Switching
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Here are short notes on X.25, ATM, and Frame Relay: a. X.25 - X.25 is a protocol suite for packet switched WANs. It establishes switched virtual circuits between DTE devices using X.121 addressing. X.25 uses LAPB for data link layer and PLP for network layer. It provides reliable data transfer over public networks. b. ATM - Asynchronous Transfer Mode is a cell switching and multiplexing technology designed for B-ISDN. It uses fixed size 53 byte cells and establishes permanent virtual circuits between endpoints. ATM supports real-time multimedia traffic using constant bit rate, variable bit rate and available bit rate. c. Frame Relay -
Switching
- 2. 2 Overview • Networks are used to interconnect many devices. —Since the invention of the telephone, circuit switching has been the dominant technology for voice communications. —Since 1970, packet switching has evolved substantially for digital data communications. It was designed to provide a more efficient facility than circuit switching for bursty data traffic. • Two types of packet switching: – Datagram (such as today’s Internet) – Virtual circuit (such as Frame Relay, ATM)
- 3. 3 Switched Communications Networks • Long distance transmission between stations (called “end devices”) is typically done over a network of switching nodes. • Switching nodes do not concern with content of data. Their purpose is to provide a switching facility that will move the data from node to node until they reach their destination (the end device). • A collection of nodes and connections forms a communications network. • In a switched communications network, data entering the network from a station are routed to the destination by being switched from node to node.
- 5. 5 Switching Nodes • Nodes may connect to other nodes, or to some stations. • Network is usually partially connected —However, some redundant connections are desirable for reliability • Two different switching technologies —Circuit switching —Packet switching
- 6. 6 Circuit Switching • Circuit switching: —There is a dedicated communication path between two stations (end-to-end) —The path is a connected sequence of links between network nodes. On each physical link, a logical channel is dedicated to the connection. • Communication via circuit switching has three phases: —Circuit establishment (link by link) • Routing & resource allocation (FDM or TDM) —Data transfer —Circuit disconnect • Deallocate the dedicated resources
- 7. 7 Circuit Switching Properties • Inefficiency —Channel capacity is dedicated for the whole duration of a connection —If no data, capacity is wasted • Delay —Long initial delay: circuit establishment takes time —Low data delay: after the circuit establishment, information is transmitted at a fixed data rate with no delay other than the propagation delay. The delay at each node is negligible. • Developed for voice traffic (public telephone network) but can also applied to data traffic. —For voice connections, the resulting circuit will enjoy a high percentage of utilization because most of the time one party or the other is talking.
- 8. 8 Public Circuit Switched Network Subscribers: the devices that attach to the network. Subscriber loop: the link between the subscriber and the network. Exchanges: the switching centers in the network. End office: the switching center that directly supports subscribers. Trunks: the branches between exchanges. They carry multiple voice-frequency circuits using either FDM or synchronous TDM.
- 9. 9 Packet Switching Principles • Problem of circuit switching —designed for voice service —Resources dedicated to a particular call —For data transmission, much of the time the connection is idle (say, web browsing) —Data rate is fixed • Both ends must operate at the same rate during the entire period of connection • Packet switching is designed to address these problems.
- 10. 10 Basic Operation • Data are transmitted in short packets —Typically at the order of 1000 bytes —Longer messages are split into series of packets —Each packet contains a portion of user data plus some control info • Control info contains at least —Routing (addressing) info, so as to be routed to the intended destination • store and forward —On each switching node, packets are received, stored briefly (buffered) and passed on to the next node.
- 12. 12 Advantages of Packet Switching • Line efficiency —Single node-to-node link can be dynamically shared by many packets over time —Packets are queued up and transmitted as fast as possible • Data rate conversion —Each station connects to the local node at its own speed • In circuit-switching, a connection could be blocked if there lacks free resources. On a packet-switching network, even with heavy traffic, packets are still accepted, by delivery delay increases. • Priorities can be used —On each node, packets with higher priority can be forwarded first. They will experience less delay than lower-priority packets.
- 13. 13 Packet Switching Technique • A station breaks long message into packets • Packets are sent out to the network sequentially, one at a time • How will the network handle this stream of packets as it attempts to route them through the network and deliver them to the intended destination? —Two approaches • Datagram approach • Virtual circuit approach
- 14. 14 Datagram • Each packet is treated independently, with no reference to packets that have gone before. —Each node chooses the next node on a packet’s path. • Packets can take any possible route. • Packets may arrive at the receiver out of order. • Packets may go missing. • It is up to the receiver to re-order packets and recover from missing packets. • Example: Internet
- 15. 15 Datagram
- 16. 16 Virtual Circuit • In virtual circuit, a preplanned route is established before any packets are sent, then all packets follow the same route. • Each packet contains a virtual circuit identifier instead of destination address, and each node on the preestablished route knows where to forward such packets. —The node need not make a routing decision for each packet. • Example: X.25, Frame Relay, ATM
- 17. 17 Virtual Circuit A route between stations is set up prior to data transfer. All the data packets then follow the same route. But there is no dedicated resources reserved for the virtual circuit! Packets need to be stored-and-forwarded.
