Business Data Communications

1. Business Data Communications Chapter Seven Wide Area Networking Fundamentals

2. Primary Learning Objectives • Understand the need for varying WAN connectivity options • Define the term switching network • Describe circuit switching networks • Describe packet switching networks • Differentiate X.25, Frame Relay, and ATM • Explain Point-to-point protocol • Identify common trunk carrier services • Distinguish between symmetric and asymmetric Digital Subscriber Line services

3. WAN Connectivity Options • There are several ways in which WAN connections can be established • Many connection options are provided by common carrier infrastructures (AT&T, MCI, Sprint, for example) • WAN connectivity options include: • Circuit switching • Packet switching: X.25, Frame Relay, ATM • Point-to-point protocols • Trunk carrier services • Digital subscriber lines

4. WAN Connectivity Options

5. Switching Networks • Composing a switching network are switches, which: • Are a series of interlinked devices • Interlinks between two communicating devices are temporary • Can be hardware, software, or a combination of both • Can be connected to multiple links • The switching network infrastructure is referred to, and graphically illustrated as, a “cloud” • Edge switches and edge routers connect the enterprise to the cloud

6. Switching Network Cloud

7. Circuit Switching Networks • Are well-suited for voice, but not data, communications • Voice communications are continuous; data communications are bursty • Create a direct connection between two communicating devices: • This direct connection is referred to as a “path” • While established, no two other devices can use the circuit path • The circuit path uses a constant rate determined by the slower of the two devices • Circuit-switched paths can be temporary or permanent

8. Circuit Switching Networks • Temporary paths: • Require a call setup • Are less expensive than permanent paths • Do not guarantee that a circuit will always be available • Permanent paths: • Do not require a call setup • Are more expensive than temporary paths • Guarantee that a circuit will always be available • Neither temporary nor permanent circuit switching prioritizes communications

9. Packet Switching Networks • Break data into units called “packets”: • Packets traverse the packet switching cloud through a series of links • Links are connected by packet switching nodes • Improve line efficiently significantly because many packet streams, from differing communications, can use the same links between packet switching nodes • Deliver packets even when the network is busy, although the packets may be delayed • Provide for data-rate link conversion between packet switching nodes

10. Data-Rate Link Conversion

11. Packet Switching Networks • Use the network implementation to determine packet size • Packets contain not only core data, but control information as well • Temporarily store and then forward packets as they pass through the packet switching cloud • Allow for prioritization of communications • May be based on datagram or virtual circuit delivery

12. Packet Switching Networks • Datagram delivery: • Is best for small messages • Treats each packet independently • Does not guarantee packet delivery • Does not sequence packets • Permits packets to be dynamically rerouted • Does not require a setup procedure between sender and receiver • Requires little overhead

13. Packet Switching Networks

14. Packet Switching Networks • Virtual circuit delivery: • Is best for data-sensitive messages • Requires a set-up procedure between sender and receiver • Requires all packets in a message to take the same route • Guarantees packet delivery • Sequences packets • Cannot dynamically reroute packets • Has significant overhead • Takes one of two forms: • Switched virtual circuit • Permanent virtual circuit

15. Packet Switching Networks • Switched virtual circuits: • Are similar in concept to a dial-up circuit switched connection • Temporarily set up the route of links a circuit takes, meaning only for the duration of the communication • Require a setup procedure for each communication

16. Packet Switching Networks • Permanent virtual circuits: • Are similar in concept to a leased circuit switched connection • Once defined, require no additional set-up procedures, meaning that the route of links does not have to be recreated • Are always available as they are permanent • Require virtual circuit identifiers for each communication

17. Packet Switching Networks

18. Packet Switching Networks • Three common forms of packet switching network are: • X.25 – The oldest • Frame Relay • ATM – The newest

19. X.25 • Uses data terminal equipment and data-circuit terminating equipment • Uses virtual circuits and statistical time division multiplexing • Supports variable length packets • Has a three-layer design: physical, frame, and packet • Uses Link Access Procedure-Balanced protocol (based on HDLC) at the frame layer • Provides for significant error checking, a drawback given today’s much improved transmission media • Was not designed with multimedia data in mind

