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Data Communications Theory Lecture-9

University of Palestine. Faculty of Information Technology. Data Communications Theory Lecture-9. Dr. Anwar Mousa. Frame Relay. Frame Relay. Three types of switching exist: circuit switching, packet switching message switching. Packet switching can use two approaches:

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Data Communications Theory Lecture-9

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  1. University of Palestine Faculty of Information Technology Data Communications Theory Lecture-9 Dr. Anwar Mousa

  2. Frame Relay

  3. Frame Relay • Three types of switching exist: • circuit switching, • packet switching • message switching. • Packet switching can use two approaches: • virtual-circuit switching • datagram. • Frame Relay is a virtual-circuit WAN with the following properties:

  4. Frame Relay • Frame Relay is available in the following speeds: 56 kbit/s, 64 kbit/s, 128 kbit/s, 256 kbit/s, 512 kbit/s, 1.544 Mbps and 44.376 Mbps • Operates in just the physical and data link layers • It can be used as a backbone network to provide services to protocols that already have a network layer protocol, such as the Internet.

  5. Frame Relay • Allows a frame size of 9000 bytes, which can accommodate all LAN frame sizes. • Less expensive than other traditional WANs • Has error detection at the data link layer only. • There is no flow control or error control. • There is no transmission policy if a frame is damaged; it is dropped

  6. Frame relay • Unlike X.25, whose designers expected analog signals, frame relay offers a fast packet technology, which means that the protocol does not attempt to correct errors. • When a frame relay network detects an error in a frame, it simply drops that frame. • The end points have the responsibility for detecting and retransmitting dropped frames. • (However, digital networks offer an incidence of error extraordinarily small relative to that of analog networks.)

  7. Frame Relay • Provide fast transmission capability for more reliable media and for those protocol that have flow and error control at the higher layer. • Frame Relay consists of an efficient data transmission technique used to send digital information quickly and cheaply in a relay of frames • to one or many destinations • from one or many end-points.

  8. Frame Relay • Network providers commonly implement frame relay for voice and data as an encapsulation technique, • used between local area networks (LANs) over a wide area network (WAN). • Each end-user gets a private line (or leased line) to a frame-relay node. • The frame-relay network handles the transmission over a frequently-changing path transparent to all end-users.

  9. Frame Relay • As of 2006 native IP-based networks have gradually begun to displace frame relay. • With the advent of dedicated broadband services such as cable modem and DSL, the end may loom for the frame relay protocol and encapsulation. • However many rural areas remain lacking DSL and cable modem services. • In such cases the least expensive type of "always-on" connection remains a 128-kilobit frame-relay line. • Thus a retail chain, for instance, may use frame relay for connecting rural stores into their corporate WAN.

  10. Frame relay description • Frame relay puts data in variable-size units called "frames" and leaves any necessary error-correction (such as re-transmission of data) up to the end-points. • This speeds up overall data transmission. • For most services, the network provides a permanent virtual circuit (PVC), Permanent virtual circuit (PVC), means that the customer sees a continuous, dedicated connection without having to pay for a full-time leased line, • while the service-provider figures out the route each frame travels to its destination and can charge based on usage.

  11. Virtual circuits • As a WAN protocol, frame relay is most commonly implemented at Layer 2 (data link layer) of the Open Systems Interconnection (OSI) seven layer model. Two types of circuits exist: • permanent virtual circuits (PVCs) which are used to form logical end-to-end links mapped over a physical network, • switched virtual circuits (SVCs), analogous to the circuit-switching concepts of the public-switched telephone network (or PSTN), the global phone network we are most familiar with today. • While SVCs exist and are part of the frame relay specification, they are rarely applied to real-world scenarios. SVCs are most often considered harder to configure and maintain and are generally avoided without appropriate justification.

