Computer Networks with Internet Technology William Stallings

1. Computer Networks with Internet TechnologyWilliam Stallings Chapter 13 Wide Area Networks

2. Frame Relay Networks • Designed to eliminate much of the overhead in X.25 • Call control signaling on separate logical connection from user data • Multiplexing/switching of logical connections at layer 2 (not layer 3) • No hop-by-hop flow control and error control • Throughput an order of magnitude higher than X.25

3. Frame Relay Architecture • X.25 has 3 layers: physical, link, network • Frame Relay has 2 layers: physical and data link (or LAPF) • LAPF core: minimal data link control • Preservation of order for frames • Small probability of frame loss • LAPF control: additional data link or network layer end-to-end functions

4. Figure 13.1 Frame Relay User-Network Interface Protocol Architecture

5. LAPF Core • Frame delimiting, alignment and transparency • Frame multiplexing/demultiplexing • Inspection of frame for length constraints • Detection of transmission errors • Congestion control

6. Figure 13.2 LAPF-core Formats

7. Frame Relay User Data Transfer • No control field, which is normally used for: • Identify frame type (data or control) • Sequence numbers • Implication: • Connection setup/teardown carried on separate channel • Cannot do flow and error control

8. Asynchronous Transfer ModeATM • 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 (physical layer) 25.6Mbps to 622.08Mbps

9. ATM Logical Connections • Virtual channel connections (VCC) • Analogous to virtual circuit in X.25 • Basic unit of switching • Between two end users • Full duplex • Fixed size cells • Data, user-network exchange (control) and network-network exchange (network management and routing) • Virtual path connection (VPC) • Bundle of VCC with same end points

10. Figure 13.3ATM Connection Relationship

11. Advantages of Virtual Paths • Simplified network architecture • Increased network performance and reliability • Reduced processing • Short connection setup time • Enhanced network services

12. VP/VC Characteristics • Quality of service • Switched and semi-permanent channel connections • Call sequence integrity • Traffic parameter negotiation and usage monitoring • VPC only • Virtual channel identifier restriction within VPC

13. Control Signaling - VCC • Done on separate connection • Semi-permanent VCC • Meta-signaling channel • Used as permanent control signal channel • User to network signaling virtual channel • 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

14. Control Signaling - VPC • Semi-permanent • Customer controlled • Network controlled

15. ATM Cells • Fixed size • 5 octet header • 48 octet information field • 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

16. Figure 13.4 ATM Cell Format

17. Header Format • Generic flow control • Only at user to network interface • Controls flow only at this point • Virtual path identifier • Virtual channel identifier • Payload type • e.g. user info or network management • Cell loss priority • Header error control

18. Header Error Control • 8 bit error control field • Calculated on remaining 32 bits of header • Allows some error correction

19. Generic Flow Control (GFC) • Control traffic flow at user to network interface (UNI) to alleviate short term overload • Two sets of procedures • Uncontrolled transmission • Controlled transmission • Every connection either subject to flow control or not • Subject to flow control • May be one group (A) default • May be two groups (A and B) • Flow control is from subscriber to network • Controlled by network side

20. Single Group of Connections (1) • Terminal equipment (TE) initializes two variables • TRANSMIT flag to 1 • GO_CNTR (credit counter) to 0 • If TRANSMIT=1 cells on uncontrolled connection may be sent any time • If TRANSMIT=0 no cells may be sent (on controlled or uncontrolled connections) • If HALT received, TRANSMIT set to 0 and remains until NO_HALT

21. Single Group of Connections (2) • If TRANSMIT=1 and no cell to transmit on any uncontrolled connection: • If GO_CNTR>0, TE may send cell on controlled connection • Cell marked as being on controlled connection • GO_CNTR decremented • If GO_CNTR=0, TE may not send on controlled connection • TE sets GO_CNTR to GO_VALUE upon receiving SET signal • Null signal has no effect

22. Use of HALT • To limit effective data rate on ATM • Should be cyclic • To reduce data rate by half, HALT issued to be in effect 50% of time • Done on regular pattern over lifetime of connection

23. ATM Service Categories • 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) • Guaranteed frame rate (GFR)

24. Real Time Services • Amount of delay • Variation of delay (jitter)

25. CBR • Fixed data rate continuously available • Tight upper bound on delay • Uncompressed audio and video • Video conferencing • Interactive audio • A/V distribution and retrieval

26. rt-VBR • Time sensitive application • Tightly constrained delay and delay variation • rt-VBR applications transmit at a rate that varies with time • e.g. compressed video • Produces varying sized image frames • Original (uncompressed) frame rate constant • So compressed data rate varies • Can statistically multiplex connections

27. nrt-VBR • May be able to characterize expected traffic flow • Improve QoS in loss and delay • End system specifies: • Peak cell rate • Sustainable or average rate • Measure of how bursty traffic is • e.g. Airline reservations, banking transactions

28. UBR • May be additional capacity over and above that used by CBR and VBR traffic • Not all resources dedicated • Bursty nature of VBR • For application that can tolerate some cell loss or variable delays • e.g. TCP based traffic • Cells forwarded on FIFO basis • Best efforts service

29. ABR • Application specifies peak cell rate (PCR) and minimum cell rate (MCR) • Resources allocated to give at least MCR • Spare capacity shared among all ARB sources • e.g. LAN interconnection

