Upload
others
View
3
Download
0
Embed Size (px)
Citation preview
Communication Networks
Chapter 7 – Connection Oriented Packet Data Networks
Communication Networks - 7. Connection Oriented PDNs 236
Overview
1. Fundamentals of Connection-Oriented Packet Switching
2. X.25
a) X.21
b) High-Level Data Link Control (HDLC)
c) X.25 Layer 3
3. Frame Relay
4. Asynchronous Transfer Mode (ATM)
Communication Networks - 7. Connection Oriented PDNs 237
7.1 Fundamentals
Packet Switching
• Circuit switching was designed for voice• Packet switching was designed for data Information transmitted in small packets Packets contain user data and control info
user data may be part of a larger message control info includes routing (addressing) info
Packets are received, stored briefly (buffered) and passed on to the next node
Communication Networks - 7. Connection Oriented PDNs 238
Packet Switching
Communication Networks - 7. Connection Oriented PDNs 239
7.1 Fundamentals
AE
SwitchingSystem
End System(User) End System
(User)Application Data
Application Data
7.1 Fundamentals
Packet-Switched Networks
• In packet-switched networks, data is packetized prior to transmission Each packet is a group of bits organized in a predetermined structure Each packet contains data bits as well as additional overhead information to ensure
error-free transmission to intended recipients Packets may be called blocks, cells, datagrams, data units, or frames
Communication Networks - 7. Connection Oriented PDNs 240
Packet-Switching Advantages and Disadvantages
Advantages• Single link shared by multiple senders and
receivers• No occupied links• Different data rates at sender and
receiver possible• Establishment of packet-priority systems• Charging by the volume of data (number
of packets) transmitted rather than connection time
Disadvantages• Variable, unforeseeable transmission
delays caused by packet processing and packet queues at packet switches
• Variable packet sizes leading to longer packet processing times at packet switches
• Overhead data in packets leading to lower data transmission efficiency and throughput than in circuit-switched networks
Communication Networks - 7. Connection Oriented PDNs 241
7.1 Fundamentals
Virtual Circuit
Communication Networks - 7. Connection Oriented PDNs 242
7.1 Fundamentals
AE
SwitchingSystem
End System(User) End System
(User)3 2 1
Virtual Circuit vs. Datagram
Virtual circuit• network can provide sequencing and
error control• packets are forwarded more quickly• less reliable in case of link or switch
failure
Datagram• no call setup phase• more flexible• more fault-tolerant, but no QoS
Communication Networks - 7. Connection Oriented PDNs 243
7.1 Fundamentals
(Real) Circuit vs. Virtual Circuit
Real Circuit• Connection-oriented• All information use the same path ordered delivery
• Resources dedicated to single connection
• Delay only depending on propagation delay of physical signal
• No overhead
Virtual Circuit• Connection-oriented• All Packets use the same path ordered delivery
• Resources shared between different connections
• Delays due to traffic on the path and delay caused by forwarding nodes
• Overhead due to packet header
Communication Networks - 7. Connection Oriented PDNs 244
7.1 Fundamentals
Event Timing
Communication Networks - 7. Connection Oriented PDNs 245
7.1 Fundamentals
According to Stallings
CR
CRCR
CC
User Data
CR
CRCR
Pkt 1Pkt 2Pkt 3
Pkt 1Pkt 2Pkt 3
Pkt 1Pkt 2Pkt 3
Pkt 1Pkt 2Pkt 3
Pkt 1Pkt 2Pkt 3
Pkt 1Pkt 2Pkt 3
ACKACK
ACK
CCCC
CC
(a) Circuit Switching (b) Virtual Circuit Packet Switching (c) Datagram Packet Switching
7.2 X.25
ITU-T Recommendation X.25
• ITU-T standard for interface between host and packet switched network:„Interface between Data Terminal Equipment and Data Circuit Terminating Equipment for Terminals Operating in the Packet Mode on Public Data Network”
• almost universal on packet switched networks and packet switching in ISDN• defines three layers
Physical Link Packet
Communication Networks - 7. Connection Oriented PDNs 246
7.2 X.25
Characteristics of X.25
• Virtual connections
