Communication Networks Winter 2017/18
Communication Networks
Chapter 7 – Connection Oriented Packet Data Networks
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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)
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Prof. Jochen Seitz 1 Communication Networks Winter 2017/18
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
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7.1 Fundamentals Packet Switching
End System (User) Application Data Application Data End System (User)
A Switching E System
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Prof. Jochen Seitz 2 Communication Networks Winter 2017/18
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
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7.1 Fundamentals Packet-Switching Advantages and Disadvantages
Advantages Disadvantages • Single link shared by multiple senders and • Variable, unforeseeable transmission receivers delays caused by packet processing • No occupied links and packet queues at packet switches • Different data rates at sender and • Variable packet sizes leading to longer receiver possible packet processing times at packet switches • Establishment of packet-priority systems • Overhead data in packets leading to • Charging by the volume of data (number lower data transmission efficiency and of packets) transmitted rather than throughput than in circuit-switched connection time networks
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Prof. Jochen Seitz 3 Communication Networks Winter 2017/18
7.1 Fundamentals Virtual Circuit
End System (User) End System 3 2 1 (User)
A Switching E System
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7.1 Fundamentals Virtual Circuit vs. Datagram
Virtual circuit Datagram • network can provide sequencing and • no call setup phase error control • more flexible • packets are forwarded more quickly • more fault-tolerant, but no QoS • less reliable in case of link or switch failure
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Prof. Jochen Seitz 4 Communication Networks Winter 2017/18
7.1 Fundamentals (Real) Circuit vs. Virtual Circuit
Real Circuit Virtual Circuit • Connection-oriented • Connection-oriented • All information use the same path • All Packets use the same path ordered delivery ordered delivery • Resources dedicated to single • Resources shared between different connection connections • Delay only depending on propagation • Delays due to traffic on the path and delay of physical signal delay caused by forwarding nodes • No overhead • Overhead due to packet header
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7.1 Fundamentals Event Timing According to Stallings
(a) Circuit Switching (b) Virtual Circuit Packet Switching (c) Datagram Packet Switching CR CR Pkt 1 CR CR Pkt 2 Pkt 1 CR CR Pkt 3 Pkt 2 CC Pkt 1 CC Pkt 3 CC Pkt 2 CC Pkt 3 Pkt 1 Pkt 2 User Data Pkt 1 Pkt 3 Pkt 2 Pkt 1 Pkt 3 Pkt 2 Pkt 3 ACK ACK ACK
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Prof. Jochen Seitz 5 Communication Networks Winter 2017/18
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
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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
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Prof. Jochen Seitz 6 Communication Networks Winter 2017/18
7.2 X.25 X.25 & the OSI Reference Model
Application Layer
Presentation Layer X.25 Protocol Stack
Session Layer Network Layer Packet Layer Protocol (PLP) Transport Layer
High-Level Data Link Access Network Layer Data Link Layer Link Control (HDLC) Procedure Balanced Data Link Layer Physical Layer X.21 Layer: RS-232 Physical Layer
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7.2 X.25 ITU-T X.25 Recommendation
ISO/OSI layers End-to-End Connections Layers 4-7 on top of X.25
Packet Interface Layer 3 X.25-3 Packet Level Procedures (Virtual Connections) Layer 2 Data Link Interface HDLC Frame Level Procedures (LAPB)
Physical Interface Layer 1 Physical Level Procedures X.21
DTE DTE/DCE Interface DCE/Network
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Prof. Jochen Seitz 7 Communication Networks Winter 2017/18
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
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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 DTE PIN CCITT-IDENTIFICATION DCE correction G/P Signal ground or common return . Circuit switching 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)
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Prof. Jochen Seitz 8 Communication Networks Winter 2017/18
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
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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)
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Prof. Jochen Seitz 9 Communication Networks Winter 2017/18
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.
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7.2 X.25 HDLC: Point-to-Point Topology
HDLC HDLC Hybrid Hybrid Station Station
Balanced link configuration
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Prof. Jochen Seitz 10 Communication Networks Winter 2017/18
7.2 X.25 HDLC: Point-to-Multipoint Topology
HDLC Primary Station
HDLC HDLC HDLC Secondary Secondary Secondary Station Station Station
Unbalanced link configuration
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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.