30. Guaranteed Frame Rate (GFR) • Designed to support IP backbone subnetworks • Better service than UBR for frame based traffic • Including IP and Ethernet • Optimize handling of frame based traffic passing from LAN through router to ATM backbone • Used by enterprise, carrier and ISP networks • Consolidation and extension of IP over WAN • ABR difficult to implement between routers over ATM network • GFR better alternative for traffic originating on Ethernet • Network aware of frame/packet boundaries • When congested, all cells from frame discarded • Guaranteed minimum capacity • Additional frames carried of not congested

31. Cellular Wireless Networks • Underlying technology for mobile phones, personal communication systems, wireless networking etc. • Developed for mobile radio telephone • Replace high power transmitter/receiver systems • Typical support for 25 channels over 80km • Use lower power, shorter range, more transmitters

32. Cellular Network Organization • Multiple low power transmitters • 100w or less • Area divided into cells • Each with own antenna • Each with own range of frequencies • Served by base station • Transmitter, receiver, control unit • Adjacent cells on different frequencies to avoid crosstalk

33. Shape of Cells • Square • Width d cell has four neighbors at distance d and four at distance d • Better if all adjacent antennas equidistant • Simplifies choosing and switching to new antenna • Hexagon • Provides equidistant antennas • Radius defined as radius of circum-circle • Distance from center to vertex equals length of side • Distance between centers of cells radius R is R • Not always precise hexagons • Topographical limitations • Local signal propagation conditions • Location of antennas

34. Figure 13.5 Cellular Geometries

35. Frequency Reuse • Power of base transceiver controlled • Allow communications within cell on given frequency • Limit escaping power to adjacent cells • Allow re-use of frequencies in nearby cells • Use same frequency for multiple conversations • 10 – 50 frequencies per cell • E.g. • N cells all using same number of frequencies • K total number of frequencies used in systems • Each cell has K/N frequencies • Advanced Mobile Phone Service (AMPS) K=395, N=7 giving 57 frequencies per cell on average

36. Characterizing Frequency Reuse • D = minimum distance between centers of cells that use the same band of frequencies (called cochannels) • R = radius of a cell • d = distance between centers of adjacent cells (d = R) • N = number of cells in repetitious pattern • Reuse factor • Each cell in pattern uses unique band of frequencies • Hexagonal cell pattern, following values of N possible •  N = I2 + J2 + (I x J), I, J = 0, 1, 2, 3, … •  Possible values of N are 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, … • D/R= • D/d =

37. Figure 13.6 Frequency Reuse Patterns

38. Increasing Capacity (1) • Add new channels • Not all channels used to start with • Frequency borrowing • Taken from adjacent cells by congested cells • Or assign frequencies dynamically • Cell splitting • Non-uniform distribution of topography and traffic • Smaller cells in high use areas • Original cells 6.5 – 13 km • 1.5 km limit in general • More frequent handoff • More base stations

39. Increasing Capacity (2) • Cell Sectoring • Cell divided into wedge shaped sectors • 3 – 6 sectors per cell • Each with own channel set • Subsets of cell’s channels • Directional antennas • Microcells • Move antennas from tops of hills and large buildings to tops of small buildings and sides of large buildings • Even lamp posts • Form microcells • Reduced power • Good for city streets, along roads and inside large buildings

40. Figure 13.7 Frequency Reuse Example

41. Operation of Cellular Systems • Base station (BS) at center of each cell • Antenna, controller, transceivers • Controller handles call process • Number of mobile units may in use at a time • BS connected to mobile telecommunications switching office (MTSO) • One MTSO serves multiple BS • MTSO to BS link by wire or wireless • MTSO: • Connects calls between mobile units and from mobile to fixed telecommunications network • Assigns voice channel • Performs handoffs • Monitors calls (billing) • Fully automated

42. Figure 13.8 Overview of Cellular System

43. Channels • Control channels • Setting up and maintaining calls • Establish relationship between mobile unit and nearest BS • Traffic channels • Carry voice and data

44. Typical Call in Single MTSO Area (1) • Mobile unit initialization • Scan and select strongest set up control channel • Automatically selected BS antenna of cell • Usually but not always nearest (propagation anomalies) • Handshake to identify user and register location • Scan repeated to allow for movement • Change of cell • Mobile unit monitors for pages (see below) • Mobile originated call • Check set up channel is free • Monitor forward channel (from BS) and wait for idle • Send number on pre-selected channel • Paging • MTSO attempts to connect to mobile unit • Paging message sent to BSs depending on called mobile number • Paging signal transmitted on set up channel

45. Typical Call in Single MTSO Area (2) • Call accepted • Mobile unit recognizes number on set up channel • Responds to BS which sends response to MTSO • MTSO sets up circuit between calling and called BSs • MTSO selects available traffic channel within cells and notifies BSs • BSs notify mobile unit of channel • Ongoing call • Voice/data exchanged through respective BSs and MTSO • Handoff • Mobile unit moves out of range of cell into range of another cell • Traffic channel changes to one assigned to new BS • Without interruption of service to user

46. Figure 13.9 Example of Mobile Cellular Call

47. Other Functions • Call blocking • During mobile-initiated call stage, if all traffic channels busy, mobile tries again • After number of fails, busy tone returned • Call termination • User hangs up • MTSO informed • Traffic channels at two BSs released • Call drop • BS cannot maintain required signal strength • Traffic channel dropped and MTSO informed • Calls to/from fixed and remote mobile subscriber • MTSO connects to PSTN • MTSO can connect mobile user and fixed subscriber via PSTN • MTSO can connect to remote MTSO via PSTN or via dedicated lines • Can connect mobile user in its area and remote mobile user

48. Required Reading • Stallings chapter 13