- 18. 18 Virtual Circuits v Datagram • Virtual circuits —Network can provide sequencing (packets arrive at the same order) and error control (retransmission between two nodes). —Packets are forwarded more quickly • Based on the virtual circuit identifier • No routing decisions to make —Less reliable • If a node fails, all virtual circuits that pass through that node fail. • Datagram —No call setup phase • Good for bursty data, such as Web applications —More flexible • If a node fails, packets may find an alternate route • Routing can be used to avoid congested parts of the network
- 21. 21 Assignments • Explain about: —X.25 protocol —ATM (Asynchronous Transfer Mode) —Frame Relay
- 22. Datagram Packet Switching • No call setup • Each packet can travel across a different route from sender to receiver • Delivery and order of packets cannot be guaranteed • Most common implementation of datagram packet switching is Internet Protocol (IP)
- 23. Virtual Circuit Packet Switching • Similar to standard circuit-switched networks • Call Setup required to define the route between Sender and Receiver • Each route is assigned a Virtual Circuit Identifier (VCI) • All packets using the same VCI will travel the same route and will arrive in sequence • Circuit is “virtual” because resources are not dedicated to a single call • Most common forms of virtual circuit packet switching are X.25 and Frame Relay
- 24. X.25 • X.25 is an ITU-T standard protocol suite for packet switched wide area network (WAN) communication. • Consists of packet-switching exchange (PSE) nodes as the networking hardware, and leased lines, PSTN connections or ISDN connection as physical links. 24
- 25. X.25 History and Overview • Designed to provide a low cost alternative for data communication over public networks —Pay only for bandwidth actually used • Ideal for “bursty” communication over low quality circuits • Standard provides error detection and correction for reliable data transfer • X.25 standard approved in 1976 by CCITT (now known as ITU) • Can support speeds of 9.6 Kbps to 2 Mbps • Can provide multiplexing of up to 4095 virtual circuits over on DTE-DCE link
- 26. X.25 Devices • Data Terminal Equipment (DTE) —Terminals, personal computers, and network hosts —Located on premises of subscriber • Data Circuit-terminating Equipment (DCE) —Modems and packet switches —Usually located at carrier facility • Packet Switching Exchange (PSE) —Switches that make up the carrier network
- 28. Packet Assembler/Disassembler (PAD) • Used for DTE devices that are too simple to implement X.25 (such as character-mode terminals) • Acts as intermediary device between DTE and DCE • Performs three functions —Buffering to store data until a device is ready to process it —Packet Assembly —Packet Disassembly
- 29. PAD in Action
- 30. X.25 mapping to OSI Model Application Presentation Session Transport Network Data Link PhysicalPhysical PLP LAPB x.21 bis, EIA/TIA-232, EIA/TIA-449, EIA-530, G.703 x.21 bis, EIA/TIA-232, EIA/TIA-449, EIA-530, G.703 Other Services X.25 Protoco l Suite
- 31. X.25 Physical Layer • Several well-known standards are used for X.25 networks —X.21bis – supports up to 2 Mbps • 15-pin connector —RS-232 (EIA/TIA-232) – supports up to 19.2 Kbps • 25-pin connector —RS-449 (EIA/TIA-449) – supports up to 64 Kbps • 37-pin connector —V.35 – supports up to 2 Mbps • 34-pin connector • Uses serial communications in either asynchronous or synchronous modes
- 32. X.25 Data Link Layer • Link Access Procedure, Balanced (LAPB) is the protocol used for this layer • LAPB is a version of HDLC —HDLC in Asynchronous Balanced Mode (ABM) —DTE and DCE are peers and can both perform all functions • LAPB manages communication and packet framing between DTE and DCE devices • Makes sure that frames are delivered in sequence and error-free —Uses sliding window of 8 or 128 frames
- 33. LAPB Frame Types • Three types of frames —I-Frames (Information Frames) • Carry data as well as Next Send (NS) and Next Receive (NR) counts —S-Frames (Supervisory Frames) • Controls flow of data with Receiver Ready (RR), Receiver Not Ready (RNR), and Reject (REJ) frames —U-Frames (Unnumbered Frames) • Establish and maintain communications with Set Asynchronous Balanced Mode (SABM), Unnumbered Acknowledgment (UA), Disconnect (DISC), Disconnect Mode (DM) and Frame Reject (FRMR)