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Prof. Jochen Seitz 11 Communication Networks Winter 2017/18
7.2 X.25 HDLC: Frame Format
8 bits 8 bits 8 bits n bits (variable) 16 bits 8 bits Data Frame Check Flag Address Control Flag (Optional Octets) Sequence
• 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
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7.2 X.25 HDLC: Control Field
01111110 8 bits 8 bits n bits (variable) 16 bits 01111110 Data Frame Check Flag Address Control Flag (Optional Octets) Sequence
I Frame 0 N(S) P/F N(R) [I=Information]
S Frame 1 0 S S P/F N(R) [S=Supervisory]
U Frame 1 1 M M P/F M M M [U=Unnumbered]
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Prof. Jochen Seitz 12 Communication Networks Winter 2017/18
7.2 X.25 HDLC: I Frame and S Frame
I Frame 0 N(S) P/F N(R) S Frame 1 0 S S P/F N(R)
• Information Frame • Supervisory Frame (both command or response) (both command or response) . I (Information) 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
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7.2 X.25 HDLC: U Frame
Unnumbered Commands Unnumbered Responses
• SNRM ...... Set NRM • UA ...... Unnumbered Acknowledgment • SARM ...... Set ARM • FRMR ...... Frame Reject • SABM ...... Set ABM • DM ...... Disconnect Mode • DISC ...... Disconnect • RIM ...... Request Initialization Mode • SNRME ...... Set extended NRM • RD ...... Request Disconnect • SARME ....……..Set extended ARM • UI ...... Unnumbered Information • SABME ...………Set extended ABM • XID ...... Exchange Identification • SIM ...... Set Initialization Mode • TEST ...... Test • UP ...... Unnumbered Poll • UI ...... Unnumbered Information • XID ...... Exchange Identification • RSET ...... Reset
• TEST ...... Test Communication Networks - 7. Connection Oriented PDNs 261
Prof. Jochen Seitz 13 Communication Networks Winter 2017/18
7.2 X.25 Notation:
Transfer of 3 I Frames Transfer of 2 I Frames incl. Acknowledgment
Transfer of 7 Additional I Frames Then Awaiting Acknowledgement
Ack without Data Data Transfer
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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
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Prof. Jochen Seitz 14 Communication Networks Winter 2017/18
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
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7.2 X.25
Transmission Time Direction LAPB Start of Frame (Flag) According to Address Control HDLC-LAPB Header Control X.25: Packet GFI X.25 • General Format Identifier (GFI) Logical Channel According to X.25 Packet Header to identify packet format Packet Control • Logical Channel Identifier (LCI) According to to identify the logical channel User Information User Information higher layers the packet belongs to and application • Packet Control to specify packet type and LAPB Frame Check Sequence According to additional control information Control HDLC-LAPB End of Frame (Flag) Trailer
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Prof. Jochen Seitz 15 Communication Networks Winter 2017/18
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: Bit 8 7 6 5 4 3 2 1 . Currently 22 different packet types GFI Logical Channel Octet 1 . Packet for data exchange: 1st Bit in Octet 3 = 0 Q D 0 1 Group (LCG) . All other packets: 1st Bit in Octet 3 = 1 Octet 2 Logical Channel Number (LCN) Octet 3 Packet Type Identification
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7.2 X.25 X.25: Encapsulation
Data
Packet Layer 3 Data Header
Layer 2 Flag Address Control Information FCS Flag
Layer 1 Bit stream according to X.21
Direction of Transmission
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Prof. Jochen Seitz 16 Communication Networks Winter 2017/18
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
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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: Logical Channel Virtual Connection 1 1 4 3 between DTE A and… 1 DTE B 2 1 3 2 DTE C DCE Bidirectional Packet Flow 3 DTE D A 4 DTE C N Packet belonging to logical channel “N”
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Prof. Jochen Seitz 17 Communication Networks Winter 2017/18
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
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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)
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Prof. Jochen Seitz 18 Communication Networks Winter 2017/18
7.2 X.25
LCI Usage 0 not available 1 … High Number PVC for permanent virtual circuits X.25: Allocation … -- of LCIs LIC (Low Number Incoming Calls) for incoming … switched virtual Incoming Calls increase the LCI HIC (High Number Incoming Calls) circuits Currently Outgoing Calls decrease the LCI … -- not used LTC (Low Number Two Way Calls) for both incoming LCIs … and outgoing HTC (High Number Two Way Calls) switched virtual circuits … -- LOC (Low Number Outgoing Calls) for outgoing … switched virtual HOC (High Number Outgoing Calls) circuits … 4095 --
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7.2 X.25 X.25: Local Character of LCIs
• Complete addresses of sender and receiver only during
1 1 ...
