Asynchronous Transfer Mode ATM EECS Instructional 异步传输模式ATM EECS教学.ppt

AAL user
CS AAL
SAR sublayer
ATM layer
Physical layer
Layer above ATM
Service dependent
Mask ATM specifics from user; universality
SAR: segmentation and reassembly. “Translates” service data from a non-ATM format into ATM cells, then back again at destination
VPC exists? Yes
No
Establish new VPC
Can QoS be Yes satisfied?
No
Block VCC or request more
capacity
Request Yes granted?
No
Reject VCC request
Make connection
ATM Adaptation Layer (AAL)
Info field, 48 bytes
53 byte cell
VC’s are not always VC’s
Virtual Channel
Transmission is connection-oriented
VC set up by some signaling protocol before any cells can be sent
Contrast with STM
“Bandwidth on demand”—Slots assigned on demand, users take any empty slot
Nothing pre-assigned, no global timing Slot Cell, fixed size of 53 bytes Arbitrary bit rates: can support T-1 using
SDU = Service Data Unit OAM = Operations, Administration, and Maintenance
ATM Cells
Small size, may reduce queuing delay of high priority cells Fixed size, more efficient switching
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 OAM segment associated cell 101 OAM end-to-end associated cell 110 Resource management cell 111 Reserved for future function
CS: convergence sublayer. Takes care of delay jitter, error checking, remove corrupted cells
ATM Service Categories
Real-time services
Constant bit rate – uncompressed audio/video info
Streamlined protocol, minimal error and flow control capabilities
Fixed packet size = ATM cell
Simplified processing, management
Synchronous Transfer Mode
Unspecified bit rate – best-effort service
e.g. text/image messaging, telecommuting
Available bit rate – bursty apps requiring reliable endto-end connection
UNI
GFC VP identifier VP identifier
VC identifier PT CLP
Header error control
5-byte header
Info field, 48 bytes
NNI
VP identifier
VC identifier PL type CLP
Header error control
bit errors in header Information – 48 bytes
Stallings, Data & Computer Communications, 6th ed., Table 11.2
PT coding ______________Interpretation____________
Pre-assigned “slots,” frame boundaries, global timing
Slots identified by position from the start of the frame
BW allocated in units of slots Idle slots wasted Efficient for Constant Bit Rate traffic
CBR, voice/video using real-time VBR, IPbased traffic using ABR and UBR, etc. Each cell must be self-identifying (overhead)
ATM cell contents
Header – 5 bytes General Flow Control (GFC) – traffic control for
Many types of traffic
Large geographic distribution
Traffic modeling, control
Virtual Path Connection (VPC), bundle of VCC’s
Physical channel
Virtual path
Virtual channels
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Logical connections
VPC = bundle of VCC’s with the same endpoints all switched together
Network management of group of connections, not many individual ones
Setup time is for a VP, adding VC’s to it involves minimal processing
Request for VCC originates
Asynchronous Transfer Mode (ATM)
EE290F 22 April 2004 Shanna Crankshaw
ATM – definition
"A transfer mode in which information is organized into cells; it is asynchronous in the sense that the recurrence of cells containing information from an individual user is not necessarily periodic".
different QoS, alleviates short-term overloads VPI – routing field for network VCI – routing to/from user Payload type Cell loss priority (CLP) Header error control (HEC) – can correct single
Efficient use of network resources Bandwidth on demand concept
Simplified network infrastructure
ATM challenges
In-network mux/buffering can lead to cell delay or loss QoS guarantees
What is it really?
Fast packet switching, high data
rates
Low-level network layer—above physical layer, below AAL (ATM adaptation layer)
Single transport mechanism for different types of traffic (voice, data, video, etc.)
Videoconferencing, TV, pay-per-view, VOD, etc.
rt-Variable bit rate
Non-real-time services
nrt-VBR – high end system QoS, critical response time
Bank transactions, airline reservations, etc.
e.g. LAN, router-to-router reliability
ATM advantages
Universality
Mixed traffic types, real-time and non-real-time
Scalability
LANs, MANs, WANs, WLANs