Ch6 COM204 Computer Networks PDF

COM204 Computer Networks Chapter 6 Local Area Networks (LANs) Prof. Mahmoud Elmesalawy Electronics and Communication Engineering Department Faculty of Engineering Helwan University  Chapter 6: Local Area Networks (LANs) 6...

COM204 Computer Networks Chapter 6 Local Area Networks (LANs) Prof. Mahmoud Elmesalawy Electronics and Communication Engineering Department Faculty of Engineering Helwan University  Chapter 6: Local Area Networks (LANs) 6.1 Introduction to Local Area Network (LAN) 6.2 IEEE Standard for LAN (IEEE802 Project) 6.3 Ethernet (IEEE802.3) LAN 6.3.1 Introduction to Ethernet 6.3.2 Ethernet Frame Format 6.3.4 Ethernet Access Method 6.3.3 Ethernet Addressing 6.4 Ethernet Implementations 6.4.1 10Base5: Thick Ethernet (Thicknet) 6.4.2 10Base2: Thin Ethernet (Cheapernet) 6.4.3 10 Base-T: Twisted-Pair Ethernet 6.4.4 10Base-F: Fiber Ethernet 6.5 Switched Ethernet 6.5.1 Switched Ethernet Network 6.5.2 Full Duplex Ethernet 6.6 Fast Ethernet 6.7 Virtual LAN (VLAN) 6.8 Wireless LAN (WLAN) 6.1 Introduction to Local Area Network (LAN) Local area network is a group of computers and associated devices (printers, etc.) connected through a wired or wireless medium by networking devices (hubs, switches and access points) as shown in Fig. 6.1. It is administered by a single organization. The main advantages of LANs are sharing information and valuable Resource (Hardware: Printers, Servers, Data storage, Processing capabilities and Software) WLAN “Wi-Fi” Wired (Ethernet) LAN Access Switch Access Point Router “Default Gateway” Fig. 6.1(a) Local area network. 6.1 Introduction to Local Area Network (LAN) Network Interface Card Workstation (NIC) Drop Cable UTP Cat 5, 5e, 6 or better RJ-45 Ethernet Access Switch Face-Plate UTP Cat 5, 5e, 6 or better Fig. 6.1(b) Ethernet LAN. 6.2 IEEE Standard for LAN (IEEE802 Project) In 1985, the Computer Society of the IEEE started a project, called Project 802, to set standards to enable intercommunication among equipment from a variety of manufacturers. Project IEEE 802 is a way of specifying functions of the physical layer and the data link layer of major LAN protocols. The IEEE has subdivided the data link layer into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC) layers as shown in Fig. 6.2. - LLC Sublayer: Responsible for flow control, error control, part of framing. provides one single data link control for all IEEE LANs. - MAC Sublayer: Defines the specific access method for each type of LAN (Ethernet–CSMA/CD, Token Ring and Token Bus-Token Passing). It also provides part of framing function. - MAC sub-layer contains a number of distinct modules; each defines the access method and the framing format specific to the corresponding LAN protocol. The following some standards that are defined by IEEE802 project: - IEEE 802.2 Ethernet Logical Link Control (LLC) sublayer. - IEEE 802.3 Ethernet Media Access Control (MAC) sublayer. - IEEE 802.5 Token Passing Media Access Control (MAC) sublayer. - IEEE 802.11 Wireless LAN [Wireless Fidelity (Wi-Fi)]. - IEEE 802.15 Wireless PAN (Bluetooth). 6.2 IEEE Standard for LAN (IEEE802 Project) Fig. 6.2 Functions of the physical and the data link layers of major LAN protocols according to IEEE 802 standard. 6.3 Ethernet (IEEE802.3) 6.3.1 Introduction to Ethernet Ethernet is one of the most widely used LAN technologies. It operates in the data link layer and the physical layer. It supports data bandwidths of 10, 100, 1000, 10,000, 40,000, and 100,000 Mbps (100 Gbps), as shown in Fig. 6.3. Family of networking technologies that are defined in the IEEE 802.2 and 802.3 standards, as shown in Fig. 6.4. Ethernet Standards define Layer 2 protocols and Layer 1 technologies. Two separate sub layers of the data link layer to operate – Logical link control (LLC) and the MAC sublayers. Fig. 6.3 Ethernet Evolution. 6.3 Ethernet (IEEE802.3) 6.3.1 Introduction to Ethernet Network LLC MAC Physical Fig. 6.4 Ethernet Evolution. 