ATMnair/courses/7344/6_atm_1_v4b.pdf · 2004. 9. 11. · Title: ATM Author: Suku Nair Created Date:...
Transcript of ATMnair/courses/7344/6_atm_1_v4b.pdf · 2004. 9. 11. · Title: ATM Author: Suku Nair Created Date:...
SMU CSE 5344/7344 1
ATM
Class 6Cell Switching
SMU CSE 5344/7344 2
Topics
• Network Architecture• ATM Protocol Architecture• Logical connections• ATM cell structure• Service levels/categories• ATM Adaptation Layer (AAL)
SMU CSE 5344/7344 3
IntroductionMoving from LAN to WAN need
– Higher speed– Traffic aggregation– Multi-protocol handling– Multiple-vendor support
Various Solutions– Frame Relay– Asynchronous Transfer Mode – aka Cell Relay– Connection oriented cell-based
• vs. packet switching• vs. frame switching• vs. telephone
SMU CSE 5344/7344 4
Background • ATM evolved from B-ISDN development efforts
– Frame Relay: high-speed WAN (1.5+ Mbps)
– ATM: very high speed WAN (155 Mbps and 622Mbps)
• ATM, like Frame Relay, was built on the assumption that the underlying physical media was reliable and flexible– minimal error and flow control capabilities
– even more streamlined, therefore faster, than Frame Relay
• Specifications developed by ITU-T and ATM Forum
SMU CSE 5344/7344 5
ATM Network Architecture
Public UNI connects ATM end-systems to a public ATM networkPrivate UNI connects ATM end-systems to a private ATM networkResidential UNI which is a variant of the public UNI for residences or small businesses
Public NNI interconnects ATM switches in the public network of the same providerPrivate NNI (PNNI) interconnects ATM switches in a private networkBroadband inter-carrier interface (B-ICI) – connects two public networks of different providers
SMU CSE 5344/7344 6
Network Interface Points
SMU CSE 5344/7344 7
Datagram (Switching & Forwarding) –Connectionless Routing
0
132
01 3
2
013
2
Switch 3 Host B
Switch 2
Host A
Switch 1
Host C
Host D
Host EHost F
Host G
Host H
SMU CSE 5344/7344 8
Connectionless Routing
Host can send a packet anywhere any time– Host does not know network congestion– Host does not know states of intervening
routers/switches– Packets are forwarded independently of each
other– Packets contain routing information– Link failures have minimal impact on packet
delivery
SMU CSE 5344/7344 9
Virtual Circuit Switching
0
13
2
01 3
2
0
13
2
Switch 3
Host B
Switch 2
Host A
Switch 1
5 11
4
7
SMU CSE 5344/7344 10
Connection-oriented RoutingHost requests to send a packet somewhere
– Host requests bandwidth which may or may not be available
– All intervening routers/switches must comply with the request
– Packets are forwarded sequentially and do not (should not!) go out of order
– Packets contain VCI information– Link failures have an impact on packet delivery
– must be torn-down & explicitly rebuiltKey difference signaling must be used to setup the
connection before traffic is admitted
SMU CSE 5344/7344 11
ATM Protocol
SMU CSE 5344/7344 12
ATM Protocol Architecture
• Fixed-size packets called cells– “cell switching” like packet switching
• 2 primary protocol layers relate to ATM functions:– Common layer providing packet transfers,
logical connections (ATM)– Service dependent ATM adaptation layer (AAL)
• AAL maps other protocols to ATM– like IP (AAL5)
SMU CSE 5344/7344 13
ATM Protocol Reference Model
Various data rates (155.52 Mbps, 622.08 Mbps) over various physical media types (Fiber Optic, SONET, UTP, etc.)
