Wide Area Networks

2 Wide Area Networks (WANs) u WAN Technologies u Ordinary telephone line and telephone modem. u Point-to-Point Leased lines u Public switched data network (PSDN) u Send your data over the Internet securely, using Virtual Private Network (VPN) technology VPN PSDN

3 Evolution of WAN Technology u Layer 1: Leased line service and networks u Layer 2: Public switched data networks (PSDN) u Layer 3: Virtual Private Networks (VPN) over the Internet and IP carrier networks

4 Carriers u Organizations have the right to lay wires in their premises u Organizations do not have right of ways between sites u Organizations must turn to a transmission carrier u Carriers have rights of way. To compensate for this power, they are regulated

5 Transmission Carriers in the U.S. u Domestic Inter-LATA Carriers (U.S.) u Domestic means within a country u Inter-LATA service (between LATAs) u Carriers are called inter-exchange carriers (IXCs). u Competition has long existed in this arena. IXC

6 Transmission Carriers Between Countries u International Carriers u Called International Common Carriers (ICCs) u Each pair of countries negotiates on what ICCs to allow, like we saw last class meeting. u When you call internationally, you use one ICC, not two--one at each end. ICC

7 POP u All competitors can interconnect their customers into an integrated system u The key to competition u Without it, new competitors could not get a critical mass of customers u With a POP, even a small customer base is no problems, because these customers can reach any other telephone customers in the world. u Trunk lines connect carrier switching offices

8 Connecting to IXCs and ICCs POP at LEC Switching Office Trunk Line IXC Switching Office IXC Switching Office ICC Switching Office The POP also links LEC and CAP subscribers to IXCs and ICCs.

9 Circuit u End-to-End Connection Between Stations u May Pass through Several Switches u May Go Through Multiple Transmission Media u Maintained throughout the call u May flow through multiple carriers u LEC, ICC, etc. Wire Satellite

10 Circuit Speeds u Voice Grade Circuits u Ordinary telephone line, except point-to-point u Analog line: high error rate u Requires modem u Worst of all, slow: Under ~35 kbps Analog

11 Circuit Speeds u 64 kbps Circuits u Digital line: low error rate u If you digitize an analog telephone system, it generates 64 kbps in data u Used to be sufficient for linking people from home u Used to be sufficient for linking branch offices u Sometimes, 56 kbps u Use to be the most widely used digital circuit u Inexpensive. In range of most demand. Digital 64 kbps 56 kbps

12 Circuit Speeds u T1 Circuits u Mbps u Designed to multiplex 24 digital voice lines u Can be used as a single high-speed data pipe u Sufficient for many uses to connect sites u Also called DS1 for the signaling format u Very widely used: In the critical speed range for many “high speed” corporate uses and not too expensive T Mbps DS1

13 Circuit Speeds u Fractional T1 Circuits u Many firms need between 64 kbps and T1 speeds u 128 kbps, 256 kbps, 384 kbps, 768 kbps common u Each vendor only offers some options u Different vendors offer different options u 768 kbps usually is the fastest offering Fractional T1 128 kbps 256 kbps 384 kbps 768 kbps

14 Circuit Speeds u T3 Circuits u 44.7 Mbps in U.S. u For firms needing very high speeds u Uncommon now but increasing u Other T-Series Speeds u There are faster T-series circuits, but they are rarely used. u There are T2 circuits, but they are not offered T Mbps

15 Circuit Speeds u E Series Circuits u Used in Europe, other areas u Created by CEPT (Conference of European Postal and Telecommunications Authorities) u E1: Mbps (faster than T1) u E3: 34.4 Mbps E Series Mbps 34.4 Mbps

16 Circuit Speeds u Higher-Speed Digital Lines (SONET/SDH) u Single world-wide standard for very high speeds u In U.S., called SONET (Synchronous Optical Network) u In Europe, elsewhere called SDH (Synchronous Digital Hierarchy) u OC circuit designations. Multiples of Mbps u OC3: 156 Mbps u OC12: 622 Mbps u Defined up to a few Gigabits per second SONET SDH

17 Types of Traditional Telephone Circuits u Dial-Up Service (Any-to-Any) u Leased Lines u Point-to-point only u Cheaper for high volumes of use Leased Line Switched Dial-Up Service Seattle Washington, D.C.