connection setup ... • Packets assigned to current virtual
49 71 DTE DTE A connection by an LCI valid only on ... X.25 Network 72 single link 2812 ...
• No relation between the network 2813 2177 B DTE ... addresses of the DTEs and the ... LCIs to be used 4095 4095 • A new virtual connection is ...... assigned a currently unused (temporary) LCI 1 2 3 52 3711 4095 several independent virtual DTE C connections between the same two DTEs possible Communication Networks - 7. Connection Oriented PDNs 273
Prof. Jochen Seitz 19 Communication Networks Winter 2017/18
7.2 X.25 X.25: Packet „Call Request”
Bit 8 7 6 5 4 3 2 1 GFI Octet 1 LCG 0 0 0 1 2 LCN Packet Type 3 0 0 0 0 1 0 1 1 Address Length of Address Length of 4 Calling DTE Called DTE Addresses of DTEs (calling, called) 0 0 0 0
0 0 Length of Performance Features
Performance Features Short information of calling user
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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 LCI Entry 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
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Prof. Jochen Seitz 20 Communication Networks Winter 2017/18
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
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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
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Prof. Jochen Seitz 21 Communication Networks Winter 2017/18
7.2 X.25
X.25: Message Interface of calling DTE/DCE Interface of called DTE/DCE Call Request Packet Incoming Sequence Chart Call Packet Call Establishment Phase Call Accepted Packet Establishment Phase Connected Packet
The arrival of a „Clear Confirmation” Data Packet Data Packet packet at the called entity might not Data Packet Data Packet necessarily be the result of a „Clear Data Phase Confirmation” packet of the calling Data Phase Data Packet entity Data Packet Clear Indication Packet Clear Request Packet Clear Clear Confirmation Disconnection Phase Confirmation Packet Packet Disconnection Phase
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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
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Prof. Jochen Seitz 22 Communication Networks Winter 2017/18
7.2 X.25 X.25: Data Packet
• Sequence numbers are Modulo 8:
Bit 8 7 6 5 4 3 2 1
Octet GFI LCG 1 Q D 0 1
2 LCN
3 P(R) M P(S) 0
4 User Information (e.g. transport layer PDU) ...
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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
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Prof. Jochen Seitz 23 Communication Networks Winter 2017/18
7.2 X.25 Important X.25 PDN Standards
X.25: defines interface between DTE and DCE in public data networks X.21: specifies the interface between user terminal equipment and PDN packet-switching nodes X.3: specifies packet assembly/disassembly processes X.28: governs asynchronous dial-up access to PDNs X.29: governs synchronous dial-up access to PDNs X.75: defines the interface between different public packet-switching networks, both domestic and international X.121: defines a global addressing scheme for PDNs
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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/)
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Prof. Jochen Seitz 24 Communication Networks Winter 2017/18
7.3 Frame Relay Frame Relay: Frame
New HDLC Variant: LAPF (Link Access Procedure, Frame Mode) Frame Header
Flag Information FCS Flag
BECN
FECN
C/R
DE
EA EA DLCI DLCI
8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 Bit DLCI = Data Link Connection Identifier C/R = Command/Response Field FECN = Forward Explicit Congestion Notification BECN = Backward Explicit Congestion Notification DE = Discard Eligibility Indicator EA = Address Field Extension Bit Communication Networks - 7. Connection Oriented PDNs 284
7.3 Frame Relay Frame Relay: Topology
LAN 4 Router
LAN 3 B C Router Frame Relay-Network A Router DLCI 3 LAN 1 Router
LAN 2
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Prof. Jochen Seitz 25 Communication Networks Winter 2017/18
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
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7.3 Frame Relay Comparison Frame Relay – X.25
X.25 Frame Relay slow fast Throughput (200 kbit/s – 2 Mbit/s) (2Mbit/s typically, 45 Mbit/s max) low Latency high (due to less overhead) Jitter No countermeasure No countermeasure Error Detection & Special mechanisms Responsibility of higher layers Correction (retransmission on layer 2 and 3) Functionality complex 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.
