Chapter 18 Virtual-Circuit Networks: Frame Relay and ATM

1. Chapter 18 Virtual-Circuit Networks: Frame Relay and ATM

2. 18-1 FRAME RELAY 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. Topics discussed in this section: ArchitectureFrame Relay Layers Extended Address FRADs VOFR LMI

3. Frame Relay • Frame Relay • Prior to FR, they were using a virtual-circuit switching network called X.25. • Drawbacks of X.25 • X.25 has a low 64 kbps data rate. • X.25 has extensive flow and error control at the data & the network layer - Flow & error control at both layers create a large overhead and slow down transmissions. • X.25 has its own network layer that user’s data are encapsulated in the network layer packets of X.25. - Internet has its own network layer. - If the Internet wants to use X.25, this doubles the overhead.

4. Feature X.25 Frame Relay Connection establishment At the network layer None Hop-by-hop flow control and error control At the data link layer None End-to-end flow control and error control At the network layer None Data rate Fixed Bursty Multiplxing At the network layer At the data link layer Congestion control Not necessary Necessary X.25 vs. Frame Relay

5. X.25 vs. Frame Relay

6. Features of Frame Relay • Operating in higher speed such as 1.544 Mbps, 45Mbps • Operating in just the physical and data link layers • Allowing bursty data • Allowing a frame size of 9000 bytes, which can accommodate all local area network fame sizes • less expensive • Error detection at the data link layer only • no flow control or error control

7. Architecture (Frame Relay) Figure 18.1 Frame Relay network

8. Virtual Circuit (Frame Relay) • A Virtual Circuit Identifier (VCI) in FR is identified by a number called a Data Link Connection Identifier (DLCI) • Permanent Virtual Circuits (PVC) • The connection setup is simple. The corresponding table entry is recorded for all switches by the administrator.

9. Virtual Circuit (Frame Relay) • Switched Virtual Circuit (SVC) • The SVC creates a temporary, short connection that exists only when data are being transferred between source and destination.

10. FR Switch • Switches • Each switch in a Frame Relay network has a table to route frames.

11. Frame Relay Layers • Frame Relay operates only at the physical and data link layers • Frame Relay does not provide flow or error control; they must be provided by the upper-layer protocols.

12. Frame Relay frame Figure 18.3 Frame Relay frame EA 0 : meaning that another address byte is to follow DE 1 : to discard this frame if there is congestion

13. Frame Relay frame (EXTENDED ADDRESS) Figure 18.4 Three address formats

14. FRAD • FRAD (Frame Relay Assembler/ Disassembler) • A FRAD assembles and disassembles frames coming from other protocols to allow them to be carried by Frame Relay frames. Figure 18.5 FRAD

15. Voice Over Frame Relay • VOFR (Voice Over Frame Relay) • VOFR sends voice through the network. • Voice is digitized using PCM and then compressed. • The result is sent as data frames over the network. • LMI (Local Management Information) • F/R was originally designed to provide PVC connections. • There was not a provision for controlling or managing intefaces. • LMI is a protocol added recently to the FR protocol to provide more management features. • A keep alive mechanism to check if data are flowing. • Multicasting mechanism • A mechanism to allow an end system to check the status of a switch.

16. Congestion Control (Frame Relay) • Congestion Avoidance • The FR protocol uses 2 bits in the frame to explicitly warn the source and the destination of the presence of congestion. • BECN(Backward Explicit Congestion Notification) • BECN bit warns the sender of congestion in the network.

17. Congestion Control (Frame Relay) • FECN(Forward Explicit Congestion Notification) • FECN bit is used to warn the receiver of congestion in the network.

18. Congestion Control (Frame Relay) • 4 cases of congestion

19. 18-2 ATM Asynchronous Transfer Mode (ATM) is the cell relay protocol designed by the ATM Forum and adopted by the ITU-T. Topics discussed in this section: Design GoalsProblemsArchitecture Switching ATM Layers

20. ATM - Design Requirements • Foremost is the need for a transmission system to optimize the use of high-data-rate transmission media, in particular optical fiber. • A technology is needed to take advantage of large bandwidth and strength to noise degradation. • The system must interface with existing systems. • Must be implemented inexpensively. • The new system must be able to work with and support the existing telecommunications hierarchies. • The new system must be connection-oriented to endure accurate and predictable delivery. • One objective is to move as many of the function to hardware as possible and eliminate as many software functions as possible.

21. Problems (ATM) • Problems associated with existing systems. • Frame networks • Different protocols use frames of varying size and intricacy. • As networks become more complex, the information that must be carried in the header becomes more extensive. • Mixed network traffic • The variety of frame size makes traffic unpredictable. • Internetworking among the different frame networks is slow and expensive at best, and impossible at worst. - The sheer size of X creates an unfair delay for frame A.

22. Cell Networks (Multiplexing Using Cells) • Cell Networks • A cell is a small data unit of fixed size. • Cell network uses the cell as the basic unit of data exchange, all data are loaded into identical cells that can be transmitted with complete predictability and uniformity. • Because each cell is the same size and all are small, the problems associated with multiplexing different-sized frames are avoided. • Despite interleaving, 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.

23. Asynchronous TDM • AsychronousTDM • ATM uses asynchronous time-division multiplexing – that is why it is called Asynchronous Transfer Mode – to multiplex cells coming from different channels.

24. Architecture of ATM Network • Architecture • ATM is a cell-switched network. • The user access devices, called the endpoints, are connected through a UNI (User- to-Network Interface) to the switches inside the network. • The switches are connected through NNI (Network- to-Network interface) .

