Wide Area Networks

2 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.

3 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

4 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 document for price examplethis document

5 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

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

7 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

8 Circuit-Switched Data Networks u ISDN u Usually circuit-switched u Both voice and data u Two 64 kbps B channels to the desktop u Sometimes can combine into a 128 kbps circuit u About twice as expensive as a telephone line u Needs a terminal adapter to connect computer, phone u Reasonably inexpensive, reasonably fast, not popular ISDN

9 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

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

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

12 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

13 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.

14 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

15 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

16 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

17 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

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

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

20 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

21 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

22 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

23 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