6.3 Ethernet (IEEE802.3) 6.3.2 Ethernet Frame Format The Ethernet frame contains seven fields: Preamble, SFD, DA, SA, length or type of protocol data unit (PDU), upper-layer data, and the CRC. Ethernet does not provide any mechanism for acknowledging received frames, making it what is known as an unreliable medium. Acknowledgments must be implemented at the higher layers. The format of the MAC frame is shown in Fig. 6.6 and described as: Figure 6.6 Ethernet 802.3 MAC frame format. 1. Preamble: The first field of the 802.3 frame contains 7 bytes (56 bits) of alternating 0s and 1s that alerts the receiving system to the coming frame and enables it to synchronize its input timing. The pattern provides only an alert and a timing pulse. The 56-bit pattern allows the stations to miss some bits at the beginning of the frame. The preamble is actually added at the physical layer and is not (formally) part of the frame. 6.3 Ethernet (IEEE802.3) 6.3.2 Ethernet Frame Format 2. Start Frame Delimiter (SFD): The second field (1 byte: 10101011) signals the beginning of the frame. The SFD warns the station or stations that this is the last chance for synchronization. The last 2 bits is 11 and alerts the receiver that the next field is the destination address. 3. Destination Address (DA): The DA field is 6 bytes and contains the physical address of the destination station or stations to receive the packet. 4. Source Address (SA): The SA field is also 6 bytes and contains the physical address of the sender of the packet. 5. Length or Type: This field is defined as a type field or length field. The original Ethernet used this field as the type field to define the upper-layer protocol using the MAC frame. The IEEE standard used it as the length field to define the number of bytes in the data field. Both uses are common today. 6. Data: This field carries data encapsulated from the upper-layer protocols. It is a minimum of 46 and a maximum of 1500 bytes, as we will see later. 7. CRC or Frame Check Sequence (FCS): The last field contains error detection information, in this case a CRC-32. 6.3 Ethernet (IEEE802.3) 6.3.3 Ethernet Access Method Standard Ethernet uses 1-persistent CSMA/CD Slot Time: In an Ethernet network, the round-trip time required for a frame to travel from one end of a maximum-length network to the other plus the time needed to send the jam sequence is called the slot time. Slot time = Round-Trip Time + Time required to send the jam sequence For proper functioning of CSMA/CD frames, the farthest parts of the network have to be able to sense the transmission before the transmission ends. If the frames could be arbitrarily short, the source could be done transmitting before farthest reaches of the network detect the transmission. i.e: There has to be a minimum limit. Minimum length of the frame is 64 bytes (512 bits). That’s 18 bytes for frame header/footer, and 46 bytes for the data. 6.3 Ethernet (IEEE802.3) 6.3.3 Ethernet Access Method Slot Time and Maximum Network Length: There is a relationship between the slot time and the maximum length of the network (collision domain). It is dependent on the propagation speed of the signal in the particular medium. In most transmission media, the signal propagates at 2 × 10^8 m/s (two-thirds of the rate for propagation in air). For traditional Ethernet, we calculate Of course, we need to consider the delay times in repeaters and interfaces, and the time required to send the jam sequence. These reduce the maximum- length of a traditional Ethernet network to 2500m, just 48 percent of the theoretical calculation. 6.3 Ethernet (IEEE802.3) 6.3.3 Ethernet Access Method Ethernet Frame Length (Size) Ethernet has imposed restrictions on both the minimum and maximum lengths of a frame, as shown in Fig. 6.7. IEEE 802.3 Ethernet standard defines the minimum frame size as 64 bytes and the maximum as 1518 bytes. Less than 64 bytes in length is considered a "collision fragment" or "runt frame” If size of a transmitted frame is less than the minimum or greater than the maximum, the receiving device drops the frame. Fig. 6.7 Frame Length. 