25. Virtual Connection of ATM Network • Virtual Connection – Connection between two endpoints is accomplished through transmission paths (TPs), Virtual paths (VPs), and Virtual circuits (VCs). • TP (Transmission Path) • TP is the physical connection (cable, satellite, and so on) between an endpoint and a switch or between two switches. • VP (Virtual Path) • VP provides a connection or a set of connections between two switches. (A TP is divided into several VPs) • VC (Virtual Circuit) • A VP is logically divided into several VCs. • Think of a VC as the lanes of highway (VP).

26. Virtual Connection of ATM

27. Example of VPs and VCs • Identifiers • VPI (Virtual Path Identifier) • The VPI defines the specific VP. • VCI (Virtual Circuit Identifier) • The VCI defines a particular VC inside the VP.

28. Connection Identifiers Note that a virtual connection is defined by a pair of numbers: the VPI and the VCI.

29. Virtual connection identifiers in UNIs and NNIs • VPI for UNI and NNI • The lengths of the VPIs for UNIs(8b) and NNIs(12b) are different. • The lengths of the VCI is the same in both interface (16bits) • Dividing a VCI into two parts is to allow hierarchical routing. • Most of the switches in typical ATM network are routed using VPIs. • The switches at the boundaries of the network, those that interact directly with the endpoint devices, use both VPIs and VCIs.

30. An ATM Cell • cells • The basic data unit in an ATM network is called a cell. • A cell is only 53 bytes long • 5 bytes allocated to the header • 48 bytes carrying the payload.

31. PVC & SVC • Connection Establishment and Release • PVC • The VPIs and VCIs are defined for the permanent connections, and the values are entered for the tables of each switch. • SVC • 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).

32. Routing with a ATM Switch • Switching • A switch routes the cell using both the VPIs and VCIs. • The routing requires the whole identifier.

33. ATM Layers • ATM Layers • AAL (Application Adaptation Layer) • ATM layer • Physical layer

34. ATM layers in endpoint devices and switches • ATM layers • The switches use only the two bottom layers. • The endpoints use all three layers.

35. Physical Layer • Physical Layer • ATM cells can be carried by any physical layer carrier. • ATM layer is divided into two parts. • Physical medium : 매체와 비트 타이밍 기능 정의 • Transmission convergence : 셀의 생성 및 전송 확인 • SONET • ATM was based on SONET as the physical layer carrier. • The high data rate of SONET reflects the philosophy of ATM. • The boundaries of cells can be clearly defined in using SONET. • Other Physical Technologies • ATM does not limit the physical layer to SONET. • Other technologies, even wireless, may be used, If the problem of cell boundaries are solved. • One solution is for the receiver to guess the end of the cell and apply the CRC to the 5-byte header.

36. ATM Layer • ATM Layer • ATM layer provides routing, traffic management, switching, and multiplexing services. • Outgoing Processes • Accepting 48-byte segment from the AAL sublayer • Transforming them into 53-byte cells by the adding of a 5-byte header.

37. ATM Headers • ATM Header Format • Format for UNI Cells : User-to-Network Interface • Format for NNI Cells : Network-to-Network Interface.

38. ATM Headers • Generic flow control (GFC) • The 4-bit GFC field provides flow control at the UNI level. • UNI level of flow control is not necessary at the NNI level, therefore, these bits are added to the VPI. • Virtual Path Identifier (VPI) • The VPI is an 8-bit field in a UNI cell and a 12-bit field in an NNI cell. • Virtual Circuit Identifier (VCI) • The VCI is a 16-bit field in both frames. • Payload Type (PT) • 3-bit PT field defines the payload as user data or managerial information. • Cell loss Priority (CLP) • The 1-bit CLP field is provided for congestion control. • A cell with its CLP bit set to 1 must be retained as long as there are cells with CLP of 0. • Header Error Correction (HEC) • The HEC is a code computed for the first 4 bytes of the header. • It is a CRC that is used to correct single-bit errors and a large class of multiple-bit errors.

39. Application Adaptation Layer (AAL) • Application Adaptation Layer (AAL) • ATM must accept any type of payload, both data frames and streams of bits. • Segmentation and Reassembly (SAR) sublayer • The AAL defines a SAR sublayer, to do so, segmentation is at the source; reassembly, at the destination. • Convergence Sublayer (CS) • Before data are segmented by SAR, they must be prepared to guarantee the integrity of the data. This is done by a CS. ATM defines four versions of the AAL : • AAL1, AAL2, AAL3/4, and AAL5

40. AAL1 • AAL1은 비디오/음성같은 고정 비트률의 정보(CBR) 전달에 응용 지원 • ATM을 기존의 음성채널과 T회선 같은 디지털 전화망에 연결할 수 있도록 허용

41. AAL2 • - 가변 비트 스트림(VBR)을 지원,-압축/비압축의 낮은 비트율 오디오와 짧은 트래픽에 사용 • - 같은 발신지 또는 여러 발신지로부터의 짧은 프레임을 하나의 셀로 캡슐화 지원

42. AAL3/4 • AAL3은 연결중심 데이터 서비스를, AAL4는 비연결형 서비스를 지원하므로 쟁점 같아 통합 • 포괄적인 순서화와 오류제어 메커니즘을 제공 – SMDS 서비스 제공(VBR패킷 전송에 사용)

43. AAL5 • - 단순하고 효과적인 응용층(SEAL : simple and efficient adaptation layer) 지원 • - 다중화 기능이 없으므로 순서화나 오류정정 메커니즘이 불필요 • - 한 메시지에 속한 모든 셀은 순차적으로 전송되며, connection-oriented VBR • 서비스를 지원