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Chapter 18 Virtual-Circuit Networks : Frame Relay and ATM. 18-1 FRAME RELAY. Packet switching can use two approaches: the virtual circuit approach and the datagram approach.
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Chapter 18 Virtual-Circuit Networks: Frame Relay and ATM
18-1 FRAME RELAY • Packet switching can use two approaches: the virtual circuit approach and the datagram approach. • Frame Relay is a virtual-circuit wide-area network that was designed in response to demands for a new type of WAN in the late 1980s and early 1990s. • Frame Relay is a relatively high-speed protocol that can provide some services not available in other WAN technologies such as DSL, cable TV, and T lines. • Frame relay, the outgrowth of the older(1970’s), slower (64Kbps), more careful, error-correcting X.25, is packet technology designed to carry variable-length frames over high-quality connections such as fiber, which was just coming into its own in the early 1990s.
WANs Based on T-1 and T-3 Lines • T-1 and T-3 Lines are leased from public service providers. • If an organization has n branches spread over an area, it needs n(n-1)/2 lines. Very costly especially when an organization uses them 10% of the time. • The services provided by T-1 and T-3 Lines assume that the user has a fixed data rate at all times. E.g. a T-1 line is designed for a user who wants to use the line at a consistent 1.544 Mbps. This type of service is not suited for the many users today that need to send bursty data. • E.g. A user may want to send data at 6Mbps for 2s, 0Mbps for 7s, and 3.44Mbps for 1s : Total 15.44Mbits during 10s i.e. average is still 1.544Mbps…but the T-1 line cannot accept this type of demand. • Bursty data require what is called bandwidth on demand.
FRAME RELAY vs. ATM • Data link layer of OSI model defines the ways of encapsulating data for transmission between two endpoints and the techniques of transferring the frames. • Both Asynchronous Transfer Mode (ATM) and Frame relay are data link layer technologies and they have connection oriented protocols. • Each technique has its own application dependent advantages and disadvantages.
FRAME RELAY vs. ATM • ATM is a network switching technology that uses a cell based methodology to quantize data. ATM data communication consists of fixed size cells of 53 bytes. An ATM cell contains a 5 byte header and 48 bytes of ATM payload. This smaller size, fixed-length cells are good for transmitting voice, image and video data as the delay is minimized. • ATM is a connection oriented protocol and therefore a virtual circuit should be established between sending and receiving points. It establishes a fixed route between two points when the data transfer starts. • Another important aspect of ATM is its asynchronous operation in time division multiplexing. Therefore cells are transmitted only when data is available to be sent unlike in conventional time division multiplexing where synchronization bytes are transferred if there data is not available to be sent. • ATM is designed to be convenient for hardware implementation and therefore processing and switching have become faster. Bit rates on ATM networks can go up to 10 Gbps. ATM is a core protocol used over the SONET/SDH backbone of the ISDN. • ATM provides a good quality of service in networks where different types of information such as data, voice, and are supported. With ATM, each of these information types can pass through a single network connection.
FRAME RELAY vs. ATM • Frame relay is a packet switching technology for connecting network points in Wide Area Networks (WAN). It is a connection oriented data service and establishes a virtual circuit between two end points. Data transfer is done in packets of data known as frames. These frames are variable in packet size and more efficient due to flexible transfers. Frame Relay was originally introduced for ISDN interfaces though it is currently used over a variety of other network interfaces as well. • In frame relay, connections are called as ‘Ports’. All the points which need to connect to the frame relay network needs to have a port. Every port has a unique Address. A frame is made of two parts which can be called as ‘actual data’ and the ‘frame relay header’. Frame architecture is same as defined for LAP-D (Link Access Procedures on the D channel) which has a variable length for information field. These frames are sent over Virtual Connections. • Frame relay can create multiple redundant connections among various routers, without having multiple physical links. Since frame relay is not specific for media, and provides means to buffer speed variations, it has the possibility to create a good interconnect medium between different types of network points with different speeds.