Permanent virtual connections
Switched virtual connections
Several virtual connections at the same time (up to 4096)
• End-to-end flow control
• According to the ISO/OSI Basic Reference Model:
Physical layer: X.21
Transmission of data frames: HDLC
Switching: X.25
Communication Networks - 7. Connection Oriented PDNs 247
X.25 & the OSI Reference Model
Communication Networks - 7. Connection Oriented PDNs 248
7.2 X.25
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
Network Layer
Data Link Layer
Physical Layer
Packet Layer Protocol (PLP)
X.21 Layer: RS-232
High-Level Data Link Control (HDLC)
Link Access Procedure Balanced
X.25 Protocol Stack
ITU-T X.25 Recommendation
Communication Networks - 7. Connection Oriented PDNs 249
7.2 X.25
Packet Level Procedures
Physical Level Procedures
Frame Level Procedures
Layers 4-7End-to-End Connections
Layer 3X.25-3
Packet Interface
(Virtual Connections)
Layer 2HDLC (LAPB)
Data Link Interface
Layer 1X.21
Physical Interface
DTE DTE/DCE Interface DCE/Network
ISO/OSI layerson top of X.25
7.2 X.25
X.25 – Physical Layer
• Interface between station node link• Two ends are distinct Data Terminal Equipment DTE (user equipment) Data Circuit-terminating Equipment DCE (node)
• Physical layer specification is X.21• Can substitute alternative such as EIA-232
Communication Networks - 7. Connection Oriented PDNs 250
7.2 X.25
X.21 Interface
• X.21 covers the first three layers of the ISO/OSI BRM: Electrical and mechanical interface Error detection and
correction Circuit switching
Communication Networks - 7. Connection Oriented PDNs 251
DTE PIN CCITT-IDENTIFICATION DCEG/P Signal ground or common return
⇒ Ga DTE common return ⇒
⇐ Gb DCE common return ⇐
⇒ T Transmit ⇒
⇐ R Receive ⇐
⇒ C Control ⇒
⇐ I Indication ⇐
⇐ S Signal element timing ⇐
⇐ B Byte timing (frame timing) ⇐
7.2 X.25
X.25 – Data Link Layer
• Link Access Protocol Balanced (LAPB) Subset of High-Level Data Link Control (HDLC) Hybrid topology
• Reliable transfer of data over one link• Information sent as a sequence of frames
Communication Networks - 7. Connection Oriented PDNs 252
7.2 X.25
High-Level Data Link Control (HDLC)
• Bit-oriented, code-transparent Data Link Protocol• Half and full duplex mode• Point-to-point and point-to-multipoint configuration• Symmetrical and asymmetrical configuration• Piggybacking of acknowledgements• Flow control based on “Sliding Window”• Variants of HDLC:
IBM‘s SDLC (Synchronous Data Link Control) LAPB (Link Access Procedure, Balanced) as used by X.25 LAPD (ISDN, Link Access Procedure for the D-Channel) LLC (IEEE 802.2, Logical Link Control) PPP (Point-to-Point Protocol)
Communication Networks - 7. Connection Oriented PDNs 253
7.2 X.25
HDLC: Configurations
• Two types of stations: Primary station (sending commands); Secondary station (sending responses). Combined (hybrid) stations are possible, too,
• Information flow alternatives: Unbalanced control: Selecting
Primary station requests secondary station to receive data. Unbalanced control: Polling
Primary station requests secondary station to send data. Balanced control
Two hybrid station may send commands or responses.
Communication Networks - 7. Connection Oriented PDNs 254
HDLC: Point-to-Point Topology
Communication Networks - 7. Connection Oriented PDNs 255
7.2 X.25
HDLCHybridStation
HDLCHybridStation
Balanced link configuration
HDLC: Point-to-Multipoint Topology
Communication Networks - 7. Connection Oriented PDNs 256
7.2 X.25
HDLCPrimaryStation
HDLCSecondary
Station
HDLCSecondary
Station
HDLCSecondary
Station
Unbalanced link configuration
7.2 X.25
HDLC: Operating Modes
• Normal Response Mode (NRM) Unbalanced configuration in which only the primary terminal may initiate data transfer. The secondary terminal transmits data only in response to commands from the primary
terminal. The primary terminal polls the secondary terminal(s) to determine whether they have data to
transmit, and then selects one to transmit.• Asynchronous Response Mode (ARM)
Unbalanced configuration in which secondary terminals may transmit without permission from the primary terminal.
