Chapter 12 Wide Area Network Services

1. Chapter 12Wide Area Network Services Part III: Wide Area Networks and Internetworking Technologies

2. Topics Addressed in Chapter 12 • WAN services fundamental concepts • Packet distribution networks • Frame relay networks • Asynchronous transfer mode (ATM) • T-1 services • SONET services • ISDN • Wireless WAN services • Choosing among WAN services Stamper and Case

3. WAN Services Fundamentals • There are two major categories of WAN connections: • Circuit-switched networks • Packet-switched networks • Switching is fundamental to both approaches • Switching technologies establish paths across networks from senders to receivers • Switching allows connections to be established and maintained between senders and receivers so that they can exchange messages and information Stamper and Case

4. Circuit-Switched Networks • In circuit-switched networks, a switched dedicated circuit is created to connect two (or more) parties • To users, it is as if a direct physical point-to-point path is established between sender and receiver • Multiple-switches may be involved is establishing a switched connection (see Figure 12-1) • There are three phases to circuit-switched communications: • Creation of the temporary circuit • Information transmission • Circuit termination • Because there is a limit to the number of switched connections that can be established at a particular point in time, circuit-switched network users may not be able to initiate communication sessions during peak usage times Stamper and Case

5. Figure 12-1 Stamper and Case

6. Packet-Switched Networks • In packet-switched networks (see Figure 12-2), data is packetized prior to transmission • Each packet is a group of bits organized in a predetermined structure • Each packet contains data bits as well as additional overhead information to ensure error-free transmission to intended recipients • Packets may be called blocks, cells, datagrams, data units, or frames • Packet assembler/disassemblers (PADs) are responsible for assembling outgoing data into packets for transmission over the packet-switching network as well as for unpacking incoming packets so that data can be delivered to intended recipients Stamper and Case

7. Figure 12-2 Stamper and Case

8. Packet Formats • Figure 12-3 illustrates the format of HDLC packets used in X.25 packet-switching networks Major overhead fields include: • Flag: used to delimit the beginning and end of a packet • Address: specifies the address of the intended packet recipient • Control: transports packet sequence numbers and retransmission requests • Frame check: used for error checking. CRC-16 or a 16-bit checksum may be used with HDLC frames Stamper and Case

9. Figure 12-3 Stamper and Case

10. Packet-Switching Advantages and Disadvantages • Relative to circuit-switching, packet-switching has a number of advantages and disadvantages. Advantages include: • A single-link between packet-switching nodes can be simultaneously shared by multiple senders and receivers; senders are not denied access to the network during peak usage periods • Packet-priority systems can be established • Subscribers to packet-switching services are often charged on the volume of data (number of packets) transmitted rather than connection time • Disadvantages include: • Variable transmission delays caused by packet processing and packet queues at packet switches • Some packet-switching networks support variable packet sizes; this contributes to longer packet processing times at packet switches • The inclusion of overhead data in packets means that data transmission efficiency and throughput is lower than that in circuit-switched networks Stamper and Case

11. Switching Alternatives in Packet-Switched Networks • Two fundamental approaches are used to route packets from senders to receivers: • Datagram approach: individual packets, even those associated with a single file, are routed independently • Two packets (datagrams) from the same source can have two different temporary circuits established to the same recipient • This type of circuit allocation is called connectionless because a dedicated connection is not established and because the packets that make up a single file do not follow each other over the same circuit from sender to receiver • Virtual circuit approach: this is similar to establishing a dedicated circuit in a circuit-switched network. Packets that comprise a single file (or message) follow the same route in sequence from sender to receiver. • This type of packet-switching is called connection-oriented • It is not identical to circuit-switched connections because the route segments in virtual circuits are shared, not dedicated Stamper and Case

12. Virtual Circuits • Call setup packets are used to establish virtual circuits; these are used to identify the best path to the destination across the network. • Virtual circuit details are stored in virtual circuit tables at packet switches • The paths followed by packets in virtual circuits are called logical channels; each packet includes a logical channel number when created by the PAD • There are two major types of virtual circuits: • Switched virtual circuits (SVCs): which are similar to temporary circuit-switched connections • Permanent virtual circuits (PVCs): which is similar to a leased, circuit-switched connection • Once a PVC is allocated, no call setup or call clearing is needed; the logical circuit is permanently stored in virtual circuit tables Stamper and Case

13. Packet Distribution Networks (PDNs) • PDN concepts were first introduced in 1964; ARPAnet (and today’s Internet) are grounded in PDN and packet-switching concepts • A PDN is sometimes called an X.25 network or public data network. • The X.25 designation stems from ITU’s recommendation X.25 which defines the interface between DTE and DCE for public data networks (see Figure 12-4) • The term value-added network (VAN) is often used in conjunction with PDNs because network proprietors offer additional services beyond mere data transmission including virtual circuits, error recovery, network management, message priorities, and store-and-forward capabilities • X.25 PDNs are more widely available outside the U.S. • In the U.S., frame relay services are more common than X.25 Stamper and Case