6.3 Ethernet (IEEE802.3) 6.3.4 Ethernet Addressing Each station on an Ethernet network (such as a PC, workstation, or printer) has its own Network Interface Card (NIC). The NIC fits inside the station and provides the station with a unique 6-byte physical address. The Ethernet address is 6 bytes (48 bits), normally written in hexadecimal notation (12-digit hexadecimal numbers), with a colon, dashes or periods between the bytes as shown in Fig.6.8. Ethernet address is known as Media Access Control (MAC) address, hardware addresses physical addresses, or NIC address. Fig. 6.8 Ethernet address format. 6.3 Ethernet (IEEE802.3) 6.3.4 Ethernet Addressing The Organizationally Unique Identifier (OUI) is assigned by the IEEE to an organization. It’s composed of 24 bits, or 3 bytes ) that is unique. The vendor assigns the other 3 bytes that represent NIC interface. MAC address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used as a network address for most IEEE 802 network technologies, including Ethernet and WiFi. Logically, MAC addresses are used in the media access control protocol sublayer of the OSI reference model. 6.3 Ethernet (IEEE802.3) 6.3.4 Ethernet Addressing Ethernet address can be Unicast, Multicast, or Broadcast Address. A source address is always a unicast address—the frame comes from only one station. The destination address, however, can be unicast, multicast, or broadcast. 1. Unicast destination MAC address defines only one recipient; the relationship between the sender and the receiver is one-to-one. If the least significant bit of the first byte in a destination address is 0, the address is unicast. Fig. 6.9 Unicast addressing. 6.3 Ethernet (IEEE802.3) 6.3.4 Ethernet Addressing 2. Multicast destination MAC address defines a group of addresses; the relationship between the sender and the receivers is one-to-many. If the least significant bit of the first byte in a destination address is 1, the address is multicast. Multicast MAC address is a Range of IPV4 multicast special value that begins with addresses is 224.0.0.0 to 01-00-5E in hexadecimal 239.255.255.255 Fig. 6.10 Multicast addressing. 6.3 Ethernet (IEEE802.3) 6.3.4 Ethernet Addressing 3. Broadcast MAC address is a special case of the multicast address in which all bits are 1s; the recipients are all the stations on the LAN. Fig. 6.11 Broadcast addressing. 6.3 Ethernet (IEEE802.3) 6.3.4 Ethernet Addressing Example 6.1 Define the type of the following destination addresses: a) 4A:30:10:21:10:1A b) 47:20:1B:2E:08:EE c) FF:FF:FF:FF:FF:FF Solution To find the type of the address, we need to look at the second hexadecimal digit from the left. If it is even, the address is unicast. If it is odd, the address is multicast. If all digits are F’s, the address is broadcast. Therefore, we have the following: a) This is a unicast address because A in binary is 1010. b) This is a multicast address because 7 in binary is 0111. c) This is a broadcast address because all digits are F’s. Example 6.2 Show how the address 47:20:1B:2E:08:EE is sent out on line. Solution The address is sent left-to-right, byte by byte; for each byte, it is sent right-to- left, bit by bit, as shown below: 6.3 Ethernet (IEEE802.3) 6.3.4 Ethernet Addressing MAC Address and IP Address 1. MAC Address  This address does not change.  Similar to the name of a person.  Known as physical address because physically assigned to the host NIC. 2. IP Address  Similar to the address of a person.  Based on where the host is actually located.  Known as a logical address because assigned logically. Both the physical MAC and logical IP addresses are required for a computer to communicate just like both the name and address of a person are required to send a letter. Address Resolution Protocol (ARP) Sending node needs a way to find the MAC address of the destination for a given Ethernet link. The ARP protocol provides two basic functions: - Resolving IPv4 addresses to MAC addresses. - Maintaining a table of mapping. 