However, the primary terminal still retains responsibility for line initialization, error recovery, and logical disconnect.
• Asynchronous Balanced Mode (ABM) Balanced configuration in which either station may initiate the transmission.
Communication Networks - 7. Connection Oriented PDNs 257
7.2 X.25
HDLC: Frame Format
• HDLC frame fields include: Flag: used to delimit the beginning and end of a packet Address: specifies the address of the intended packet recipient or sender Control: transports packet type, sequence numbers and retransmission requests Frame check: used for error checking. CRC-16 or a 16-bit checksum may be used
with HDLC frames
Communication Networks - 7. Connection Oriented PDNs 258
Flag Address Control Frame CheckSequence FlagData
(Optional Octets)
8 bits 8 bits 8 bits 16 bits 8 bitsn bits (variable)
HDLC: Control Field
Communication Networks - 7. Connection Oriented PDNs 259
7.2 X.25
Flag Address Control Frame CheckSequence FlagData
(Optional Octets)
01111110 8 bits 8 bits 16 bits 01111110n bits (variable)
1 0 S SS Frame[S=Supervisory]P/F N(R)
1 1 M MU Frame[U=Unnumbered]P/F M M M
0I Frame[I=Information]P/FN(S) P/F N(R)
HDLC: I Frame and S Frame
I Frame
• Information Frame(both command or response) I (Information)
S Frame
• Supervisory Frame(both command or response)containing sequence number RR (Receive Ready) S S = 0 0 RNR (Receive not ready) S S = 0 1 REJ (Reject) S S = 1 0 SREJ (Selective Reject) S S = 1 1
Communication Networks - 7. Connection Oriented PDNs 260
7.2 X.25
0 N(S) P/F N(R) 1 0 S S P/F N(R)
HDLC: U Frame
Unnumbered Commands
• SNRM .............Set NRM• SARM ............. Set ARM• SABM ............. Set ABM• DISC ............... Disconnect• SNRME ...........Set extended NRM• SARME ....……..Set extended ARM• SABME ...………Set extended ABM• SIM .................Set Initialization Mode• UP .................. Unnumbered Poll• UI ................... Unnumbered Information• XID ................. Exchange Identification• RSET ...............Reset• TEST ............... Test
Unnumbered Responses
• UA .................. Unnumbered Acknowledgment• FRMR ............. Frame Reject• DM .................Disconnect Mode• RIM ................ Request Initialization Mode• RD .................. Request Disconnect• UI ................... Unnumbered Information• XID ................. Exchange Identification• TEST ............... Test
Communication Networks - 7. Connection Oriented PDNs 261
7.2 X.25
HDLC: MSC
Communication Networks - 7. Connection Oriented PDNs 262
7.2 X.25
Primary Station Secondary Station
Set NormalResponse Mode
Transfer of 3I Frames
Transfer of 7Additional
I FramesThen Awaiting
Acknowledgement
Data Transfer
Acknowledgment
Transfer of 2 I Framesincl. Acknowledgment
Ack without Data
Notation: <Command/Response, [N(S),] P/F, N(R)>half duplex mode,
Modulo 8, Window Size 7
7.2 X.25
X.25 – Packet Switching
• Provides logical connections (virtual circuit or virtual connection) between subscribers
• All data in this connection form a single stream between the end stations• All packets follow the same path through the network• Established permanently or on demand
Communication Networks - 7. Connection Oriented PDNs 263
7.2 X.25
Virtual Circuits
• Call setup packets Used to establish virtual circuits Identify the (currently) best path to the destination through the network
• Virtual circuit details stored in virtual circuit tables at packet switches• Logical channel = path followed by the packets in a virtual circuit Identified by logical channel number when created
• Two major types of virtual circuits: Switched virtual circuits (SVCs): similar to temporary circuit-switched connections Permanent virtual circuits (PVCs): similar to a leased, circuit-switched connection
Once a PVC is allocated, no call setup or call clearing is needed the logical circuit is permanently stored in virtual circuit tables
Communication Networks - 7. Connection Oriented PDNs 264
X.25: Packet• General Format Identifier (GFI)
to identify packet format• Logical Channel Identifier (LCI)
