Transcript Chapter 2
Chapter 2 Wide Area Network Technologies What is a WAN ? • The set of connecting links between local area networks • These links can be made using: – – – – – – – Public switched telephone network-PSTN Leased telephone lines (T1-1.5 Mbps, T3-44.7 Mbps) Fiber-optic cable Satellite links Packet radio waves Microwaves Internet What is a WAN ? – Continued • Most WANS are private and owned by the company that uses them • Internet is the largest WAN in the world • Companies are forming private WANs through encrypted communications over the Internet • WANs suffer from extremely limited bandwidths – many times slower than LANs • WAN links are good for short messages Some PSTN Services and Transmission Rates • • • • X.25 56Kpbs T1 1.5Mbps T3 44.7Mbps ATM 44.7Mbps SLIP • Serial Line Internet Protocol • Common protocol used to transmit IP packets over serial lines and telephone connections • Designed to handle just TCP/IP traffic – not other protocols • Must know your IP address and the IP address of the remote computer • SLIP does not provide for data compression PPP • Point-to-Point Protocol • Common protocol used to transmit IP packets over serial lines and telephone connections • PPP can transmit TCP/IP traffic as well as IPX, AppleTalk, etc. simultaneously • Offers data compression • Offers enhanced security • PPP has slowly replaced SLIP How IP Datagrams are Encapsulated by a Windows 2000 that Uses WAN Technology • Encapsulation done at Data Link Layer prior to being sent onto the physical medium • Delimiters – distinguishes each frame at the Data Link Layer as well as frame’s payload from header and trailer How IP Datagrams are Encapsulated – Cont • Protocol Identification – Used to distinguish different protocols on the WAN link such as TCP/IP, IPX, or AppleTalk • Addressing – Destination must be identified if multiple destinations are supported • Bit-level integrity check – Checksum process used Categories of WAN Encapsulations for IP Datagrams • Point-to-point links – Maximum of two nodes – No Data Link Layer addressing needed – Examples include analog phone lines, ISDN lines, T-1, T-3 • Non-Broadcast Multiple Access (NBMA) – Segment has more than 2 nodes – No facility to broadcast a single IP datagram to multiple locations – X.25, ATM SLIP Encapsulation • Provides only frame delimitation services • Does not provide protocol identification or bitlevel integrity verification • Uses a special character called the END character (0xC0) • END is placed at the beginning and the end of each IP datagram • Character stuffing technique used to prevent the occurrence of the END character within the IP datagram • See Figure 2-1 for SLIP encapsulation Character Stuffing • Any occurrence of the END character inside the datagram is replaced with a sequence beginning with another special character called ESC (0xDB) • END is replaced with 0xDB-DC • ESC in original datagram is replaced by 0xDB-DD • See Figure 2-2 for example PPP Encapsulation • Flag – Indicates start and end of a PPP frame (1 byte) • Address – Used as a destination address on a multi-point network. On point-to-point links, the destination is the other node. The value of 0xFF is used here – the broadcast network (1 byte) • Control – always set to the same value (0x03) to indicate an unnumbered frame (1 byte) • Protocol – Used to identify the upper layer protocol of the frame (2 bytes) – 0x00-21 indicates IP datagram – 0x00-2B indicates IPX datagram PPP Encapsulation – Cont. • IP datagram – data from Network Layer • Frame Check Sequence – 2 byte FCS used for error detection. PPP frame is silently discarded if the sent FCS does not equal calculated FCS