  12. FRAME RELAY LAYERS • Frame relay has only physical and data link layers • Physical layer • No specific protocol is defined • It is left to the implementer to use whatever is available • Data link layer • Use a simple protocol that does not support flow or error control

  13. Frame relay usage • Frame relay complements and provides a mid-range service between ISDN, which offers bandwidth at 128 kbit/s, and Asynchronous Transfer Mode (ATM), which operates in somewhat similar fashion to frame relay but at speeds from 155.520 Mbit/s to 622.080 Mbit/s. • Frame Relay originated as an extension of Integrated Services Digital Network (ISDN). Its designers aimed to enable a packet-switched network to transport the circuit-switched technology. • Frame relay has its technical base in the older X.25 packet-switching technology, designed for transmitting analog data such as voice conversations.

  14. Frame relay usage • Frame relay often serves to connect local area networks (LANs) with major backbones as well as on public wide-area networks (WANs) • It requires a dedicated connection during the transmission period. • Frame relay does not provide an ideal path for voice or video transmission, both of which require a steady flow of transmissions. • However, under certain circumstances, voice and video transmission do use frame relay.(VOFR: Voice Over Frame Relay )

  15. VOFR: Voice Over Frame Relay VOFR: Voice Over Frame Relay • Voice is digitized using PCM and then compressed. • The result is sent as data frame over the network. • This feature allows inexpensive sending of voice over long distances. • However, the quality of voice is not as good as voice over a circuit switched network as PSTN. • Also, the varying delay sometimes corrupts real-time voice.

  16. Frame relay Assembler/Disassembler (FRAD) • To handle frames arriving from other protocols (such as Ethernet,ATM and X.25) • Assembles and disassembles frames comming from other protocols to allow them to be carried by Frame Relay frames. • A FRAD can be implemented as a separate device or as a part of a switch

  17. Frame Relay versus X.25 • The design of X.25 aimed to provide error-free delivery over links with high error-rates. • Frame relay takes advantage of the new links with lower error-rates, enabling it to eliminate many of the services provided by X.25. • The elimination of functions and fields, combined with digital links, enables frame relay to operate at speeds 20 times greater than X.25. • X.25 specifies processing at layers 1, 2 and 3 of the OSI model, while frame relay operates at layers 1 and 2 only. • This means that frame relay has significantly less processing to do at each node, which improves throughput.

  18. Frame Relay versus X.25 • X.25 prepares and sends packets, while frame relay prepares and sends frames. • X.25 packets contain several fields used for error and flow control, none of which frame relay needs. • The frames in frame relay contain an expanded address field that enables frame relay nodes to direct frames to their destinations with minimal processing . • X.25 has a fixed bandwidth available. It uses or wastes portions of its bandwidth as the load dictates. • Frame relay can dynamically allocate bandwidth during call setup negotiation at both the physical and logical channel level.

  19. Asynchronous Transfer Mode (ATM)

  20. ATM • Asynchronous Transfer Mode (ATM) is a cell relay, packet switching (virtual circuits) network and data link layer protocol • Encodes data traffic into small (53 bytes; 48 bytes of data and 5 bytes of header information) fixed-sized cells. • This differs from other technologies based on packet-switched networks (such as the Internet Protocol or Ethernet), in which variable sized packets (known as frames when referencing layer 2) are used. • ATM is a connection-oriented technology, in which a logical connection is established between the two endpoints before the actual data exchange begins.

  21. ATM • ATM has proved very successful in the WAN scenario and numerous telecommunication providers have implemented ATM in their wide-area network cores. • Also many ADSL implementations use ATM. However, ATM has failed to gain wide use as a LAN technology • Currently it seems likely that gigabit Ethernet implementations (10Gbit-Ethernet, Metro Ethernet) will replace ATM as a technologyof choice in new WAN implementions.

  22. Asynchronous TDM • ATM uses asynchronous Time-Division Multiplexing-that is why it is called Asynchronous Transfer Mode- to multiplexed cells coming from different channels. • It uses fixed-size slots (size of a cell). • ATM multiplexers fill a slot with a cell from any input channel that has a cell. • The slot is empty if none of the channel has a cell to send.