6.3 Ethernet (IEEE802.3) 6.3.4 Ethernet Addressing ARP Request - Layer 2 broadcast to all devices on the Ethernet LAN. - The node that matches the IP address in the broadcast will reply. - If no device responds to the ARP request, the packet is dropped because a frame cannot be created. ARP Table - ARP Table is used to find the data link layer address that is mapped to the destination IPv4 address. - As a node receives frames from the media, it records the source IP and MAC address as a mapping in the ARP table. 6.3 Ethernet (IEEE802.3) 6.3.4 Ethernet Addressing ARP Operation Fig. 6.12(a) Example 6.3 Host A wants to send data to IP address: 10.10.0.3. 6.3 Ethernet (IEEE802.3) 6.3.4 Ethernet Addressing Step #1 ARP Operation Example 6.3 Solution Step #1 Host A broadcast to all devices on the Ethernet LAN. Fig. 6.12(b). 6.3 Ethernet (IEEE802.3) 6.3.4 Ethernet Addressing Step #2 ARP Operation Example 6.3 Solution Step #2 The node that matches the IP address in the broadcast will reply. Fig. 6.12(c). 6.3 Ethernet (IEEE802.3) 6.3.4 Ethernet Addressing Step 3 ARP Operation Example 6.3 Solution Step #3 ARP Table is used to find the data link layer address that is mapped to the destination IPv4 address. As a node receives frames from the media, it records the source IP and MAC address as a mapping in the ARP table Fig. 6.12(d). 6.3 Ethernet (IEEE802.3) 6.3.4 Ethernet Addressing ARP Operation Step 4 Example 6.3 Solution Step #4 The data is sent from the source to destination. Fig. 6.12(e). 6.3 Ethernet (IEEE802.3) 6.3.4 Ethernet Addressing Example 6.4: In the network shown in Fig. 6.13(a), assume a communication process is running between device P and device A, where data transmitted from device P to device A. Show the packet header in network layer and frame header in data link layer at each link shown in the figure (L1, L2, L3 and L4). Consider physical address of each device is denoted by number, and logical address is denoted by letter. Fig. 6.13(a). 6.3 Ethernet (IEEE802.3) 6.3.4 Ethernet Addressing Example 6.4 Solution : The packet header in network layer and frame header in data link layer at each link are shown in Fig. 6.13(b). Fig. 6.13(b). 6.4 Ethernet Implementations Standard Ethernet implementations are as follows: 6.4 Ethernet Implementations 6.4.1 10Base5: Thick Ethernet (Thicknet) 10 Base 5 was the first Ethernet specification to use a bus topology with an external transceiver (transmitter/receiver) connected via a tap to a thick coaxial cable as in Fig. 6.14. The transceiver is responsible for transmitting, receiving, and detecting collisions. The transceiver is connected to the station via a transceiver cable that provides separate paths for sending and receiving. This means that collision can only happen in the coaxial cable. The maximum length of the coaxial cable must not exceed 500 m, otherwise, there is excessive degradation of the signal. If a length of more than 500 m is needed, up to five segments, each a maximum of 500-meter, can be connected using repeaters. Note The following: - Maximum Length per segment is 500 meters. - Bus topology. - Minimum distance between transceivers is 2.5 feet. - Maximum length of standard AUI transceiver drop cable is 50 meters. - Maximum length of office transceiver cable is 12.5 meters. - Repeaters may be used to extend the signal. - Both ends of the cable segment must be terminated with a 50 ohm terminator. 6.4 Ethernet Implementations 6.4.1 10Base5: Thick Ethernet (Thicknet) Fig. 6.14 10 Base 5:Thick Ethernet implementation. 6.4 Ethernet Implementations 6.4.2 10Base2:Thin Ethernet (Cheapernet) The second implementation is called 10Base2, thin Ethernet, or Cheapernet. 