1. 1 Computer Networks Course: CIS 3003 Fundamental of Information Technology

2. 2 Objectives OSI model Local area network (LAN) Topologies of LAN Wireless LAN

3. 3 OSI Model Open Systems Interconnection model Layer abstractions Protocol/technology at each layer can be changed with minimal impact to other layers

4. 4 OSI Model Hardware: Fiber optic, twisted pair cables Link: 802.11 Network: routing protocols Transport: TCP/IP, UDP Application: HTTP, FTP User: Internet Explorer, Firefox

5. 5 OSI Model

6. 6 Accessing LAN Network Interface Controller (NIC) Each NIC has unique physical address – MAC-48, EUI-48, EUI-64; Ethernet NIC address has 6 byte or 48 bits – First 3 bytes: manufacture identifier – Last 3 bytes: device unique identifier; serial no. NIC breaks data into smaller frames and add control information NIC sends data over physical medium.

7. 7 Ethernet Data Format Ethernet preamble: 10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011 Source and destination address are NIC addresses Padding: minimum data size is 46 bytes FCS – error checking

8. 8 Local Area Network (LAN) Confined to a local area such as building or home. Shared access to devices such as printer, internal servers. Logical Topologies: – Ring – Star – Bus Wired and wireless

9. 9 Token Ring A token is passed from one computer to next. Whichever computer has the token can transmit data. Computers can be connected to a hub that provides a ring topology.

10. 10 Token Ring Deterministic through use of token. One fail device can break an entire ring unless hub bypasses the device. Use special designed cable and network card; more expensive. Token management.

11. 11 FDDI Fiber Distributed Data Interface Connected by fiber optic cables. Dual ring topology – redundancy fault tolerance 100 Mbps: faster than token ring but not faster than latest Ethernet (100 and 1000 Mbps).

12. 12 Token Ring Frame

13. 13 Bus One computer transmits, all computers can hear the information Need a medium access control mechanism to prevent two computers transmit at the same time.

14. 14 Bus One computer transmits, all computers can hear the information Need a medium access control mechanism to prevent two computers transmit at the same time.

15. 15 Bus Computers can be connected to a hub that provides a bus topology. Hub is a simple device. Add more computers to the bus increases chance of collision. Cost advantage of hub over switch is diminished.

16. 16 CSMA/CD & CSMA/CA Carrier Sense Multiple Access with Collision Detection Used in LAN Node senses the medium first. When node detects collision, it stops transmission and retransmits after a random delay. Carrier Sense Multiple Access with Collision Avoidance Used in wireless RTS/CTS packets – A node sends Request to Send packet to a receiver. – The receiver replies with a Clear to Send if it does not sense a busy wireless channel.

17. 17 Switched Network Each computer connects directly to a switch Switch manages the data to be sent out to the network. Switch performs faster than hub due to lack of collision.

18. 18 WLAN

19. 19 WLAN Standards IEEE 802.11a — Wireless technology operating at a frequency of 5 GHz with a maximum data rate of 54 Mbps IEEE 802.11b — Wi-Fi wireless technology operating at a frequency of 2.4 GHz with a maximum data rate of 11 Mbps IEEE 802.11g — Backward compatible with 802.11b, operating at a frequency of 2.4 GHz and a maximum data rate of 54 Mbps

20. 20 WLAN Standards IEEE 802.11i—Standards for wireless security mechanisms IEEE 802.15—Known as WPAN, or wireless personal area networks; blue tooth. IEEE 802.16—WiMAX, an emerging technology that seeks to provide high-speed wireless access over much longer distances than implementations of the IEEE 802.11 standards

21. 21 Security Information transmitted over air; no physical security as in a wired network. Signal can be overheard by other Wi-Fi enabled computers within the range. Rogue access point, jamming wireless channel, overhearing. Turn on WEP/WPA encryption, change default access point password and SSID, which shows models of the access point.

22. 22 Summary Local area networks can have a ring, bus, or switch topology. Token ring provides a deterministic method to access shared medium. However, it has the complexity of token management. Bus hubs used to cost less than a switch but there are increased collision when more devices connected to a bus. Switches are inexpensive and provide easy setup and faster speed than a bus topology.

23. 23 Summary WLAN is similar to a bus topology where every node shares the same wireless medium. WLAN employs RTS/CTS to avoid collision. WLAN is less secure than a switch network. There are many ways to protect against attacks in a WLAN.