to identify the logical channel the packet belongs to
• Packet Controlto specify packet type and additional control information
Communication Networks - 7. Connection Oriented PDNs 265
7.2 X.25
Start of Frame (Flag)AddressControl
Logical ChannelPacket Control
Frame Check Sequence
End of Frame (Flag)
User Information
TransmissionDirection
According toHDLC-LAPB
According to X.25
According tohigher layersand application
According toHDLC-LAPB
LAPBControlHeader
X.25Packet Header
User Information
LAPBControl
Trailer
Time
GFI
7.2 X.25
X.25: Packet Header
• General Format Identifier, GFI: Bit 5 and 6 = Sequence numbering scheme: 01 = Modulo 8; 10 = Modulo 128 Bit 7 = Delivery Confirmation Bit Bit 8 = Data Qualifier Bit (for important control information) Bits 7 and 8 only relevant for special packet types
• Logical Channel Identifier, LCI: Divided into group and number (which is not important…) 212 = 4096 logical channels might be simultaneously active on one subscriber line
• Packet Type Identification: Currently 22 different packet types Packet for data exchange: 1st Bit in Octet 3 = 0 All other packets: 1st Bit in Octet 3 = 1
Communication Networks - 7. Connection Oriented PDNs 266
Octet 1
Octet 2
Octet 3
Bit 8 7 6 5 4 3 2 1GFI
Q D 0 1Logical Channel
Group (LCG)Logical Channel Number (LCN)
Packet Type Identification
X.25: Encapsulation
Communication Networks - 7. Connection Oriented PDNs 267
7.2 X.25
Data
DataPacketHeader
InformationControlAddressFlag FCS Flag
Bit stream according to X.21
Direction of Transmission
Layer 3
Layer 2
Layer 1
7.2 X.25
Error Correction
• Node-to-node (aka hop-to-hop or point-to-point) error detection and correction• Each packet is checked for errors at each packet switch before being forwarded to
the next hop on its path• If no errors are detected, an ACK is sent to the previous hop• If errors are detected, a NAK is sent to the previous hop which triggers
retransmission of the packet• Store-and-forward: packets are stored at switching nodes until positive
acknowledgements are received
Communication Networks - 7. Connection Oriented PDNs 268
7.2 X.25
X.25: Network Access Protocol
• Packet multiplexing on layer 3 Packets are delivered in statistical time multiplex in demand 4096 different layer 3 virtual connections use one physical link and one HDLC connection Virtual connections are differentiated by the Logical Channel Identifier
• Packet multiplexing at the interface between DCE and DTE:
Communication Networks - 7. Connection Oriented PDNs 269
Bidirectional Packet Flow
Packet belonging to logical channel “N”
DCE
1 1 4 3
2 1 3
N
Logical Channel Virtual Connection between DTE A and…
1 DTE B
2 DTE C
3 DTE D
4 DTE CA
7.2 X.25
X.25: Logical Channels and Identifiers
• Logical Channel Identifiers, LCIs LCIs group all packets that belong to a virtual X.25 connection on a network link Both directions use the same identifier LCIs are valid on a single link only The subscriber only knows the LCIs on the subscriber line LCIs are used for packet multiplexing
• On all links of a virtual connection, different LCIs are used• All LCIs are independent of each other
Communication Networks - 7. Connection Oriented PDNs 270
7.2 X.25
X25: Assignment of LCI
• Assignment of Logical Channel Identifiers: For each link independently During connection establishment by the initiating entity
• Call collision: Incoming Call Request and Outgoing Call Request using the same LCI Avoidance by a special LCI allocation method
• Logical Channel Identifiers are grouped for Permanent Virtual Circuits (PVC) for incoming only Switched Virtual Circuits (SVC) for outgoing only Switched Virtual Circuits (SVC) for incoming and outgoing Switched Virtual Circuits (SVC)
Communication Networks - 7. Connection Oriented PDNs 271
X.25: Allocation of LCIsIncoming Calls increase the LCIOutgoing Calls decrease the LCI
Communication Networks - 7. Connection Oriented PDNs 272
7.2 X.25
LCI Usage
0 not available
1 … High Number PVC for permanentvirtual circuits
… --
LIC (Low Number Incoming Calls)…