10Base2 which sees a bus topology, but the cable is much thinner and more flexible as in Fig. 6.15. The transceiver is normally part of the network interface card (NIC), which is installed inside the station. This implementation is more cost effective than 10Base5 because thin coaxial cable is less expensive than thick coaxial and the tee connections are much cheaper than taps. Installation is simpler because the thin coaxial cable is very flexible. However, the length of each segment cannot exceed 185 m (close to 200 m) due to the high level of attenuation in thin coaxial cable. Note The following: - Uses an RG-58A/U coaxial cable. - Wired in a bus topology. - Each device on the network is connected to the bus by a BNC "T" adapter. - End of the bus must have a 50 Ohm terminator attached. - Node on the bus must be a minimum of 0.5 meters (1.5 feet) apart. - Length of the bus must be less than 185. 6.4 Ethernet Implementations 6.4.2 10Base2:Thin Ethernet (Cheapernet) 10 base 2 &10 base 5 Advantages 10 base 2 &10 base 5 disadvantages - Simple - Specialized Cable - Relatively Inexpensive - Difficult Troubleshooting - Noise Immunity - Difficult To Change - Fault Intolerant Fig. 6.15 10 Base 2:Thin Ethernet implementation. 6.4 Ethernet Implementations 6.4.3 10 Base-T: Twisted-Pair Ethernet The third implementation is called 10Base-T or twisted-pair Ethernet. 10Base-T uses a physical star topology. The stations are connected to a hub via two pairs of twisted cable. The two pairs of twisted cable create two paths (one for sending and one for receiving) between the station and the hub. Any collision here happens in the hub. Compared to 10Base5 or 10Base2, we can see that the hub actually replaces the coaxial cable as far as a collision is concerned. The maximum length of the twisted cable here is defined as 100 m, to minimize the effect of attenuation in the twisted cable. Note The following: - RJ45 connector is connected to cabling and is run from the hub to the NIC. - Pins 1 and 2 transmit data and pins 3 and 6 receive data (the other pins are not used). 6.4 Ethernet Implementations 6.4.3 10 Base-T: Twisted-Pair Ethernet Unshielded Twisted-Pair (UTP ) Cable Shielded Twisted-Pair (STP) cable Fig. 6.16 10 Base-T: Twisted-pair Ethernet implementation. 6.4 Ethernet Implementations 6.4.4 10 Base-F: Fiber Ethernet 10Base-F uses a star topology to connect stations to a hub. The stations are connected to the hub using two fiber-optic cables Fig. 6.17 10 Base-F: Fiber Ethernet. Summary of Standard Ethernet implementations 6.5 Switched Ethernet 6.5.1 Switched Ethernet Network A layer 2 switch is an N-port bridge with additional sophistication that allows faster handling of the packets. Evolution from a bridged Ethernet to a switched Ethernet was a big step that opened the way to an even faster Ethernet. Fig. 6.18 Switched Ethernet network. 6.5 Switched Ethernet 6.5.2 Full Duplex Ethernet One of the limitations of 10Base5 and 10Base2 is that communication is half- duplex (10Base-T is always full-duplex); a station can either send or receive, but may not do both at the same time. The next step in the evolution was to move from switched Ethernet to full-duplex switched Ethernet. The full-duplex mode increases the capacity of each domain from 10 to 20 Mbps. Fig. 6.19 shows a switched Ethernet in full-duplex mode. Note that instead of using one link between the station and the switch, the configuration uses two links: one to transmit and one to receive. Fig. 6.19 Full Duplex Ethernet network. 6.5 Switched Ethernet 6.5.2 Full Duplex Ethernet In full-duplex switched Ethernet, there is no need for the CSMA/CD method. In a full-duplex switched Ethernet, each station is connected to the switch via two separate links. Each station or switch can send and receive independently without worrying about collision. Each link is a point-to-point dedicated path between the station and the switch. There is no longer a need for carrier sensing; there is no longer a need for collision detection. The job of the MAC layer becomes much easier. The carrier sensing and collision detection functionalities of the MAC sublayer can be turned off. Auto-MDIX detects the type of connection required (Full or Half Duplex) and configures the interface accordingly. Fig. 6.20 Full-duplex switched Ethernet. 