Data Communications

1. Data Communications Asynchronous Transfer Mode and Frame Relay

2. What is ATM? • A packet switched, connection-oriented service • Local area, metro area, and wide area service • Can support real-time traffic and non-real-time traffic (data arrives in order, low delay) • Can support various levels of service (continuous, variable, available, and unspecified) • Very fast (up to 622 Mbps) • A complex technology and typically expensive

3. More What is ATM? • Similarities between ATM and packet switching • Transfer of data in discrete chunks • Multiple logical connections over single physical interface • In ATM flow on each logical connection is in fixed sized packets called cells • Minimal error and flow control • Reduced overhead • Data rates - 25.6Mbps to 622.08Mbps (155.5 Mbps necessary for full-motion video)

4. Overview of ATM Network • ATM is similar to IP – a mesh network of “routers” (ATM switches) • Two types of links in ATM • NNI (network – network interface) connects two ATM switches; UNI (user – network interface) connects switch to user device • ATM is connection-oriented • User must create a virtual circuit thru the ATM network (using virtual circuit ID); signals create circuit, maintain circuit, dissolve circuit

5. Protocol Architecture

6. Protocol Architecture • User plane • Provides for user information transfer along with flow control and error control • Control plane • Performs call and connection control functions • Management plane • Plane management • Management functions related to system as a whole; make sure the various planes coordinate their activities properly • Layer management • Provides operations, administration, and maintenance (OAM) services thru info packets that switches exchange to keep system running effectively

7. Protocol Architecture • Physical plane • Designed to run over SONET but can also run over FDDI, T-1, and T-3 • ATM Layer • Defines the cell format and how to respond to info found in the header. Also responsible for setting up and releasing connections, and performs congestion control • ATM Adaptation Layer (AAL) • Provides the interface between applications and the ATM layer

8. ATM Logical Connections • Virtual channel connections (VCC) • Analogous to virtual circuit in X.25 • Basic unit of switching between two end users • Full duplex • VCCs used for data, user-network exchange (control), and network-network exchange (network management and routing)

9. ATM Logical Connections • Two types of virtual circuits • Permanent virtual circuit – analogous to a leased telephone line • Switched virtual circuit – created using a connection protocol based on ITU-T Q.2931 • Virtual path connection (VPC) • Bundle of VCCs with the same end points

10. ATM Connection Relationships

11. Advantages of Virtual Paths • Simplified network architecture • Network transport functions can be applied to a channel or a path of channels • Increased network performance and reliability • Network deals with fewer entities • Reduced processing and short connection setup time • Much work is setting up path, reserving capacity for future channels • Enhanced network services • Path is used internally but also visible to user

12. Call Establishment Using VPs

13. What Are VCCs Used For? • Between end users • Used to carry end to end user data, control signals • VPC provides overall capacity, VCC organization done by users • Between end user and network • Used to carry control signaling between user and network (typos top of page 353 – VPC should be VCC) • Between network entities • Used to carry network traffic management and routing information

14. Control Signaling – VCC • Done on separate connection ; Four methods for establishing a VCC: • Semi-permanent VCC – no control signaling necessary • Meta-signaling channel - used as permanent control signal channel – this channel is used to set up other VCC signaling channels between user and network • User-to-network signaling virtual channel – Used for control signaling - Used to set up VCCs to carry user data • User-to-user signaling virtual channel • Within pre-established VPC • Used by two end users without network intervention to establish and release user-to-user VCC

15. ATM Cells • Fixed size • 5 octet header (cell tax) • 48 octet information field • Why so small? • Small cells reduce queuing delay for high priority cells • Small cells can be switched more efficiently • Easier to implement switching of small cells in hardware • Fixed-size makes programming more easy

16. ATM Cell Format

17. Header Format • Generic flow control • Used at user to network interface • Controls flow of data from user device into the ATM network only • Essentially two classes of connections – controlled and uncontrolled • Controlled – network provides info to user regarding how many cells it can send – like a credit mechanism for flow control • Uncontrolled – network simply enables or disables sending of cells – like X-ON/X-OFF flow control