HIC (High Number Incoming Calls)
for incomingswitched virtualcircuits
… --
LTC (Low Number Two Way Calls)…
HTC (High Number Two Way Calls)
for both incomingand outgoingswitched virtual circuits
… --
LOC (Low Number Outgoing Calls)…
HOC (High Number Outgoing Calls)
for outgoingswitched virtualcircuits
… 4095 --
Currentlynot used
LCIs
7.2 X.25
X.25: Local Character of LCIs
• Complete addresses of sender and receiver only during connection setup
• Packets assigned to current virtual connection by an LCI valid only on single link
• No relation between the network addresses of the DTEs and the LCIs to be used
• A new virtual connection is assigned a currently unused (temporary) LCI several independent virtual connections between the same two DTEs possible
Communication Networks - 7. Connection Oriented PDNs 273
X.25 Network
1 2 3 52 3711 4095DTE C
... ... ...
1
49
28122813
4095
......
...
DTE A
1
7172
2177
4095
......
...
DTE
B
X.25: Packet „Call Request”
Communication Networks - 7. Connection Oriented PDNs 274
7.2 X.25
LCGGFI0 0 0 1
LCNPacket Type
0 0 0 0 1 0 1 1Address Length of
Called DTEAddress Length of
Calling DTE
Addresses of DTEs (calling, called) 0 0 0 0
Length of Performance Features0 0
Performance Features
Short information of calling user
Octet 1
2
3
4
8 7 6 5 4 3 2 1Bit
7.2 X.25
X.25: Connection Setup (1)
• Connection setup at the calling DTE: Selection of a currently unused LCI according to allocation rules Creation of a „Call Request“ packet:
Entry of LCIEntry of (complete) DTE addresses
− Destination Address and Source Address− Length of Address might go up to 15 digits
Entry of requested performance features, e.g.:− Reverse Charging Acceptance− Throughput Class− Transmission Rate
Short User Data
Communication Networks - 7. Connection Oriented PDNs 275
7.2 X.25
X.25: Connection Setup (2)
X.25 “Call Request“ packet encapsulated into an HDLC Information Frame Packet sent to the DCE to which the initiating DTE is connected DCE is forwarding the packet into the network which is responsible for delivering it to
the receiving DCE Receiving DCE is transmitting an X.25 „Incoming Call” Packet with an accordingly
selected LCI to the called DTE
Communication Networks - 7. Connection Oriented PDNs 276
7.2 X.25
X.25: Phases
• Three Phases: Connection Establishment Phase:
finished with the arrival of a „Call Connected” Packet at the calling DTE Data Phase:
Full duplex, bidirectional data transmission Disconnection Phase:
finished with the arrival of a „Clear Confirmation” Packets at the DTE that wanted to disconnect
Communication Networks - 7. Connection Oriented PDNs 277
Establishment PhaseEstablishment Phase
X.25: Message Sequence Chart
The arrival of a „Clear Confirmation” packet at the called entity might notnecessarily be the result of a „Clear Confirmation” packet of the calling entity
Communication Networks - 7. Connection Oriented PDNs 278
7.2 X.25
Interface of calling DTE/DCE Interface of called DTE/DCE
Call Request Packet IncomingCall Packet
Call Accepted PacketCallConnected Packet
Data Packet
Data Packet
Data Packet
Data Packet
Data PacketData Packet
ClearIndication Packet Clear Request Packet
Clear ConfirmationPacket
ClearConfirmation Packet Disconnection Phase
Data Phase
Disconnection Phase
Data Phase
7.2 X.25
X.25: Data Packet
• Sequence numbers either Modulo 8 or Modulo 128• Virtual connections always end-to-end: Sequence numbers for sequence guarantees and acknowledgements just like in HDLC Sequence numbers used end-to-end and not only on one link
• Control information of the data packet: P(S):
Packet Send Sequence Number
P(R): Packet Receive Sequence Number
M-Bit (More Data Bit): Several packets belong to the same context E.g. a segmented transport layer PDU
Communication Networks - 7. Connection Oriented PDNs 279
7.2 X.25
X.25: Data Packet
• Sequence numbers are Modulo 8:
Communication Networks - 7. Connection Oriented PDNs 280
Octet GFIQ D 0 1 LCG
LCN
P(R) M P(S) 0
Bit 8 7 6 5 4 3 2 1
User Information(e.g. transport layer PDU)
2
3
4
1
...