6.6 Fast Ethernet IEEE created Fast Ethernet under the name 802.3u. Fast Ethernet is backward-compatible with Standard Ethernet, but it can transmit data 10 times faster at a rate of 100 Mbps. The goals of Fast Ethernet can be summarized as follows: – Upgrade the data rate to 100 Mbps. – Make it compatible with Standard Ethernet. – Keep the same 48-bit address. – Keep the same frame format. – Keep the same minimum and maximum frame lengths. A main consideration in the evolution of Ethernet from 10 to 100 Mbps was to keep the MAC sublayer untouched. For the star topology, there are two choices: half duplex and full duplex. In the half-duplex approach, the stations are connected via a hub; in the full- duplex approach, the connection is made via a switch with buffers at each port. The access method is the same (CSMA/CD) for the half-duplex approach; for full-duplex Fast Ethernet, there is no need for CSMA/CD. However, the implementations keep CSMA/CD for backward compatibility with Standard Ethernet. 6.6 Fast Ethernet Auto negotiation A new feature added to Fast Ethernet is called autonegotiation. It allows a station or a hub a range of capabilities. Autonegotiation allows two devices to negotiate the mode or data rate of operation. It was designed particularly for the following purposes: – To allow incompatible devices to connect to one another. For example, a device with a maximum capacity of 10 Mbps can communicate with a device with a 100 Mbps capacity (but can work at a lower rate). – To allow one device to have multiple capabilities. – To allow a station to check a hub’s capabilities. Topology for Fast Ethernet Fast Ethernet is designed to connect two or more stations together. If there are only two stations, they can be connected point-to-point. Three or more stations need to be connected in a star topology with a hub or a switch at the center, as shown in Fig. 6.21. Fig. 6.21 Fast Ethernet Topology. 6.6 Fast Ethernet Fast Ethernet Implementations Fast Ethernet implementation at the physical layer can be categorized as either two-wire or four-wire. The two-wire implementation can be either category 5 UTP (100Base-TX) or fiber-optic cable (100Base-FX). The four-wire implementation is designed only for category 3 UTP (100Base-T4). See Fig. 6.22. Fig. 6.22 Fast Ethernet Implementations. 6.7 Virtual LAN (VLAN) 6.7.1 Introduction to VLAN VLANs are created to provide segmentation services traditionally provided by physical routers in LAN configurations. VLANs create separate broadcast domains within the switch. A VLAN is a broadcast domain created by one or more switches. VLAN = Subnet Before VLANs: All switch ports: Single broadcast domain, multiple collision domain. After VLANs: Each VLAN is a single broadcast domain and one logical subnet. Routers or layer3 switches are needed to pass information between different VLANs One link per VLAN or a single VLAN Trunk (later) Without 10.1.0.0/16 10.1.0.0/16 VLANs With VLANs 10.2.0.0/16 10.2.0.0/16 10.3.0.0/16 10.3.0.0/16 Fig. 6.23 Virtual LAN. 6.7 Virtual LAN (VLAN) 6.871 Introduction to VLAN VLAN Features VLANs are configured through software rather than hardware, which makes them extremely flexible. The whole idea of VLAN technology: divide a LAN into logical, instead of physical, segments. Each VLAN is a workgroup in the organization. One of the biggest advantages is that when a station moves from one group to another, without any hardware reconfiguration. All members belonging to a VLAN can receive broadcast messages sent to that particular VLAN. Stations in a VLAN communicate with one another as though they belonged to a physical segment. VLAN technology even allows the grouping of stations connected to different switches in a VLAN. 6.7 Virtual LAN (VLAN) 6.7.2 VLAN Membership 1) Static VLAN membership - Assign certain port to a certain VLAN ( port based VLAN). - By default, all ports of the switch are assigned to VLAN 1. 