  23. ATM Features • Error & flow control is moved to the network boundary • No error control on data field within the network • No flow control on links within the network • Connection oriented at the lowest level • All information is transferred in a virtual circuit assigned for the duration of the connection • Fixed cell (packet) size • Permits high speed switching nodes • Efficient cell structure • 48 bytes of data • 5 byte header

  24. Why cells? • The motivation for the use of small data cells was the reduction of jitter (delay variance, in this case) in the multiplexing of data streams; • reduction of this (and also end-to-end round-trip delays) is particularly important when carrying voice traffic. • This is because the conversion of digitized voice back into an analog audio signal is an inherently real-time process,

  25. Why cells? • If the next data item is not available when it is needed, the codec has no choice but to produce silence. • and if the data is late, it is useless, because the time period when it should have been converted to a signal has already passed. • At the time ATM was designed, 155 Mbit/s SDH (135 Mbit/s payload) was considered a fast optical network link, and many PDH links in the digital network were considerably slower, ranging from 1.544 to 45 Mbit/s in the USA (2 to 34 Mbit/s in Europe).

  26. ATM-SDH • At this rate, a typical full-length 1500 byte (12000-bit) data packet would take 77.42 µs to transmit.{ = (1/155)*12000} • In a lower-speed link, such as a 1.544 Mbit/s T1 link, a 1500 byte packet would take up to 7.8 milliseconds. • A queueing delay induced by several such data packets might be several times the figure of 7.8 ms, in addition to any packet generation delay in the shorter speech packet. • This was clearly unacceptable for speech traffic, which needs to have low jitter in the data stream being fed into the codec if it is to produce good-quality sound.

  27. Why 48 bytes? • ATM was designed to implement a low-jitter network interface. However, to be able to provide short queueing delays, but also be able to carry large datagrams, it had to have cells. • ATM broke up all packets, data, and voice streams into 48-byte chunks, adding a 5-byte routing header to each one so that they could be reassembled later. • The choice of 48 bytes was, as is all too often the case, political instead of technical.

  28. Why 48 bytes?// • When the CCITT was standardizing ATM, parties from the United States wanted a 64-byte payload because having the size be a power of 2 made working with the data easier and this size was felt to be a good compromise between larger payloads optimized for data transmission and shorter payloads optimized for real-time applications like voice; • parties from Europe wanted 32-byte payloads because the small size (and therefore short transmission times) simplify voice applications with respect to echo cancellation. • Most of the interested European parties eventually came around to the arguments made by the Americans, but France and a few allies held out until the bitter end.

  29. Why 48 bytes?// • With 32 bytes, France would have been able to implement an ATM-based voice network with calls from one end of France to the other requiring no echo cancellation. • 48 bytes (plus 5 header bytes = 53) was chosen as a compromise between the two sides, but it was ideal for neither and everybody has had to live with it ever since. • 5-byte headers were chosen because it was thought that 10% of the payload was the maximum price to pay for routing information. • ATM multiplexed these 53-byte cells instead of packets. • Doing so reduced the worst-case queuing jitter by a factor of almost 30, removing the need for echo cancellers

  30. Why virtual circuits? • ATM is Virtual circuits (VCs) network. • Connection between two endpoints is accomplished through transmission paths (TPs), virtual paths (VPs) and virtual circuits(VCs) • Every ATM cell has an 8- or 12-bit Virtual Path Identifier(VPI) and 16-bit Virtual Channel Identifier (VCI) pair defined in its header. • Together, these identify the virtual circuit used by the connection.

  31. VPI/VCI • The length of the VPI varies according to whether the cell is sent on the user-network interface UNI (on the edge of the network, VPI=8 bits), or if it is sent on the network-network interface NNI (inside the network, VPI=12 bits). • VPI is the same for all virtual connections that are bundled (logically ) into one VP • Most of the switches in a typical ATM network are routed using VPIs. The switches at the boundary of the network, those that interact directly with the endpoint devices, uses both VPIs and VCIs.