Framing, cell structure& Logical Connections
Map data to the ATM cellstructure
SMU CSE 5344/7344 14
Protocol Model3 Application Function Planes
– User – provides user (station) information transfer and associated controls (flow control, congestion control) – traffic
– Control – performs call control and connectioncontrol functions – signaling
– Management – provides plane management and layer management and coordination functions
• ATM Adaptation Layer (AAL)• ATM Layer• Physical Layer
SMU CSE 5344/7344 15
User Plane Layers
AAL
ATM
User information
User information
AAL
ATM
PHYPHY
ATM
PHY
ATM
PHY
…
End system End systemNetwork
SMU CSE 5344/7344 16
Virtual Circuits
SMU CSE 5344/7344 17
Logical Connections• VCC (Virtual Channel Connection)
– a logical connection analogous to a virtual circuit in X.25, or Frame Relay data link connection
• full-duplex flow between end users• user-network control signaling• network-network management/routing
• VPC (Virtual Path Connection)– a bundle of VCCs that
• Have the same end points (not necessarily same end-users)
• Are switched along the same virtual path(es)
SMU CSE 5344/7344 18
ATM Connection Relationships
Physical Transmission Medium
SMU CSE 5344/7344 19
ATM VPC/VCC
c ATMSw1
ATMSw4
ATMSw2
ATMSw3
ab
de
VP3 VP5
VP2
VP1
a
bc
de
ATMDCC
VP6
Sw = switch
DCC = Cross-connect switch
SMU CSE 5344/7344 20
VCC (logical connection) Uses
• Exchange between end users– user data– control signaling (more later)
• Exchange between an end user and a network entity– control signaling (more later)
• Exchange between 2 network entities– traffic management– routing functions
SMU CSE 5344/7344 21
Advantages of Virtual Paths• Simplified network architecture
– allows separation of functionality into into individual logical connections and related groups of logical connections
• Increased network performance and reliability– network consists of fewer aggregated entities
• Reduced processing and short connection setup time– complex setup tasks are in virtual paths– simplifies setup of new virtual channels over existing virtual path
• Enhanced network services – supports user-specified closed groups/networks of VC bundles
SMU CSE 5344/7344 22
Cell-based Switching within VCs
SMU CSE 5344/7344 23
Cell Format and Switching• Virtual Path Identifier (VPI)• Virtual Channel Identifier (VCI)
– ATM LANs do all the switching using VPs and VCs are used for multiplexing/de-multiplexing
• 5 byte header and 48 byte payload• Cell loss priority, Header error checksum
4 16 3 18
Payload
384848 bytes5 bytes
GFC HEC (CRC-8)VPI VCI CLPType
SMU CSE 5344/7344 24
ATM Cell Format
UNI NNI
SMU CSE 5344/7344 25
Header Format
• Generic flow control (more )
• Virtual path identifier (VPI)• Virtual channel identifier (VCI)• Payload type
– (3 bits: identifies cell as user data or network management cell, presence of congestion, SDU type)
• Cell loss priority (0: high don’t lose; 1: low)• Header error control (more )
SMU CSE 5344/7344 26
Generic Flow Control
• Controls traffic flow at UNI– alleviates short-term overload conditions– Note: not employed in network core
• When GFC is enabled at the UNI, two procedures are used:– Uncontrolled transmission
• not subject to flow control– Controlled transmission
• flow control constraints (using GFC) are in force
SMU CSE 5344/7344 27
Header Error Control• 8-bit CRC• error detection• in some cases, error correction of single-
bit errors in header• 2 modes:
– Error detection– Error correction
SMU CSE 5344/7344 28
HEC Operation at Receiver
Based on recognition of fact that bit errors occur in bursts