14. Figure 12-4 Stamper and Case

15. PDNs and the OSI Reference Model • Only three layers of the OSI reference model are described for PDNs (see Figure 12-5) because a PDN is only responsible for message delivery: • Packet Layer Protocol (PLP) in the X.25 protocol stack corresponds to OSI’s network layer • HDLC (High-Level Data Link Control) and LAPB (Link Access Protocol—Balanced) are X.25 data link layer protocols. LAPB plays a key role in error recovery • X.21 is the physical layer standard for X.25 networks; RS232 serves as the physical layer protocol for communications between terminals and packet-switching nodes Stamper and Case

16. Figure 12-5 Stamper and Case

17. Important X.25 PDN Standards • X.25: defines interface between DTE and DCE in public data networks • X.21: specifies the interface between user terminal equipment and PDN packet-switching nodes • X.3: specifies packet assembly/disassembly processes • X.28: governs asynchronous dial-up access to PDNs • X.29: governs synchronous dial-up access to PDNs • X.75: defines the interface between different public packet-switching networks, both domestic and international • X.121: defines a global addressing scheme for PDNs • Several of these are illustrated in Figure 12-7 Stamper and Case

18. Figure 12-7 Stamper and Case

19. PDN Error Correction Processes • PDNs employ node-to-node (aka hop-to-hop or point-to-point) error detection and correction (see Figure 12-8) • Each packet is checked for errors at each packet switch before being forwarded to the next hop on its path • If no errors are detected, an ACK is sent to the previous hop • If errors are detected, a NAK is sent to the previous hop which triggers retransmission of the packet • This process means that PDNs are store-and-forward networks; packets are stored at switching nodes until positive acknowledgements are received Stamper and Case

20. Figure 12-8 Stamper and Case

21. Frame Relay Networks • Frame relay is an outgrowth of X.25 networks • It is designed to eliminate much of the transmission overhead associated with X.25 networks and to take advantage of higher-speed, less error-prone, and more reliable digital circuits to connect switching nodes. • To improve throughput over X.25, frame relay networks point-to-point error detection and end-to-end error correction (see Figure 12-10) • Frames with errors are discarded by frame relay switches; missing frames cause recipients to request retransmission • Many ISPs, especially those in urban areas, support frame-relay-based Internet access • Higher transmission speeds make frame relay more attractive than X.25 for interconnecting geographically distributed LANs Stamper and Case

22. Figure 12-10 Stamper and Case

23. Frame Relay Technologies • Key technologies in frame relay networks are illustrated in Figure 12-9. These include: • Frame assembler/dissembler devices (FRADs) which like X.25 PADs are responsible for building outgoing frames and unpacking incoming frames • Frame relay switches which are responsible for accepting frames, checking them for errors, and transmitting them to their next hops in the network • Both switched and permanent virtual circuits are supported in frame relay networks • Frame relay circuits. Frame relay switches are typcially connected by DS-1 (T-1) or DS-3 (T-3) circuits. The Frame Relay Form (FRF) has addressed connections up to 622 mbps (OC-12) Stamper and Case

24. Figure 12-9 Stamper and Case

25. Frame Formats • Frames are formatted by FRAD devices or software • Variable length frames may be supported; some may include up to 8,000 characters • Figure 12-11 shows a LAPD (Link access Procedure—D channel) frame relay • The address field carries the recipient’s network address as well as a data link connection identifier (DLCI) that serves the same purpose as a virtual circuit identifier in X.25 (see Figure 12-12) • The BCEN, FCEN, and DE fields are used to address network congestion during peak usage periods Stamper and Case

26. Figure 12-11 Figure 12-12 Stamper and Case

27. Asynchronous Transfer Mode (ATM) • ATM is a high-bandwidth, low-delay, packet-switching and multiplexing technology that can handle many types of network traffic and WAN services • ATM represents a step in the evolution of frame relay by using frames (called cells) that do not vary in size • The use of small fixed-size packets translates into easier switching and faster transmission rates. • By 2002, ATM transfer rates of 38.813 gbps had been achieved over OC-768 circuits • Virtual channels are used in ATM to establish logical connections between senders and receivers (see Figure 12-13) • Once setup up, full-duplex variable-rate transmission is possible over the connections • Virtual paths are also supported. These are bundles of virtual channels with the same end-points that are switched as a set. Each channel can carry a different type of data Stamper and Case