- 34. LAPB Frame Format Flag FlagAddress Control Data FCS Flag:Flag: (8 bits) Indicates start and end of frame (01111110)(8 bits) Indicates start and end of frame (01111110) Address:Address: (8 bits) DTE address is maintained in higher(8 bits) DTE address is maintained in higher layer so this field is used to identify command andlayer so this field is used to identify command and responses between DTE and DCE. A value of 0x01responses between DTE and DCE. A value of 0x01 indicates a command from DTE and responses from DCEindicates a command from DTE and responses from DCE while a value of 0x03 indicates commands from DCE andwhile a value of 0x03 indicates commands from DCE and responses from DTE.responses from DTE. Control:Control: (8 bits) Contains sequence numbers,(8 bits) Contains sequence numbers, commands and responses for controlling data flowcommands and responses for controlling data flow
- 35. X.25 Network Layer • Packet Layer Protocol (PLP) is the X.25 network layer protocol • PLP manages calls between a pair DTE devices using a Permanent Virtual Circuit (PVC) or a Switched Virtual Circuit (SVC) • PLP handles segmentation, reassembly, bit padding and error and flow control • PLP uses X.121 Addressing Scheme to setup a virtual circuit
- 36. PLP Operates in Five Modes • Call Setup —Used to setup virtual circuit for SVC • Data Transfer —Used for transferring data with both SVC and PVC • Idle —Used when SVC call has been established but no data is currently being transferred • Call Clearing —Used to end communication between DTEs for a SVC • Restarting —Used to synchronize DTE and DCE for all virtual circuits that exist between them
- 37. PLP Frame Format GFI LCI PTI User Data General Format Indicator:General Format Indicator: (4 bits) Identifies packet(4 bits) Identifies packet parameters, such as whether the packet carries user dataparameters, such as whether the packet carries user data or control information, what kind of windowing is beingor control information, what kind of windowing is being used, and whether delivery confirmation is requiredused, and whether delivery confirmation is required Bit 1 – 0=User Data, 1=Data for PAD Bit 2 – 0=Local Ack, 1=Remote Ack Bits 3 & 4 – 00=Reserved, 01=Window Size 8, 10=Window Size 128, 11=Extended Format Logical Channel Identifier:Logical Channel Identifier: (12 bits) Identifies the virtual(12 bits) Identifies the virtual circuit (1-4095) across the local DTE to DCE interfacecircuit (1-4095) across the local DTE to DCE interface. This field consists of a 4-bit Logical Channel GroupThis field consists of a 4-bit Logical Channel Group
- 38. X.121 Addressing • PLP uses X.121 addressing during the call setup phase to establish a virtual circuit between DTEs • Only used for SVC calls • Address consists of up to 14 digits —3 digits for Country Code —1 digit for Network Number (only 10 per country) —Up to 10 digits to define the terminal number on the network PSNCountry National Terminal Number
- 39. X.25 Call Request Packet 0 D X X LGCN LCN 0 0 0 0 1 0 1 1 Called Address Length Calling Address Length Called Address Calling Address Facilities Length Facilities* Call User Data *Facilities field contains*Facilities field contains control information tocontrol information to setup call specificsetup call specific features such asfeatures such as reverse charging,reverse charging,
- 40. X.25 Clearing Packet 0 D X X LGCN LCN 0 0 0 1 0 0 1 1 Clearing Cause Clearing Diagnostic Address in Call Request format if used Facilities in Call Request format if used Clear User Data
- 41. X.25 Data Packet Q D X X LGCN LCN Pr3 Pr2 Pr1 M Ps3 Ps2 Ps1 0 User Information or Higher Layer Protocol PrXPrX is a 3-bit Send Sequence Numberis a 3-bit Send Sequence Number PsXPsX is a 3-bit Receive Sequenceis a 3-bit Receive Sequence NumberNumber MM indicates whether packets are part ofindicates whether packets are part of a sequencea sequence
- 42. X.25 Call Setup Call Setup Phase Data Transfe r Phase Call Clearin g Phase DTE to DCE Interface DCE to DTE Interface Call Request Call Connected Data Packet Incoming Data Clear Request Clear Confirm Incoming Call Call Accepted Incoming Data Data Packet Clear Indication Clear Response
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- 44. 44 Write short notes on: a.X.25 b.ATM c.Frame Relay