SMU CSE 5344/7344 29
Effect of Error at Cell Header
SMU CSE 5344/7344 30
ATM Adaptation Layer (AAL)
• Support higher-level protocols– e.g., PCM voice, LAPF, IP
• AAL Services– Handle transmission errors– Segmentation/reassembly (SAR)– Handle lost and misinserted cell
conditions– Flow control and timing control
SMU CSE 5344/7344 31
Applications of AAL and ATM
• Circuit Emulation (e.g., T-1 synchronous TDM circuits)
• VBR voice and video• General data services• IP over ATM• Multiprotocol encapsulation over ATM
(MPOA)• LAN Emulation (LANE)
SMU CSE 5344/7344 32
Segmentation and Reassembly
AAL
ATM
AAL
ATM
… …
SMU CSE 5344/7344 33
ATM Adaptation Layer Types
AAL provides different services according to:– Time relationship between source and
destination – Bit rate - fixed vs. variable– Connection mode – connection oriented
vs. connectionless• Types
– AAL1, AAL2, AAL3/4, AAL5
SMU CSE 5344/7344 34
AAL Protocol and Services
Basis for classification:• requirement for a timing relationship between source and destination
• requirement for a constant bit rate data flow• connection or connectionless transfer
SMU CSE 5344/7344 35
AAL Protocols
• AAL layer has 2 sublayers:– Convergence Sublayer (CS)
• Supports specific applications using AAL• Users attach via the Service Access Point
(like port number)• Common part (CPCS) and application service-
specific part (SSCS)– Segmentation and Reassembly Sublayer
(SAR)• Packages data from CS into ATM cells and
unpacks at other end
SMU CSE 5344/7344 36
AAL Protocols and PDUs
SMU CSE 5344/7344 37
AAL Protocol Types
SMU CSE 5344/7344 38
Segmentation and Reassembly PDUs
SMU CSE 5344/7344 39
AAL Type 1
• Constant-bit-rate source• SAR simply packs bits into cells and
unpacks them at destination• One-octet header contains 3-bit SC
field to provide an 8-cell frame structure
• No CS PDU structure is defined since CS sublayer primarily for clocking and synchronization
SMU CSE 5344/7344 40
AAL Type 2• Intended for use with applications with low
variable bit-rate service on multiple channels (multiplexing)– Mini-cells
• Initial specification has been withdrawn
SMU CSE 5344/7344 41
AAL Type 3/4
• Intended for applications that generate bursty data and demand low loss
• Originally delineated as– Connectionless (AAL4) or – Connection (AAL3) oriented
• May be message mode or streaming mode
SMU CSE 5344/7344 42
AAL 3/4 Example (message mode)
SMU CSE 5344/7344 43
Adaptation Layer 3/4CS PDU
CPI Btag BASize Pad 0 Etag Len
8 16 0– 24 8 8 16< 64 KB8
User data
ATM Cell
2 44 25
ATM header Length CRC-10
40 2 4
SEQ MIDType Payload
352 (44 bytes)10 6 10
AAL 3/4
53
SMU CSE 5344/7344 44
Encapsulation and Segmentation
CS-PDUheader
CS-PDUtrailerUser data
44 bytes 44 bytes 44 bytes ≤ 44 bytes
ATM headerAAL header
Cell payloadAAL trailer
Padding
244
2
5
SMU CSE 5344/7344 45
AAL Type 5
• Streamlined transport for connection oriented protocols– Reduce protocol processing overhead– Reduce transmission overhead– Ensure adaptability to existing transport
protocols– primary function is segmentation and
reassembly of higher-level PDUs
SMU CSE 5344/7344 46
AAL5 Example
SMU CSE 5344/7344 47
CPCS PDUs: AAL 3/4 , AAL5
SMU CSE 5344/7344 48
AAL5CRC-32
< 64 KB 0– 47 bytes 16 16
ReservedPad Len
32
Data
CS-PDUtrailerUser data
48 bytes 48 bytes 48 bytes
ATM headerCell payload
Padding
SMU CSE 5344/7344 49
ATM Layer
• Assigns header to the segment streams generated by AAL
• Routing and multiplexing of cells over VCs
• Maintains VCI table
SMU CSE 5344/7344 50
Topics for Next Time
• ATM Switch design• ATM LAN emulation