M08 LANs and Wireless LANs Ch 7

• LAN Components

  
• LAN Components (cont’d.)

  •Clients: Devices on the network that request information from servers •Servers: Devices on the network that deliver information or provide services to clients •Network interface cards (NIC): Also called network cards and network adapters ◦Operate at layers 1 and 2 ◦Commonly built into motherboards ◦Ethernet NICs contain unique MAC address.
• Some Important Network Cable Standards

  -Category 3

  UTP

  10 Mbps

  10BASE-T

  -Category 5

  UTP/ST

  100 Mbps

  100BASE-T

  -Category 5e

  UTP/STP

  1 Gbps

  1000BASE-T

  Category 6/6a

  UTP/STP

  10Gbps

  10GBASE-T

  -OM1 (62.5/125 µm)

  Fiber

  10 Gbps

  1000BASE-SX

  -OM3 (50/125 µm)

  Fiber

  100 Gbps

  10GBASE-SR

  
• Hubs and Switches 


  •Link cables from different devices, sometimes more than one type of cabling •Act as repeaters, reconstructing and strengthening incoming signals

  
• Access Points


  •Access points (APs) use radio waves to connect wireless clients to the wired network (instead of connecting using hubs/switches). ◦Many APs use power over Ethernet (PoE) for electricity. ◦No external power is needed. ◦Power flows over unused twisted pair wires. ◦Some IP cameras and phones use access points. ◦Modern consumer APs are typically packaged with routing hardware/software and sold as “wireless routers”
• LAN Software


  •Network operating system (NOS): Runs on devices and manages networking functions◦ e.g., Novel NetWare, Microsoft Windows Server, Linux◦e.g., Cisco IOS or JUNOS on routers. •Clients devices typically have network software components included with OS installation: e.g., TCP/IP included in Windows, OS X, and Linux ◦Allows clients to view and access available network resources. Provides directory services about LAN resources ◦Network profiles specify resources that devices and users can access.
• Wired Ethernet – Standards and Protocols


  •Originally developed at Xerox PARC and standardized by a consortium of Digital Equipment Corp., Intel and Xerox (DIX) •IEEE 802.3 standards •Used by nearly all LANs today •Layer 2 protocol, but physical layer must also meet protocol requirements
• Wired Ethernet – Topology 


  •Topology: layout of a network. •Types: Logical Topology: how the network works conceptually◦Physical Topology: how the network is physically installed
• Wired Ethernet – Hub-Based Ethernet


  •Also called shared or traditional Ethernet •Logical bus topology: all devices receive every frame as if they were connected to the same circuit •Hub: a multiport repeater •Uses a physical star topology
• Wired Ethernet – Hub-Based Ethernet (cont’d.)

  
• Wired Ethernet – Switch-Based Ethernet 


  •Logical star topology: only the destination receives the frame ◦Switch: an intelligent device with a small computer built in that is designed to manage a set of separate point-to-point circuits ◦Not shared with any other devices ◦Reads destination address of the frame and only sends it to the interface (physical port) connected to a circuit. •Uses a physical star topology
• Wired Ethernet – Switch-Based Ethernet (cont’d.) 

  
• Wired Ethernet – Switch Operation


  •Switches learn which MAC address is associated with an interface (physical port) by reading the source address on a frame: ◦Breaks up the collision domain ◦When new frame received, the switch reads the destination MAC address ◦Looks up destination address in forwarding tables (also called MAC or CAM tables), which are similar to routing tables ◦If found, forwards frame to the corresponding interface ◦If not found, broadcasts frame to all devices (like a hub)
• Wired Ethernet – Switching Modes


  •Store and forward switching – frames retransmitted after entire frame is received and error check is complete ◦Slower, but fewer errors. •Cut-through switching – frames retransmitted as soon as destination address read: ◦Low latency, but some capacity wasted. •Fragment-free switching – frames retransmitted once the header (first 64 bytes) is received and has no errors ◦Compromise between store and forward and cut-through
• Wired Ethernet – Media Access Control