7.2 X.25
Issues with X.25
• Key features include: Call control packets: in band signaling Multiplexing of virtual circuits at layer 3 Both layers 2 and 3 include flow and error control
• Considerable overhead• Not appropriate for modern digital systems with high reliability
Communication Networks - 7. Connection Oriented PDNs 281
7.2 X.25
Important X.25 PDN Standards
X.25: defines interface between DTE and DCE in public data networksX.21: specifies the interface between user terminal equipment and
PDN packet-switching nodesX.3: specifies packet assembly/disassembly processesX.28: governs asynchronous dial-up access to PDNsX.29: governs synchronous dial-up access to PDNsX.75: defines the interface between different public packet-switching
networks, both domestic and internationalX.121: defines a global addressing scheme for PDNs
Communication Networks - 7. Connection Oriented PDNs 282
7.3 Frame Relay
Advancement: Frame Relay
• Influenced by: Requests for higher throughput / bandwidth Lower error rates on the physical medium supersedes extensive error detection and
correction mechanisms as in X.25
• Standardization: Initial proposals for the standardization of Frame Relay presented to the Consultative
Committee on International Telephone and Telegraph (CCITT) in 1984 ITU-T Standard I.122: „Packet Mode Bearer Services for ISDN”
In 1990, Cisco, Digital Equipment Corporation (DEC), Northern Telecom, and StrataCom formed a consortium to focus on Frame Relay technology development:“Frame Relay Specification with Extensions”
Establishment of Frame Relay Forum, which is now the Broadband Forum (http://www.broadband-forum.org/)
Communication Networks - 7. Connection Oriented PDNs 283
Frame Relay: Frame
Communication Networks - 7. Connection Oriented PDNs 284
7.3 Frame Relay
Flag Information FCS Flag
Frame Header
DLCI
8 7 6 5 4 3 2 1
C/REA
8 7 6 5 4 3 2 1 Bit
DLCI
FECNBECN
DE EADLCI = Data Link Connection IdentifierC/R = Command/Response FieldFECN = Forward Explicit Congestion NotificationBECN = Backward Explicit Congestion NotificationDE = Discard Eligibility IndicatorEA = Address Field Extension Bit
New HDLC Variant: LAPF (Link Access Procedure, Frame Mode)
Frame Relay: Topology
Communication Networks - 7. Connection Oriented PDNs 285
7.3 Frame Relay
Frame Relay-Network
A
BC
Router
Router
Router
Router
LAN 1
LAN 2
LAN 3
LAN 4
DLCI 3
7.3 Frame Relay
Frame Relay: Explicit Congestion Notification
Congestion avoidance policy:• Forward Explicit Congestion Notification FECN bit set to 1 to indicate that congestion was experienced in the direction
of the frame transmission, so it informs the destination that congestion has occurred
• Backwards Explicit Congestion Notification BECN bit set to 1 to indicate that congestion was experienced in the network
in the direction opposite of the frame transmission, so it informs the sender that congestion has occurred
Communication Networks - 7. Connection Oriented PDNs 286
Comparison Frame Relay – X.25
Communication Networks - 7. Connection Oriented PDNs 287
7.3 Frame Relay
X.25
Throughput slow(200 kbit/s – 2 Mbit/s)
Latency
JitterError Detection &
CorrectionFunctionality
high
No countermeasure
Special mechanisms(retransmission on layer 2 and 3)
complex
Frame Relayfast
(2Mbit/s typically, 45 Mbit/s max)low
(due to less overhead)
No countermeasure
Responsibility of higher layers
simple
• Frame Relay discards frames (in congestion or with wrong FCS) – higher layers have to correct this problem.• Isochronous transmission is not possible with both protocols.