Difference between ATM and Frame Relay • Although both techniques are based on end to end delivery of quantized data, there are many differences in terms of sizes of the data quanta, application network types, controlling techniques etc. • 2. Although ATM uses fixed size packets (53 bytes) for data communication, frame relay uses variable packet sizes depending on the type of information to be sent. Both information blocks have a header in addition to data block and transfer is connection oriented. • 3. Frame Relay is used to connect Local Area Networks (LAN) and it is not implemented within a single area network contrast to ATM where data transfers are within a single LAN. • 4. ATM is designed to be convenient for hardware implementation and therefore, cost is higher compared to frame relay, which is software controlled. Therefore frame relay is less expensive and upgrading is easier. • 5. Frame relay has a variable packet size. Therefore it gives low overhead within the packet which results it an efficient method for transmitting data. Although fixed packet size in ATM, can be useful for handling video and image traffic at high speeds, it leaves a lot of overhead within the packet, particularly in short transactions.
Frame Relay Features • FR operates at a higher speed (1.544Mbps and recently 44.376 Mbps) • FR operates in just the physical and data link layers. This means it can be used as a backbone network to provide services to protocols that already have a network layer protocol, such as the Internet. • FR allows bursty data • FR allows a frame size of 9000bytes, which can accommodate all local area network frame sizes. • FR is less expensive than other traditional WANs. • FR has error detection at the data link layer only. No flow control or error control. FR was designed in this way to provide fast transmission capability for those protocols that have flow and error control at the higher layers.
Figure 18.1 Frame Relay network • FR provides permanent virtual circuits and • switched virtual circuits. • The FR WAN is used as one link in the global • Internet. Switch Table matches an incoming port-DLCI combination with an outgoing port-DLCI combination. (as in Chap.8 VCIs are replaced by DLCIs.)
Note VCIs in Frame Relay are called DLCIs. Data Link control identifiers
Permanent vs. Switched Virtual Circuits (PVC vs. SVC) • In PVC the connection setup is very simple. The corresponding table entry is recorded for all switches by the administrator (remotely and electronically). An outgoing DLCI is given to the source, and an incoming DLCI is given to the destination. • PVCs have 2 drawbacks: • Costly…pay for connection all the time • A connection is created from one source to one single destination. If a source needs connections with several destinations, it needs a PVC for each connection. • SVC creates a temporary, short connection that exists only when the data are being transferred between source and destination. SVC requires establishing and terminating phases (Chapter 8).
Figure 18.2 Frame Relay layers No flow or error control, only an error detection Mechanism.
Note Frame Relay operates only at the physical and data link layers.
Note Frame Relay does not provide flow or error control; they must be provided by the upper-layer protocols.
Figure 18.5 FRAD (Frame Relay Assembler Disassembler) FRAD assembles and disassembles frames coming from other protocols to allow them to be carried by FR frames. A FRAD can be implemented as a separate device or as part of a switch.
Congestion Control and Quality of Service • One of the nice features of FR is that it provides Congestion Control and Quality of Service (QoS), two important aspects of networking.
18-2 ATM • ATM was designed in the 1980s to deliver five distinct levels of QoS, so users could send traffic with greater or less delay. • Asynchronous Transfer Mode (ATM) is the cell relay protocol designed by the ATM Forum and adopted by the ITU-T. • Key to ATM’s charm was that it could emulate direct circuits and guarantee bandwidth, a shortcoming of frame relay. • Frame relay won in the WAN. ATM lived on, though in carrier core networks, where it is slowly being decommissioned. • ATM has been accepted universally as the transfer mode of choice for Broadband Integrated Services Digital Networks(BISDN). ATM can handle any kind of information i.e. voice, data, image, text and video in an integrated manner.
Design Goals • The need for a transmission system to optimize the use of high-data-rate transmission media, in particular optical fiber. • The system must interface with existing systems and provide wide-area interconnectivity between them. • The design must be implemented inexpensively. If ATM is to become the backbone of international communications, it must be available at a low cost. • The new system must be able to work with and support the existing telecommunications hierarchies (local loops, local providers, long-distance carriers, etc.) • The new system must be connection-oriented to ensure accurate and predictable delivery. • One objective is to move as many of the functions to hardware as possible (for speed) .