20. Frame Relay • Utilizes two layers: physical and data link • Supports variable length packets • Requires Frame Relay Access Devices (FRADs) to disassemble and reassemble packets • Requires that subscribers negotiate a committed information rate with a common carrier • A committed burst rate can also be negotiated • Uses the Link Access Procedure-Function protocol (based on HDLC) • Does not provide for flow and error control • These are left to higher-level services

21. ATM – Asynchronous Transfer Mode • Is a universal integrated carrier of voice, data, audio, and video • Has a significant implementation cost and high degree of configuration complexity • Utilizes two layers: physical and data link • Is a point-to-point solution • Uses virtual path and virtual channel identifiers • Requires fixed-length packets, called cells, of 53 bytes • 5 bytes of header, 48 bytes of data

22. ATM – Asynchronous Transfer Mode

23. Point-to-Point Protocol • Was developed by the IETF • Creates a physical serial link between two devices • Is inexpensive, but provides limited bandwidth capacity • Is the most common protocol used by dial-up modems • Replaced Serial Link Internet Protocol (SLIP) • Requires no media access control • Uses protocols such as PAP and CHAP for authentication • Has a five-phase sequence

24. Point-to-Point Protocol – Five Phases

25. Trunk Carrier Services • Most commonly used forms include: T-1, T-3, and Fractional T-1; the “T” stands for trunk services • The higher the “T” level designation, the greater the possible number of communication channels • Significantly more costly than point-to-point protocol, but offers much higher bandwidths • Digital leased lines have a range of bandwidths referred to as digital signal (DS) speeds • Subscribers negotiate with common carriers the level of service quality, for a cost

26. Trunk Carrier Services

27. Trunk Carrier Services • Usually used in conjunction with a: • Channel service unit (CSU): • The end-point of the digital link • Keeps the link open and active • Data service unit (DSU): • Converts signals from a connecting device into the type of signal required by the leased line • Connecting devices may be switches, routers, or Private Branch Exchanges

28. Trunk Carrier Services • Usually used in conjunction with a: • Private Branch Exchange (PBX): • A common connecting device used with a CSU and a DSU • An on-site switching facility used to interconnect telephones to a Public Switched Telephone Network

29. Trunk Carrier Services

30. DSL – Digital Subscriber Line • Supports simultaneous voice and data communications • Has a relatively low cost, with high transmission speeds • Is technically referred to as “xDSL”, whereby the “x” indicates the type of DSL service provided, for example: • ADSL • HDSL • SDSL • IDSL • VDSL

31. DSL – Digital Subscriber Line • Two terms associated with DSL are Downstream and Upstream: • Downstream describes the transmitting of data from a remote location to a local device • Upstream describes the transmitting of data from a local device to a remote location • Most users of DSL primarily utilize downstream transmission • Downstream and upstream transmission rates can vary or be the same, depending on the DSL service • Asymmetric versus Symmetric DSL

32. DSL – Digital Subscriber Line • The most common form of DSL is ADSL, or Asymmetric Digital Subscriber Line • With ADSL, downstream transmissions are faster than upstream transmission, for example: • Up to 1 Mbps downstream • Up to 512 Kbps upstream • Users located less than 18,000 feet from a local provider’s point-of-presence are good candidates for ADSL • DSL users also need to select between static and dynamic IP addresses

33. DSL – Digital Subscriber Line

34. DSL – Digital Subscriber Line

35. In Summary • There are a variety of WAN connectivity solutions • Circuit switched networks are most appropriate for voice • Packet switched networks are most appropriate for data • Common packet switched networks include X.25, Frame Relay, and ATM • Point-to-Point Protocol, while inexpensive, offers low bandwidth • Trunk services, particularly T-1, T-3, and Fractional T-1, are popular business WAN solutions • Forms of Digital Subscriber Line are becoming increasingly implemented as home and business solutions