ATM and xDSL

Chapter 12: Asynchronous Transfer Mode

     Is a dedicated connection switching technology that organizes digital data into 53-byte cell units and transmits them over a physical medium using digital signal technology. Individually, a cell is processed asynchronously relative to other related cells and is queued before being multiplexed over the transmission path. 

     Because ATM is designed to be easily implemented by hardware , faster processing and switch speed are possible. The prespecified bit rates are either 155.520 MBps or 622.080Mbps. Speeds on ATM networks can reach 10 Gbps. Along with Synchronous Optical Network(SONET) and other technologies, ATM is a key component of Broadband ISDN(BISDN).




Comparing Network Features

Feature	Data    Communications	Telecommunications	ATM
Traffic support	Data	Voice	Data, voice, video
Transmission unit	Packet	Frame	Cell
Transmission length	Variable	Fixed	Fixed
Switching type	Packet	Circuit	Cell
Connection type	Connectionless or Connection-oriented	Connection-oriented	Connection-oriented
Time sensitivity	None to some	All	Adaptive
Delivery	Best effort	Guaranteed	Defined class or guaranteed
Media and operating rate	Defined by protocol	Defined by class	Scalable
Media access	Shared or dedicated	Dedicated	Dedicated

ATM provides a mechanism for merging voice data and video onto LANs, CANs, MANs and WANs. you can gain an appreciation for how ATM accomplishes this by learning about its architecture.




Why does ATM uses cells?




Consider a speech signal reduced to packets, and forced to share a link with burst data traffic. No matter how small the speech packets could be made, they would have always encounter full size data packets, and under normal queuing conditions, might experience maximum queuing delays. That is why all packets or cells should have the same small size. in addition the fixed cell structure means that ATM can be always readily switched by hardware without the inherent delays introduced by the software switched and routed frames.




ATM SERVICES

Guarantees	ATM Service Feedback	Metrics	Loss	Delay	Bandwidth
Constant Bit Rate (CBR)	PCR, CDVT	Yes	Yes	Yes	No
Variable Bit Rate–Real Time (VBR–RT)	PCR, CDVT, SCR, BT	Yes	Yes	Yes	No
Variable Bit Rate–Non-Real Time (VBR-NRT)	PCR, CDVT, SCR, BT	Yes	Yes	Yes	No
Unspecified Bit Rate (UBR)	Unspecified	No	No	No	No
Available Bit Rate (ABR)	PCR, CDVT, MCR	Yes	No	Yes	Yes

Legend:


PCR = Peak Cell Rate
CDVT = Cell Delay Variation Tolerance
SCR = Sustained Cell Rate
BT = Burst Tolerance
MCR = Minimum Cell Rate




ATM Layered Architecture

Application Layer
User Layers
ATM Adaptation Layer:	Convergence Sublayer
	Segmentation and Reassembly Sublayer
ATM Layer
Physical Layer:	Transmission Convergence Sublayer
	Physical Medium Dependent Sublayer

The Physical Layer defines the medium for transmission, any medium-dependent parameters, and framing used to find the data contained within the medium.


The ATM Layer provides the basic 53-bytes cell format, by defining the 5-byte ATM header for each 48-byte payload segment handed down by the ALL.


The ATM Adaptation Layer adapts the higher level data into  formats compatible with the ATM Layer requirements; this layer segments the data and adds appropriate  error control information as necessary. It is dependent on the type of services being transported by the higher layer.




Several AAL protocols have been defined for specific types of data. these are loosely associated with the various classes of data. however, no AAL is restricted to a specific data class or type, all types of data could conceivable be handled by any of the AALs.


AAL 1


     Constant bit rate, connection-oriented, synchronous traffic.


AAL 2


    Definition never completed undefined, but envisioned to be for variable bit rate, connection-oriented,synchronous traffic.


AAL 3/4


    Variable bit rate,connection-oriented,asynchronous traffic or connectionless packet data with an additional 4 byte header in the information payload of the cell.


AAL 5


   Similar to AAL 3/4 with a simplified information header scheme that requires only one header per data unit and uses the PTI bit to indicate the last cell in a transmission. AAL 5 is the most widely used ATM Adaptation Layer Protocol.




5-byte Header ATM Cell

8	7	6	5	4	3	2	1
Generic Flow Control				Virtual Path Identifier
Virtual Path Identifier				Virtual Channel Identifier
Virtual Channel Identifier
Virtual Channel Identifier				Payload Type ID		CLP
Header Error Control
INFORMATION PAYLOAD

Generic Flow Control(GFC)


    The GFC field of the Header is only defined across the UNI. It is intended to control the traffic flow across the UNI to alleviates short term overload conditions. It is currently undefined and these 4 bits must be set to 0's.


Virtual Path Identifier(VPI)


    The VPI, an 8 bit field for the UNI and 12 bit for NNI, is used to identify virtual paths. In an idle cell, the VPI is set to all 0's.


Virtual Channel Identifier(VCI)


    This 16 bit field is used to identify a virtual channel. For idle cells, the VCI is set to all 0's. Together with the VPI, the VCI provides a unique local identification for the transmission.


Payload Type Identifier(PTI)


   The 3 bit for PTI are used for different purposes. Bit 4 is set to 1 to identify operation, administration, or maintenance cells. Bit 3 is set to 1 to indicate that congestion was experienced by a data cell in transmission and is only valid when bit 4 is set to 0. Bit 2 is used by AAL 5 to identify the data as type 0 when bit 4 is set to 0.


Cell Loss Priority(CLP)


   The 1 bit CLP field is used for explicit indication of the priority of the cell. It may be set by the AAL Layer to indicate cells to discard in cases of congestion, or by the network as part of the traffic management on commercial subscriber networks.


Header Error Control(HEC)


   The HEC is compared by each Switch as the ATM cell is received and all cells with HEC discrepancies are discarded. Cells with a single bit error may be subject to error correction.