Figure 18.6 Multiplexing using different frame sizes The variety of frame sizes makes traffic unpredictable. Switches, multiplexers, and routers must incorporate elaborate software systems to manage various sizes of frames. Internetworking among the different frame networks is slow and expensive. Problem: Providing consistent data rate delivery when frame sizes are unpredictable and can vary dramatically. To get the most out of broadband technology, traffic must be time-multiplexed onto shared paths. E.g. :
Note A cell network uses the cell as the basic unit of data exchange. A cell is defined as a small, fixed-size block of information. Because each cell is the same size and all are small, the problems associated With multiplexing different-sized frames are avoided.
Figure 18.7 Multiplexing using cells The cells are interleaved so that none suffers a long delay. A cell network can handle real-time transmissions, such as a phone call, without the parties being aware of the segmentation or multiplexing at all.
Figure 18.8 ATM multiplexing ATM uses asynchronous time-division multiplexing. It uses fixed-size slots (size of a cell).
Figure 18.9 Architecture of an ATM network ATM is a cell-switched network. The user access devices, called endpoints, are connected thru a user-to-network interface UNI. to the switches inside the network. Network-to-network Interface
Virtual Connection • Between two endpoints is accomplished thru transmission paths (TPs), virtual paths (VPs), and virtual circuits (VCs). • A TP is the physical connection (wire, cable, satellite,..etc.) between an endpoint and a switch or between two switches. • A TP is divided into several VPs. • A VP provides a connection or a set of connections between two switches. • Cell networks are based on VCs. • All cells belonging to a single message follow the same virtual circuit and remain in their original order.
Note Note that a virtual connection is defined by a pair of numbers: the VPI and the VCI.
Figure 18.12 Connection identifiers • In a UNI, the VPI is 8 bits, whereas in an NNI • the VPI is 12 bits. The length of the VCI is the • same in both interfaces (16 bits). • Hence a virtual connection is identified by • 24 bits in a UNI and by 28 bits in an NNI. • The idea behind dividing a VCI into 2 parts • is to allow hierarchical routing.
Figure 18.13 Virtual connection identifiers in UNIs and NNIs
Figure 18.14 An ATM cell The basic data unit in an ATM network is called a cell.
Connection Establishment and Release • Like FR, ATM uses two types of connections : PVC and SVC • PVC : a permanent virtual-circuit is established between two endpoints by the network provider. The VPIs and VCIs are defined for the permanent connection and the values are entered for the tables of each switch. • SVC : In a switched virtual-circuit connection, each time an endpoint wants to make a connection with another endpoint, a new virtual circuit must be established. ATM cannot do the job by itself, but needs the network layer addresses and the services of another protocol (such as IP).
Figure 18.15 Routing with a switch ATM uses switches to route the cell from a source endpoint to the destination endpoint.
Figure 18.16 ATM layers Application Adaptation Layer
Figure 18.17 ATM layers in endpoint devices and switches • SONET : The original design of ATM was based on SONET as the physical layer carrier: • First, the high data rate of SONET • Second, in SONET, the boundaries of cells can be clearly defined. • Other Physical Technologies : ATM does not limit the physical layer to SONET. • Other technologies such as wireless may be used. Problem : cell boundaries !.. but there is a solution
Figure 18.18 ATM layer The ATM layer provides : routing, traffic management, switching, and multiplexing services. It processes outgoing traffic by accepting 48-byte segments from the AAL sublayers and transforming them into 53-byte cells by the addition of a 5-byte header.
Figure 18.20 AAL1 Supports applications that transfer information at constant bit rates such as video and voice. It allows ATM to connect existing digital telephone networks such as voice channels and T lines.
Figure 18.21 AAL2 It is used for low bit rate traffic and short-frame traffic such as audio, video, or fax. Ex. In mobile telephony
18-3 ATM LANs ATM is mainly a wide-area network (WAN ATM); however, the technology can be adapted to local-area networks (ATM LANs). The high data rate of the technology has attracted the attention of designers who are looking for greater and greater speeds in LANs.
ATM Applications • There are several practical applications using ATM Technology. • ATM is the Backbone Network for many broadband • applications including Information SuperHighway. • Some of the key applications can be mentioned as follows: • Video Conferencing • Desktop Conferencing • Multimedia Communications • ATM Over Satellite Communications • Mobile Computing over ATM for Wire-less Networks