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Prof. Jochen Seitz 26 Communication Networks Winter 2017/18
7.3 Frame Relay
Comparison Frame Relay – X.25 Taken from „The Basic Guide to Frame Relay Networking“ (Frame Relay Forum, http://www.frforum.com)
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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
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Prof. Jochen Seitz 27 Communication Networks Winter 2017/18
7.4 ATM Traffic
STM – Synchronous Transfer Mode ATM – Asynchronous Transfer Mode
. Isochronous data traffic (constant data . Transport of small data packets rate and constant latency) (ATM cells) . Example: Narrowband-ISDN . Data rate and latency are variable, but . Circuit-switched communication guarantees are possible channels . Packet-switched communication (connection-oriented: virtual channels)
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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: Cell Header User Data
5 Octets 48 Octets
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Prof. Jochen Seitz 28 Communication Networks Winter 2017/18
7.4 ATM ATM: Multiplexing
• Advantages of small size of ATM cells: Constant Variable . Easy Multiplexing of different streams Bit Rate Bit Rate . 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
ATM ist a fundamental concept, 155 Mbit/s on which different networks can be based
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7.4 ATM ATM: Cell Header (UNI)
• GFC (Generic Flow Control): local control functions such Cell Header User Data as identification of several terminals connected to the same ATM interface 5 Octets 48 Octets • 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 GFC VPI 1 be preferentially deleted or not in case of a transmission VPI VCI 2 bottleneck. Cells with CLP=0 have higher priority than cells with CLP=1. VCI 3 • HEC (Header Error Control) VCI PT CLP 4 HEC 5 At NNI, the VPI field is 4 bits longer, as the GFC is not needed there [Byte]
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Prof. Jochen Seitz 29 Communication Networks Winter 2017/18
7.4 ATM ATM: Virtual Channel and Virtual Connection
VC VC VP ... Transmission Path VP VC
• 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)
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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): ABR an average cell rate is specified, which can peak at a certain level for a maximum interval before being UBR problematic (typical e.g. for MPEG streams) • ABR (available bit rate): a minimum guaranteed rate is specified; if the rate VBR cannot be sustained, the sender is notified CBR • UBR (unspecified bit rate): no guarantees, just best effort transmission t
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Prof. Jochen Seitz 30 Communication Networks Winter 2017/18
7.4 ATM ATM and AAL
End System A End System B Service-dependent AAL connections AAL AAL
Service-independent ATM ATM ATM connections physical physical layer layer
Application ATM-Network
• ATM Layer: service-independent transport of ATM cells, multiplexing and demultiplexing • AAL Layer: ATM Adaptation Layer – support of different services
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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-network in 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
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Prof. Jochen Seitz 31 Communication Networks Winter 2017/18
7.4 ATM ATM: Support of Voice and Data Services
Services
Bit Stream Bit Stream Packets Packets ATM Adaptation Layer Cells Cells Cells Cells ATM-Switch ATM Subnetwork
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7.4 ATM B-ISDN Reference Model (I)
• 3-dimensional reference model • Signaling information is transferred separately • Three vertical planes from the user information (Out-of-Band- Signalling) . User Plane . Control Plane Management Plane PlaneManagement
. Management Plane Control User LayerManagement • Three hierarchical Layers Plane Plane . Physical Layer Higher Layers . ATM Layer ATM Adaptation Layer . ATM Adaptation Layer ATM Layer
Physical Layer Layers Planes
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Prof. Jochen Seitz 32 Communication Networks Winter 2017/18
7.4 ATM B-ISDN Reference Model (II)
Management plane
Control User plane plane
Higher Class Class Class Class Signaling layers A B C D
ATM Signaling AAL3/4 AAL1 AAL2 adaptation layer AAL or AAL5
ATM layer
Physical layer
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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)
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Prof. Jochen Seitz 33 Communication Networks Winter 2017/18
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
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