Topic 5 - Wired and wireless LANs

Question 1

Wired LANs

Answer 1

– The dominant standard: Ethernet (802.3x)
– Operations: Topologies and media access control (MAC)
– Types of Ethernet

Question 2

Wireless LANs

Answer 2

– The dominant standard: 802.11x (NOT Ethernet-compliant but Ethernet-compatible)
– Operations: Topologies and media access control (MAC)
– Security (updated!), site survey and other design considerations

Question 3

Circuits

Answer 3

Carry signals
– Wired LANs can use UTP or STP or optical fiber cables whereas wireless LANs use radio waves
– Key characteristics and applications covered in Topic 3

Question 4

Connectivity devices used at the access layer

Answer 4

– Hub or switch: Connect computers via cables and act as repeater for incoming signals
– Access point (AP): Use radio waves to connect wireless clients to a wired network (AP ≠ wireless router, see next slide)

Question 5

Operating Systems for servers

Answer 5

– Includes Windows Server or Unix, Linux, etc.
– Manages a server’s networking functions including traffic control and security (i.e., network profiles)

Question 6

Physical topology

Answer 6

How a network looks physically => Hub = Switch

Question 7

Logical topology

Answer 7

How a network works conceptually (how data flow) => Hub ≠ Switch

Question 8

Hubs

Answer 8

– A hub is a half-duplex device that broadcasts frames it receives from one port to all other ports but only the computer with the MAC address matching that of the destination reads that frame
– A hub is a Layer-1 device and creates a multipoint circuit (or collision domain)
- Hubs create a logical bus topology
- Create collision domains

Question 9

Switches

Answer 9

• create a logical star topology
• A switch is a full-duplex device that reads destination address of frames and sends them to the intended destination only using entries in its forwarding table
• A switch is a Layer 2-device (other are Layer-3) and creates point-to-point circuits
• Elminate collisions

Three switching modes:
* Cut-through switching: Frames retransmitted as soon as destination address read (low latency but no error check)
* Store-and-forward switching: Frames retransmitted after entire frame is received and error check is complete (higher latency)
* Fragment-free switching: Frames retransmitted once the header (first 64 bytes) is received and has no errors (compromise between cut-through and store-and-forward)
=> Most switches today use cut-through or fragment-free switching

Question 10

Three security mechanisms for Wireless LANs

Answer 10

– Encryption: 3 successive protocols based on encryption keys + algorithm
* Wired Equivalent Privacy (WEP): Encryption key is dynamically produced but relatively easy to crack
* Wi-Fi Protected Access (WPA): Improved encryption due to a key that is (i) longer and (ii) changed for every frame transmitted
* WPA2 (aka 802.11i): Current method that improves encryption by using a stronger encryption algorithm (Advanced Encryption Standard or AES)
– Authentication: Access control based on user ID, passwords and permissions
– MAC address filtering: Access control based on their MAC address (but MAC addresses can also be spoofed)