Comparison Frame Relay –X.25Taken from „The Basic Guide toFrame Relay Networking“ (Frame Relay Forum, http://www.frforum.com)
Communication Networks - 7. Connection Oriented PDNs 288
7.3 Frame Relay
7.4 ATM
Broadband Applications According to ITU-T
• Interactive Services Conversational Services
e.g. video conferencing, interactive work on shared documents Messaging Services
e.g. transfer of multimedia messages (videos, high resolution pictures) Retrieval Services
e.g. video on demand, access to high resolution pictures• Distribution Services ... without User Individual Presentation Control (Broadcast Services)
e.g. television broadcast ... with User Individual Presentation Control
e.g. Broadband videotext
Communication Networks - 7. Connection Oriented PDNs 289
Traffic
STM – Synchronous Transfer Mode
Isochronous data traffic (constant data rate and constant latency)
Example: Narrowband-ISDN Circuit-switched communication
channels
ATM – Asynchronous Transfer Mode
Transport of small data packets(ATM cells)
Data rate and latency are variable, but guarantees are possible
Packet-switched communication(connection-oriented: virtual channels)
Communication Networks - 7. Connection Oriented PDNs 290
7.4 ATM
7.4 ATM
ATM: Principle
• Technology for backbone networks• Statistical (asynchronous, demand-driven) time multiplex (ATDM)• Cell header includes identifier of virtual connection• Mixing of different cell rates possible
Different throughput for different virtual connections• Support of variable data rates:
Guaranteed base rate Bursty traffic possible, if there are enough resources
• ATM cell:
Communication Networks - 7. Connection Oriented PDNs 291
Cell Header User Data
5 Octets 48 Octets
7.4 ATM
ATM: Multiplexing
• Advantages of small size of ATM cells: Easy Multiplexing of different streams Support for different bit rates Support of different services
• Easy processing of ATM cells: High data rates Complex end systems, network equipment can be kept simple
Communication Networks - 7. Connection Oriented PDNs 292
ATM ist a fundamental concept,on which different networks can be based
ConstantBit Rate
VariableBit Rate
155 Mbit/s
7.4 ATM
ATM: Cell Header (UNI)
• GFC (Generic Flow Control): local control functions such as identification of several terminals connected to the same ATM interface
• VPI, VCI (see next slide)• PT (Payload Type): type of data in the information field
(user data or control data)• CLP (Cell Loss Priority): determines whether a cell can
be preferentially deleted or not in case of a transmission bottleneck. Cells with CLP=0 have higher priority than cells with CLP=1.
