1 Computer Networks Course: CIS 3003 Fundamental of Information Technology

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

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

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

5 OSI Model

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

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 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 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 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 Token Ring Frame

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 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 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 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 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 WLAN

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

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

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 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 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.