  •Wired Ethernet uses a contention-based technique called carrier sense multiple access with collision detection (CSMA/CD): ◦Carrier sense (CS): ◦A device “listens” to determine if another computer is transmitting ◦Only transmits when no other computer is transmitting ◦Multiple access (MA): ◦Many devices have access to transmit on the network medium. ◦Collision detection (CD): ◦Collisions occur when multiple devices transmit simultaneously. ◦If a collision is detected, wait a random amount of time and resend. •Relies on collision detection not avoidance
• Wired Ethernet – Types of Ethernet


  •Ethernet is moving towards gigabit speeds: 1000Base-F (which runs at 1 Gbps and is sometimes called 1 GbE), 10 GbE (10 Gbps), 40GbE (40 Gbps), and 100 GbE (100 Gbps).

  
• Wireless Ethernet


  •Commonly called Wi-Fi •A family of standards developed by IEEE formally called 802.11 •Uses radio frequencies to transmit signals through the air (instead of cables) •Wi-Fi has many benefits: ◦Provides network connections where cabling is impossible or undesirable ◦Allows device and user mobility ◦Potentially more economical than wired networks
• Wireless Ethernet – Topology 


  •A physical star and a logical bus •Access points (APs) ◦Antenna type ◦Omnidirectional ◦Directional◦Active vs. passive scanning •Wireless NICs
• Wireless Ethernet – Media Access Control


  •Uses CSMA/CA (CSMA with collision avoidance) •Two methods ◦Distributed coordination function (DCF) ◦Point coordination function (PCF) ◦Solves hidden node problem; not widely implemented •join WLAN, computer must first associate with a specific AP ◦Searches for available AP (called scanning); active/passive ◦During active scanning, NIC transmits special frame called probe frame on active channels on frequency range ◦During passive scanning, NIC listens all channels for special frame called beacon frame sent w/ APDistributed coordination function (DCF)
• Distributed Coordination Function


  •Also called physical carrier sense method •With DCF, each frame in CSMA/CA is sent using stop-and-wait ARQ •After the sender transmits one frame, it immediately stops and waits for an ACK from the receiver before attempting to send another frame •The time interval between a frame and the matching ACK is so short that no other computer has the opportunity to begin transmitting
• Point Coordination Function


  •Also called the virtual carrier sense method •Computers far away may t sense the other transmission and transmit at the same time causing a collision at the APoThis is called the hidden node problem •In this case, the AP is the only device guaranteed to be able to communicate with all computers on the WLAN •Therefore, the AP must manage the shared circuit using a controlled-access technique, not the contention-based approach of traditional Ethernet
• Point Coordination Function (cont’d)


  •Any computer wishing to transmit first sends a request to send (RTS) to the AP •If no other computer is transmitting, the AP responds with a clear to send (CTS), specifying the amount of time for which the circuit is reserved for the requesting computer •Controlled-access methods provide poorer performance in low-traffic networksControlled-access techniques work better in high-traffic WLANs
• Wireless Ethernet – Frame Layout


  •802.11 Frame◦Includes four address fields◦Two addresses have the same meaning as in wired Ethernet; the others are used communicating with APs and other devices.

  
• Wireless Ethernet – Frequency Ranges


  •WiFi devices transmit and receive within frequency ranges. ◦These frequency ranges are divided into “channels”. •Larger frequency range → higher potential bandwidth •Higher frequency → greater attenuation (i.e., shorter range) •Overlapping channels should be minimized.
• Wireless Ethernet – Types of Wi-Fi


  •802.11n: obsolete version, but still used because it is cheap. •IEEE 802.11ac: a current version ◦Two different frequency spectrums simultaneously (2.4 and 5 GHz) to provide very high speed data rates. ◦Eight-channel version (433 Mbps with a maximum range of 100 meters) and user-configured version available •802.11ax is the newest version (called Wi-Fi 6) ◦Runs in three different frequency spectrums to provide very high speed data rates.
• Wireless Ethernet – Security


  •Security is particularly important for WLANs because they are easy to discover •Security protocols ◦Wired Equivalent Privacy (WEP) ◦Insecure and easy to bypass ◦WPA and WPA2 (802.11i) ◦WPA2 is currently recommended  •MAC address filtering◦May prevent casual users from connecting