28. Figure 12-16 Stamper and Case

29. Figure 12-13 Stamper and Case

30. ATM Cell Formats • Two cell formats have been specified for ATM (see Figure 12-14): • User-network interface (UNI): UNI cells carry data between the user and the ATM network • Network-network interface (NNI): NNI cells carry network control information between ATM switches • NNI also enables network control information to be exchanged between different ATM networks Stamper and Case

31. Figure 12-14 Stamper and Case

32. ATM Protocol Stack • Link other packet-switching networks, the ATM protocol stack corresponds to the layers 1, 2, and 3 of the OSI reference model (see Figure 12-15) • Two ATM layers correspond to OSI’s data link layer: • User inputs (such as voice, data, and video) are processed into 53-octet cells at the ATM adaptation layer (AAL) before being passed to ATM switches for delivery to recipients • There are multiple AAL sub-layers (see Table 12-1); these are responsible for segmenting and reassembling different types of user inputs. Five different quality of service levels are identified • The ATM layer is responsible for final cell formatting. ATM typically uses SONET (OC-3, OC-12, etc.) for framing and error correction out over the wire. ATM switches convert cells to SONET frames and frames to cells at the port interface Stamper and Case

33. Figure 12-15 Stamper and Case

34. Table 12-1 Stamper and Case

35. T-1 Services • T-1 lines are among the most widely used leased digital circuits in the U.S. • Each T-1 line has a bandwidth of 1.54 mbps and can be subdivided into twenty-four 64 kbps channels • Channels are differentiated via a TDM adaptation called periodic framing • Each T-1 frame consists of 192 data bits (8-bits per channel X 24 channels and a framing bit (193 bits total)---see Figure 12-17. 8000 frames per second can be transmitted (8000 x 193 bits = 1.54 mbps) • The 24 channels can be reallocated to users when there are fewer than 24 simultaneous users • Reallocation of a single T-1 line among two or more subscribers is known as fractional T-1 (FT-1) Stamper and Case

36. Figure 12-17 Stamper and Case

37. T-1 Service Access Technologies • Businesses use a variety of services to access T-1 services (see Figure 12-18). These include: • T-1 CSU/DSUs • T-1 multiplexors • T-1 channel banks • T-1 switches Stamper and Case

38. Figure 12-18 Stamper and Case

39. SONET Services • Synchronous optical network (SONET) is an optical transmission interface/specification for high-speed digital transmission over optical fiber • SONET specifications define a hierarchy of standardized data transfer rates over optical media. An abbreviated set is provided in Table 12-2 • Each level is capable of carrying multiple lower-speed signals. An STS-1 channel, for example, is capable of carrying multiple DS-1 (T-1) signals • STS-1 frames are the fundamental data transmission format in SONET (see Figure 12-19) • Each consists of 810 octets that can logically be depicted as a matrix of 9 rows with 90 octets in each row • 87 octets in each row carry data and can be flexibly allocated to lower bandwidth channels such as DS-0, DS-1, and DS-2 • SONET service access technologies include add-drop multiplexors, cross-connect switches, and broadband bandwidth managers Stamper and Case

40. Table 12-2 Figure 12-19 Stamper and Case

41. ISDN • Integrated Services Digital Network (ISDN) is widely used by business to provide digital WAN services among geographically dispersed operating locations • ISDN switches are the core of the ISDN network (see Figure 12-20) • Two major categories of ISDN are: • Narrowband ISDN. This is essentially a circuit-switched digital network service that allows temporary connections to be dynamically created and terminated among ISDN subscribers. Two narrowband service levels exist: • Basic rate interface (BRI) that supports two 64 kbps bearer channels and one 16 kbps data channel (2B+D) • Primary rate interface (PRI) with 23 64 bps bearer channels and a 64 bps data channel (23B+D) • Broadband ISDN (B-ISDN) which may be described as ATM over SONET Stamper and Case

42. Figure 12-20 Stamper and Case

43. Wireless WAN Services • Increasingly, organizations are turning to wireless WAN services to satisfy their data communication needs including: • Circuit-switched cellular systems (see Figure 12-21) • Cellular digital packet data (CDPD) • ARDIS (Advanced Radio Data Information Service) • Mobitex • Metricom (see Figure 12-22) • Personal communication services (PCS) • Broadband wireless services (such as wireless T-1 service) Stamper and Case

44. Figure 12-21 Stamper and Case

45. Figure 12-22 Stamper and Case

46. Choosing Among WAN Services: Business Considerations • Businesses have many issues to consider when choosing WAN services including: • Availability • Data transmission rates • Costs • Other factors include: reliablity, security, expandability, and support for mobile users • The Internet is also affecting business choices • Many businesses are leveraging ISDN, frame relay, and T-1 services to access the Internet via ISPs • Businesses are also seeking ways to bypass traditional WAN services to route voice, data, and video over the Internet Stamper and Case

47. Chapter 12Wide Area Network Services Part III: Wide Area Networks and Internetworking Technologies