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Transcript of 1 Fall 2005 Virtual Circuit Switching and ATM: Asynchronous Transfer Mode Qutaibah Malluhi CSE...
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Fall 2005
Virtual Circuit Switching andATM: Asynchronous Transfer
Mode
Qutaibah MalluhiCSE DepartmentQatar University
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Types of WANs
Dedicated-circuit Networks
Switched Networks– Circuit-switched and packet-switched Networks
» Virtual Circuit approach
Local Loop Technologies– E.g., DSL, Cable Modem
Wireless WAN (Cellular Network)
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Dedicated Circuit Networks
Lease circuits from common carriers All connections are point-to-point. A flat fee per month. Unlimited use of the circuit Several standards specified by the telephone
industry in each country– T-Series Carrier Services (US)
» T1: 1.5 Mpbs, T2: 6.312 Mbps, T3: 44.736 Mbps
– E Services (Europe & Qatar)» E1: 2.048 Mbps, E2: 8.448 Mbps, E2 : 34.368 Mbps
– Other high-speed Services » E.g., Optical Carrier (OC) Standard:
51.840 Mbps (OC-1) to 2,488.320 Mbps (OC-48)
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Switched Networks
Circuit-switched networks – Form a dedicated connection (circuit) between two points
– Guaranteed capacity but high-cost (cost is fixed and is independent of the traffic)
– Circuit-switched networks operates over the PSTN (public switched telephone network)
– No link sharing
– E.g. Plain Old Telephone Service (POTS) and Integrated Services Digital Network (ISDN)
Packet switched networks– Data divided into small packets. Each packet is sent
individually
– Link is shared by multiple transmissions
– E.g. IP datagram delivery
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Packet Switching
Datagram Switching– Each packet contains the destination address and
sequence number to each packet– A route is independently chosen for EACH packet.– The packets may arrive out of sequence.– E.g., Ethernet, IP
Virtual Circuit Switching– Similar to dedicated circuits unlike circuit switching,
which is a physical layer technology, it is a Data link layer technology.
– A preplanned route is established before data transmission.
– All packets for one transmission take the same route– Each packet contains a virtual circuit identifier.– E.g. Frame Relay and ATM
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VCI: Virtual Circuit Identifier
VCI is used to identify a virtual circuit between the sender and the receiver
A small number that has the switch scope Used by a frame between two switches VCI in a data frame changes from one switch to
another
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Switch Forwarding Table
Has a table entry for each VC 4 columns: (In Port, In VCI), (Out VCI, Out Port) Maps an incoming (port, VCI) into an outgoing
(port, VCI)
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Communication Phases
Sender and receiver go through three phases– Setup
» create virtual circuit forwarding table entries
– Data transfer– Teardown
» Delete VC virtual circuit forwarding table entries
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Types of Virtual Circuits
Permanent Virtual Circuit (PVC)– Forwarding table entries are setup manually by the
administrator– Like a point-to-point leased telephone circuit– No dialing is needed, circuit is always up and ready– Costly (pay even if you are not using it)
Switched Virtual Circuit (SVC)– Dynamic on-demand creation of connections– Exists when data is being transferred. Tear down when
session ends.– Connection (dialing) phase is required (Also called
Signaling)
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Asynchronous Transfer Mode (ATM)
Designed by phone companies Single technology meant to handle
– Voice– Video– Data
Intended as LAN or WAN
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ATM Characteristics
End-to-end (application to application)
Connection-oriented interface:– Establish “connection”– Send data– Teardown connection
Performance guarantees (statistical) Uses cell switching
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ATM Switch
Building block of ATM network Connections to
– Computers—User-Network Interface (UNI)– Other ATM switches– Network-Network Interface
(NNI) Accepts and forwards cells
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ATM Design Issues
Different traffic has different demands Variable packet size introduces more
jitter (variance in delivery time) Even sending at a constant rate,
contention can result jitter Small packets incur less jitter and
delay, but less efficient Large packets more efficient, delay
and jitter is more serious (packet loss)
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ATM Cell
A cell network uses the cell as the basic unit of data exchange.
A cell is a small, fixed-sized block of information. Size chosen as compromise between voice
(small) and data (large)– 5 octet header– 48 octet payload
Note: size not optimal for any application
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ATM Cell Format
GFC: Flow control – local flow control (user-to-network only). doesn't appear in
network-to-network interface VPI/VCI: virtual circuit identification
– together provide identification of the cell connection (see later)– Only difference between NNI and UNI cells is that NNI VPI is
a larger field PT - Payload type
– indicates the type of the cell (e.g. user data cell, resource management cell, operation and maintenance cell)
CLP: Cell Loss Priority– one bit specifying whether or not the cell can be dropped (e.g.
voice/video is loss tolerant) HEC - Header Error Control
– 8-bit CRC
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Cell Forwarding
Virtual Circuit Switching Also label switching: Uses label in cell
– Label is used to identify the virtual circuit– Label is specified by the pair: Virtual Path
Identifier/Virtual Channel Identifier (VPI/VCI)
Like standard virtual circuit switching, VPI/VCI is rewritten at each switch
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TP, VP and VC
TP: Transmission Path– Physical connection
VP: Virtual Path– specified by VPI– Set of connections
between two end devices– Path the VC follows
through the network VC: Virtual Circuit
– specified by VCI– All cells of the same
message follow the same VC
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Example II Of VPI/VCI Rewriting
Sender uses VPI/VCI 3 Receiver uses VPI/VCI 6 Intermediate VPI/VCIs are established within
each switch
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ATM Layers
Physical Layer– Can be one of several physical layer technologies (e.g.
SONET) ATM Layer
– Routing, traffic management, switching and multiplexing
– Receives 48 byte segment from AAL sublayers and transform it into a 53 byte cell
AAL: Application Adaptation Layer– Depend on type of application: data frames, stream of
bits, CBR
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Application Adaptation Layers AAL1:
– Constant bit rate (CBR) – Example: audio
AAL2: – Variable bit rate (VBR)
» Example: video with adaptive encoding– Low bit rate and short packet traffic
» Audio, video, and fax AAL3/4:
– conventional packet switching with sequencing and error control
AAL5: – Used for sending data (not audio/video)– Simple and efficient adaptation layer– No sequencing: assumes cells corresponding to a message
travel sequentially– No error control: left to upper layers (e.g. TCP/IP)
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ATM Quality Of Service
Specified when connection established Endpoint specifies
– Type of data transfer– Throughput desired– Maximum packet burst size– Maximum delay tolerated
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Issues
More expensive than traditional LAN technology More connection setup time Cell tax (header/data ~= 10%) Need to specify service requirement at the
connection, some may not know which to specify Lack of efficient broadcast Complexity of QoS (Quality of Service): one can
specify the request, but hard to enforce it Assumption of homogeneity