18. Header Format • Virtual path identifier • An 8-bit (UNI) or 12-bit (NNI) path ID • Virtual channel identifier • A 16-bit channel ID. Together, VPI and VCI identify a logical connection • Payload type • Various types of user info or network management info • For example: leftmost bit identifies payload as user data or OAM info; second bit indicates whether cell has passed thru any congested switches; third bit might be used to indicate last cell in a sequence of cells

19. Header Format • Cell loss priority • CLP bit indicates a cell’s priority level • If congestion occurs, ATM has option of deleting cells to relieve congestion. Cells with CLP = 1 go first. • Header error control • See the following slides

20. Header Error Control • Provides for error checking on the header only • Payload is unprotected. Is this a good idea? • Fiber optic used – so low error rates • Some other layer can error detect the payload • Does it really make sense to error detect real-time traffic? • ATM needs the speed! • Uses x8 + x2 + x + 1 checksum • Allows some error correction (single-bit errors, which AT&T says happens 99.5% of time)

21. HEC Operation at Receiver

22. Header Error Control • HEC can also be used for providing synchronization • Apply error-checking method using 40 consecutive bits. If it does not generate a result consistent with the last 8 bits, shift one bit and try again. • Repeat above step until a consistent result is found. Could it be a coincidence? Try it three more times. All four succeed? You are in sync.

23. ATM Service Categories • An ATM network can support many types of traffic: • Real time • Constant bit rate (CBR) • Real time variable bit rate (rt-VBR) • Non-real time • Non-real time variable bit rate (nrt-VBR) • Available bit rate (ABR) • Unspecified bit rate (UBR)

24. CBR • Fixed data rate continuously available • Tight upper bound on delay • Can support uncompressed audio and video • Video conferencing • Interactive audio • A/V distribution and retrieval • Tightly controlled by Peak Cell Rate (PCR), Cell Transfer Delay (CTD), and Cell Delay Variation (CDV) • $$$$

25. rt-VBR • Time sensitive application • Tightly constrained delay and delay variation • rt-VBR applications transmit at a rate that varies with time • Examples include bursty voice and video • Can statistically multiplex connections • Parameters include Peak Cell Rate, Sustainable Cell Rate, and Maximum Burst Size • $$$

26. nrt-VBR • Non-real time VBR • Intended for bursty traffic with no tight constraints on delay and delay variation • Examples include airline reservations, banking transactions • Parameters include Peak Cell Rate, Sustainable Cell Rate, Maximum Burst Size, Cell Loss Ratio, Cell Transfer Delay • $$$

27. ABR • Application specifies Peak Cell Rate (PCR) and Minimum Cell Rate (MCR) • Resources allocated to give at least MCR • Spare capacity shared among all ABR sources • Examples include LAN interconnection and basic critical data transfer systems such as banking, defense information • (flying standby) • $$

28. UBR • For application that can tolerate some cell loss or variable delays (non-critical apps) • Cells forwarded on FIFO basis • Do not specify traffic related service guarantees • Examples include text/data/image transfer, messaging, remote terminals • Best effort service (wear your parachute) • $

29. ATM Bit Rate Services

30. ATM Adaptation Layer • Essentially the “translation layer” between ATM layer and other layers, such as PCM and IP: • PCM (voice) • Assemble bits into cells • Re-assemble into constant flow • IP • Map IP packets onto ATM cells • Fragment IP packets • Use LAPF over ATM to retain all IP infrastructure

31. AAL Protocols

32. Adaptation Layer Services • Handle transmission errors • Segmentation and re-assembly • To enable larger blocks of data to be carried in the information field of ATM cells • Handle lost and misinserted cells (cells routed the wrong way) • Perform flow control and timing control

33. Supported Application types • Four AAL protocols defined: • AAL 1: CBR traffic, e.g. circuit emulation (T-1 over ATM), voice over ATM, real-time video • AAL 2: rt-VBR traffic, e.g. MPEG voice and video • AAL 3/4: nrt-VBR traffic, e.g. general data service (not really used by anyone) • AAL 5 (successor to AAL 3/4): e.g. nrt-VBR: voice on demand; nrt-VBR: frame relay, ATM; UBR: IP over ATM