• HEC (Header Error Control)
At NNI, the VPI field is 4 bits longer, as the GFC is not needed there
Communication Networks - 7. Connection Oriented PDNs 293
GFC VPI
VPI VCI
VCI
VCI PT CLP
HEC
1
2
3
4
5
[Byte]
Cell Header User Data
5 Octets 48 Octets
7.4 ATM
ATM: Virtual Channel and Virtual Connection
• Definition of two different connection types: Virtual Channel (VC):
unidirectional virtual connection: Virtual Path (VP):
Bundle of virtual channels with the same end points• Accordingly identified in the cell header:
Virtual Channel Identifier (VCI) Virtual Path Identifier (VPI)
Communication Networks - 7. Connection Oriented PDNs 294
VCVC
VC
... Transmission PathVP
VP
7.4 ATM
ATM: Traffic Engineering
• CBR (constant bit rate): a peak cell rate (PCR) is specified, which is constant, and a maximum jitter value
• VBR (variable bit rate):an average cell rate is specified, which can peak at a certain level for a maximum interval before being problematic (typical e.g. for MPEG streams)
• ABR (available bit rate):a minimum guaranteed rate is specified; if the rate cannot be sustained, the sender is notified
• UBR (unspecified bit rate):no guarantees, just best effort transmission
Communication Networks - 7. Connection Oriented PDNs 295
CBR
VBR
UBR
ABR
t
7.4 ATM
ATM and AAL
• ATM Layer: service-independent transport of ATM cells, multiplexing and demultiplexing• AAL Layer: ATM Adaptation Layer – support of different services
Communication Networks - 7. Connection Oriented PDNs 296
physicallayer
ATM
AAL
physicallayer
ATM
AAL
End System A End System B
ATM-Network
Service-dependentAAL connections
Service-independentATM connections
Application
7.4 ATM
ATM Networks and Broadband-ISDN
• ATM networks provide data transmission services for realtime applications for applications with variable bit rates
• Broadband-ISDN was realized as an ATM-networkin Germany: T-Net-ATM
• ATM is also applied in broadband data networks with high bit rates• ATM provides the infrastructure for IP networks
LANE (LAN Emulation) CIP (Classical IP over ATM) MPOA (Multi-Protocol over ATM)
• Standardization: ITU-T, ATM-Forum, IETF
Communication Networks - 7. Connection Oriented PDNs 297
ATM: Support of Voice and Data Services
Communication Networks - 7. Connection Oriented PDNs 298
7.4 ATM
ATM-SwitchATMSubnetwork
ATMAdaptationLayer
Services
Cells Cells Cells Cells
Packets PacketsBit Stream Bit Stream
B-ISDN Reference Model (I)
• 3-dimensional reference model• Three vertical planes
User Plane Control Plane Management Plane
• Three hierarchical Layers Physical Layer ATM Layer ATM Adaptation Layer
• Signaling information is transferred separately from the user information (Out-of-Band-Signalling)
Communication Networks - 7. Connection Oriented PDNs 299
7.4 ATM
Physical Layer
ATM Layer
ATM Adaptation Layer
Higher Layers
Management Plane
ControlPlane
UserPlane
Layer Managem
ent
Plane Managem
ent
PlanesLayers
B-ISDN Reference Model (II)
Communication Networks - 7. Connection Oriented PDNs 300
7.4 ATM
Physical layer
ATM layer
SignalingAAL AAL1 AAL2 AAL3/4
or AAL5
Signaling ClassA
ClassB
ClassC
ClassD
Controlplane
User plane
Management plane
Higherlayers
ATMadaptation layer
7.4 ATM
ATM Adaptation Layer AAL
• Different AAL for different service types• AAL0 : empty AAL = no adaptation required• AAL consists of sublayers Segmentation and Reassembly Sublayer (SAR-Sublayer) Convergence Sublayer (CS)
Common Part Convergence Sublayer (CPCS) Service Specific Convergence Sublayer (SSCS)
Communication Networks - 7. Connection Oriented PDNs 301
References
References
• Kasera, Sumit (2007): ATM Networks. Concepts and Protocols. New York: McGraw-Hill (McGraw-Hill Communications).
• Stallings, William; Case, Thomas (2012): Business Data Communications. Infrastructure, Networking and Security. 7th edition. Upper Saddle River, N.J., London: Prentice Hall; Pearson Education.
• Stallings, William (2014): Data and Computer Communications. 10th edition. Upper Saddle River, N.J.: Pearson.
• Stallings, William (1999): ISDN and Broadband ISDN with Frame Relay and ATM. 4th edition. Upper Saddle River,N.J.: Prentice Hall.
• Stamper, David A.; Case, Thomas (2003): Business Data Communications. 6th edition. Princeton, N.J.: Prentice Hall.
Communication Networks - 7. Connection Oriented PDNs 302