of management and routing
information.
! A Virtual Path Connection (VPC) is a bundle of VCCs that have thesame endpoints.
! All the cells flowing over all VCCs in a single VPC are switchedtogether.
! To set up a VCC, there must first be a VPC to the required destinationnode with sufficient available capacity to support the VCC with theappropriate QOS.
! The virtual path control mechanisms include:- calculating routes- allocating capacity- storing connection state information
ATM-5Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
Representation of VP and VC Switching Hierarchy
ATM-6Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
ATM Cell Format
! Cell Structure:Header (5 Octets) Information Field (48 Octets)
! User-Network Interface (UNI) Header Format:8 7 6 5 4 3 2 1
Generic Flow Control Virtual Path IdentifierVPI cont.
Virtual Channel IdentifierPayload Type CLP
Header Error Control (HEC)
Notes:GFC - Enables a local switch to regulate (flow control) the entry of
cells by a user into the network.CLP - Cell Loss Priority - provides guidance to the network in the
event of congestion:- 0 indicates cell of relatively high priority.- 1 indicates cell is subject to discard.
PT: 000 - user data, no congestion, SDU type 0001 - user data, no congestion, SDU type 1010 - user data, congestion, SDU type 0011 - user data, congestion, SDU type 1100 - OAM segment associated cell101 - OAM end-to-end associated cell110 - Resource management cell111 - Reserved for future function
ATM-7Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
ATM Cell Format cont.
! Network-Network Interface (NNI) Header Format:8 7 6 5 4 3 2 1
Virtual Path Identifier
Virtual Channel IdentifierPayload Type CLP
Header Error Control (HEC)
ATM-8Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
ATM Service Categories
! Real-Time Service:- Constant Bit Rate (CBR)- Real-Time Variable Bit Rate (rt-VBR)
! Non-Real-Time Service:- Non-Real-Time Variable Bit Rate (nrt-VBR)- Available Bit Rate (ABR)
Application specifies a peak cell rate (PCR) that it will use anda minimum cell rate (MCR) that it requires.
- Unspecified Bit Rate (UBR)This is a best-efforts service using capacity not allocated toany other service.
ATM-9Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
AAL Sublayers
! Convergence Sublayer (CS):Performs a convergence function between the service offered at thelayer interface and that provided by the underlying ATM layer.
! Segmentation and Reassembly (SAR) Sublayer:Provides cell segmentation and reassembly functions.
ATM-10Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
CS & SAR PDU Formats for AAL 5
AAL 5
! Known as the Simple and Efficient Adaptation Layer (SEAL).! AAL 5 is also used as the AAL in the C plane (Signaling AAL).
! CS PDU and SAR PDU Formats:
Notes:- There is no header in the CS PDU.- The CS PDU is padded to an integral multiple of 48 octets.- The UU fields enables the two correspondent user layers to relate
the AAL SDU to a particular SAP.- The use of the CPI field is yet to be defined.- The Length field indicates the number of octets in the data field.- There is no header or trailer in the SAR PDU.- The SAR protocol is said to be null.- Segments relating to the same CS PDU are identified using the
field in the header of the ATM cell that is used to transport thesegment.
- The SDU Type Bit in user data cells, is used to indicate BOM, COM(type 0) or EOM (type 1)
ATM-11Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.
Example of AAL 5 Transmission