1 ITGD4103 Data Communications and Networks Lecture-14: Frame Relay and ATM week 14- q-2/ 2008 Dr. Anwar Mousa University of Palestine Faculty of Information Technology

Frame Relay

3 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, Mbps and 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 Mbit/s to 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 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.

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 technology of 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 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 µs to transmit.{ = (1/155)*12000} In a lower-speed link, such as a 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 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. –Interoperability

45 Architecture of LANE