34. AAL 1 • AAL 1 is the interface between a real-time uncompressed byte stream and ATM • Got to be fast! • No convergence sublayer, only SAR sublayer • AAL 1 takes 46 or 47 bytes of data and puts a one or two byte header on front

35. AAL 1 continued • AAL 1 header consists of following: • One bit pointer – tells whether this is a one byte header or a two byte header. If second byte is included, this byte tells where the data starts within the payload (in case the payload does not contain a full 46 bytes of data) • Three-bit sequence number – used to tell if a cell is lost or mis-inserted (which may be too late anyway for real-time) • Four bits of error checking on preceding 3-bit sequence number (yikes!)

36. AAL 2 • AAL 2 format is used for compressed data, which needs to indicate where each frame of compressed data ends and begins • Similar to AAL 1 – no convergence sublayer, only the SAR sublayer • Unlike AAL 1, AAL 2 adds a header and a trailer

37. AAL 2 continued • The AAL 2 format has the following fields: • Sequence number – same as AAL 1 • Type field – helps identify message boundaries by indicating when a cell corresponds to the first, last, or intermediate cell of a message • Length field – specifies the number of bytes in the payload • Checksum – applied to the entire cell, including the data!

38. AAL 5 • AAL 5 packets can be very large – up to 65,535 byte payload • AAL 5 not designed for real-time traffic • SAR sublayer takes the potentially large convergence sublayer packets and breaks them into 48 byte chunks, ready for the ATM layer • SAR sublayer also adds a 32-bit CRC at the end of the packet, which is applied to the entire packet (see next slide for example)

39. Example AAL 5 Transmission

40. Frame Relay • What is it? • A high-speed communications technology that is used in hundreds of networks throughout the world to connect LAN, SNA, Internet, and even voice traffic. • Designed to be more efficient than X.25 • Developed before ATM • Larger installed base than ATM • ATM now of more interest on high speed networks

41. Recall X.25 • Call control packets, inband signaling • Multiplexing of virtual circuits at layer 3 • Layer 2 and 3 include flow and error control • Considerable overhead! • Not appropriate for modern digital systems with high reliability

42. Frame Relay - Differences • Call control carried in separate logical connection • Multiplexing and switching at layer 2 • Eliminates one layer of processing • No hop-by-hop error or flow control • End-to-end flow and error control (if used) are done by higher layer • Single user data frame sent from source to destination and ACK (from higher layer) sent back

43. Advantages and Disadvantages • Lost hop-by-hop error and flow control • Increased reliability makes this less of a problem • Streamlined communications process • Lower delay • Higher throughput • Tulsa, OK to NYC and back: • X.25: 1 sec delay round trip • Frame relay: 70 msec delay round trip

44. Protocol Architecture

45. Control Plane • Between subscriber and network • Separate logical channel used • Similar to common channel signaling for circuit switching services • Data link layer • LAPD (Q.921) • Reliable data link control • Error and flow control between user (TE) and network (NT) • Used for exchange of Q.933 control signal messages

46. User Plane • End to end functionality • Transfer of info between ends • LAPF (Link Access Procedure for Frame Mode Bearer Services) Q.922 • Frame delimiting, alignment and transparency • Frame mux and demux using addressing field • Ensure frame is integral number of octets (zero bit insertion/extraction) • Ensure frame is neither too long nor short • Detection of transmission errors • Congestion control functions

47. Frame Format

48. Frame Fields • DLCI – Denotes the port to which the destination LAN (or device) is attached • The routing tables at each intervening frame relay switch use the DLCI to route the frames to the proper destination • FECN and BECN – Congestion control techniques • DE – Discard Eligibility bit – Have you exceeded your data rate + burst rate for more than two seconds?

49. Frame Relay Operation • Each frame relay switch performs following: • 1. Check integrity of frame (FCS) • 2. Look up DLCI in a table • 3. Relay frame out appropriate port or trunk

50. Any Problems? • Just discard the frame! • Frame check error? Discard frame • Congestion? Discard frame • Invalid DLCI? Discard frame • Who informs the sender that a frame was discarded? • Not frame relay! (Let TCP do it)