  32. VPI/VCI • As these cells traverse an ATM network, switching is achieved by changing the VPI/VCI values. • All cells belonging to a single message follow the same virtual circuit (VC) and remain in their original order until they reach their destination. • Multiple VCIs may be used for a multi-component service e.g. sound and video over separate VCIs in video-telephony

  33. Virtual Paths

  34. Structure of an ATM cell • An ATM cell consists of a 5 byte header and a 48 byte payload. • The payload size of 48 bytes was a compromise between the needs of voice telephony and packet networks, obtained by a simple averaging of the US proposal of 64 bytes and European proposal of 32, said by some to be motivated by a European desire not to need echo-cancellers on national trunks. • ATM defines two different cell formats: NNI (Network-Network Interface) and UNI (User-Network Interface).

  35. Structure of an ATM cell

  36. ATM Layered Model

  37. ATM Physical Layer • Physical layer consists of two sublayers • Physical Medium (PM) sublayer • correct transmission and reception of bits • medium dependent (optical, electrical) • Transmission Convergence (TC) sublayer • Maps recovered bitstream into the ATM cells • Maps the cells into the transmission mode e.g. SDH, PDH, cell-based • Two options for cell transmission • At the NNI and within the network SDH is preferred transport mechanism (PDH in early versions) • At the UNI a cell-based transport is preferred • Data rates for both options - 155Mbps 622Mbps

  38. ATM Layer • ATM layer is fully independent of physical medium • Four main Functions • Multiplexing and demultiplexing from different connections (using VCI) into a single cell stream • Cell header extraction/insertion for communication with Adaptation Layer • Translation of VCI at ATM switching nodes • Implementation of flow control mechanism upon the UNI

  39. Application Adaptation Layer -AAL • AAL has two sublayers • Segmentation and Reassembly (SAR) of the Protocol Data Units (PDU) from the higher layer • ATM must accept any type of payload, both data frame and streams of bits. • Whether the data are a data frame or a stream of bits, the payload must be segmented into 48-byte segments to be carried by a cell. • At the destination, these segments need to be reassembled to recreate the original payload. • Convergence sublayer (CS) - service specific functions • Prepare the data to guarantee their integrity

  40. ATM LAN • ATM is mainly a wide-area network, however the technology can be adapted to local-area network. • Two ways exist to incorporate ATM technology in LAN architecture: • Creating a pure ATM LAN • Making a legacy ATM LAN

  41. Pure ATM LAN • An ATM switch is used to connect the stations in a LAN. • Stations can exchange data at one of two standard rates of ATM technology (155 and 652 Mbps) • The stations uses a VPI and VCI instead of a source and destination address. • Drawback • The system needs to be built from the ground up; existing LANs cannot be upgraded into pure ATM LANs

  42. Legacy ATM LAN • Use ATM technology as a backbone to connect traditional LANs • Stations on the same LAN can exchange data at the same rate and format of traditional LANs (Ethernet, Token ring,…) • When two stations on two different LANs need to exchange data, they can go through a converting device that changes the frame format. • Output from several LANs can be multiplexed together to create a high data rate input to the ATM switch.

  43. Mixed Architecture • Mix the two previous architectures. • Keeping the existing LANs and at the same time allowing new stations to be directly connected to an ATM switch. • Allow the gradual migration migration of legacy LANs onto ATM LANs • By adding more and more directly connected stations to the switch. • Problem • How can a station in a traditional LAN communicate with a station directly connected to the ATM switch or vice versa?

  44. ATM LAN Emulation (LANE) • Many issues need to be resolved: • Connectionless versus connected-oriented. • Physical address versus virtual-circuit identifiers. • Multicasting and broadcasting delivery • Interoperability

  45. Architecture of LANE

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