18 Leased Line Service Customer Premises ACustomer Premises B Switching Office Switching Office Switching Office Trunk Line Trunk Line Local Loop Local Loop Leased Lines May Pass Through Multiple Switches, Even Multiple Carriers

19 Leased Lines u Limited to 2 points u Cheaper than dial-up on high-volume routes u Companies can build enterprise networks from meshes of leased lines between sites Leased Line Corporate-owned Switch See this web site for price examplethis web site

20 Data Networks u Data Networking Alternatives u Use the telephone network and modems (slow) u Lease lines, add own switching (complex) u Data Networks u Optimized for data transmission u Customer only has to connect to the data network u Carrier handles transmission, switching, management u Shown as cloud to indicate lack of need to know details u Two types: circuit-switched and packet-switched Data Network

21 Circuit-Switched Data Networks u Switched for any-to-any communication u Just dial the number of the party being called u Very flexible

22 Circuit-Switched Data Networks u Dedicated Capacity u Circuit is maintained during the duration of the call u Capacity is always available u You must pay for this constant capacity u Most data transmission is burst, with long silences between transmission u Utilization of the line may be as low as 5% u So circuit-switched services is inherently expensive

23 ADSL versus Business-Class Symmetric Digital Subscriber Line (DSL) Services

24 Packet-Switched Data Networks u Messages are Broken into Small Pieces (Packets) u Flow through the network more easily than long messages, like sand in an hourglass Packet

25 Packet Switches u Packet Switched Networks have Switches u Route the packets through the network Switch

26 Packet Switching is Efficient u Packets from several stations multiplexed over trunk lines between switches u No costly dedicated transmission capacity Trunk Line

27 Error Checking in Packet-Switched Data Networks u The Process u Sender transmits the packet u Sender maintains the packet in memory u Receiver checks the packet for errors u If there is an error, asks for a retransmission u Sender retrieves from memory, retransmits 21

28 Error Checking in Packet-Switched Data Networks u Considerations in u Adds delay (latency) every time it is done u Places a heavy load on the switch, lowering throughput u Not often needed, because there are very few errors on modern transmission lines.

29 Reliable Packet-Switched Data Networks u Check for Errors at Each Hop u Have reduced throughput u Have latency (delays) Error Check Error Check Error Check Error Check Error Check

30 Unreliable Packet-Switched Data Networks u No Error Check at Each Packet Switch u Check only once, at receiving host u Low latency, load on switches Error Check No Error Checks at Switches

31 Unreliable Service u Most Packet Switched Networks Today are Unreliable u Little Need: Error rates are low with modern lines, switches u Reduces delays: critical for some applications u Low load on the switches for high throughput u Better to check once, on the receiving host, than at every switch

32 Connectionless Service u Routing Decision for each packet at each switch u Places a heavy load on switches u Unnecessary work: subsequent packets usually travel same path, because conditions rarely change between packets Decision See some Level 3 servicesLevel 3 services

33 Connection-Oriented Service u Routing decision is made once, at start of connection Decision

34 Connection-Oriented Service u First decision establishes a path (virtual circuit) u All subsequent packets follow the virtual circuit Virtual Circuit

35 Connection-Oriented Packet-Switched Data Networks u All Commercial Packet Switched Networks are Connection-Oriented u Reduces loads on the switches for higher throughput u Lower latency because of less work at each switch u When marketers say “packet switched,” they now automatically include the concept of connection orientation

36 Connections in Packet-Switched Data Networks u Permanent Virtual Circuits (PVCs) u Established for long durations u Set up weeks or months ahead of time u If your firm has four sites, need 6 PVCs u Makes packet switched networks like network of leased lines Site 1 Site 3 Site 4 Site 2 PVC

37 Connections in Packet-Switched Data Networks u Switched Virtual Circuits u Established at call setup u Only available in some packet switched networks u Will provide the any-to-any flexibility of circuit- switched data networks AND the efficiency of connection-oriented packet switching

38 OSI Layering u Connectionless Service u OSI Layer 3 (Networking) u Routing across a series of packet switches u Alternative Routing u Connection-Oriented Service u OSI Layer 2 (Data Link) u Reduces network to a single path u Loses flexibility of alternative routing after virtual circuit is established