1) Dynamic VLAN membership - Assign certain MAC to a certain VLAN ( MAC based VLAN). - Even if the PC changes its port on the switch, the PC still be connected to its VLAN. - This is done by using VMPS (VLAN membership policy server). Fig. 6.24. Port-based Vlan Membership 6.8 Wireless LAN (WLAN) Wireless Fidelity (Wi-Fi) is an IEEE 802.11 WLAN standard; provides network access to home and corporate users, to include data, voice and video traffic, to distances up to 0.18 mile (300m). The following table shows different IEEE802.11 standards for WLAN 6.8 Wireless LAN (WLAN) Components of WLAN Wireless deployment requires: – End devices with wireless NICs. – Infrastructure device, such as a wireless router or wireless Access Point (AP) Fig. 6.25 Components of WLAN. 6.8 Wireless LAN (WLAN) 802.11 WLAN Topologies and Modes Independent Basic Service Set (IBSS) Topology: Devices interconnected directly without the use of AP or wireless router as shown in Fig. 6.26. This scenario operate as Ad Hoc mode. Fig. 6.26 Independent Basic Service Set (IBSS) topology. 6.8 Wireless LAN (WLAN) 802.11 WLAN Topology Modes Basic Service Set (BSS) Topology: Devices interconnect using the service of AP or wireless router as shown in Fig. 6.27. This scenario is operate as infrastructure mode with one AP. Fig. 6.27 Basic Service Set (BSS) topology. 6.8 Wireless LAN (WLAN) 802.11 WLAN Topology Modes Extended Service Set (ESS) Topology: Devices interconnect using the service of AP or wireless router as shown in Fig. 6.28. This scenario is operate as infrastructure mode with more than one AP. Fig. 6.28 Extended Service Set (ESS) topology. 6.8 Wireless LAN (WLAN) CSMA/Collision Avoidance (CSMA/CA) media access method for WLAN Device examines the media for the presence of data signal - if the media is free, the device sends a notification across the media of its intent to use it. The device then sends the data. Step 1 Listen to ensure that the medium (space) is not busy (no radio waves currently are being received at the frequencies to be used). Step 2 Set a random wait timer before sending a frame to statistically reduce the chance of devices all trying to send at the same time. Step 3 When the random timer has passed, listen again to ensure that the medium is not busy. If it isn’t, send RTS and then send the frame after receiving CTS. Step 4 After the entire frame has been sent, wait for an acknowledgment. Step 5 If no acknowledgment is received, resend the frame, using CSMA/CA Logic to wait for the appropriate time to send again. Fig. 6.29 WLAN. access method. 6.8 Wireless LAN (WLAN) WLAN Operation Wireless Clients and Access Point Association Fig. 6.30 Wireless clients and access point association. 6.8 Wireless LAN (WLAN) WLAN Operation Discovering APs Passive mode - AP advertises its service by sending broadcast beacon frames containing the SSID, supported standards, and security settings. -The beacon’s primary purpose is to allow wireless clients to learn which networks and APs are available in a given area. Active mode - Wireless clients must know the name of the SSID. - Wireless client initiates the process by broadcasting a probe request frame on multiple channels. - Probe request includes the SSID name and standards supported. - May be required if an AP or wireless router is configured to not broadcast beacon frames. Authentication Open authentication – A NULL authentication where the wireless client says “authenticate me” and the AP responds with “yes.” Used where security is of no concern. Shared key authentication – Technique is based on a key that is pre-shared between the client and the AP. 6.8 Wireless LAN (WLAN) WLAN Operation Authentication Fig. 6.31 Authentication. 6.8 Wireless LAN (WLAN) WLAN Operation Association Parameters – SSID – Unique identifier that wireless clients use to distinguish between multiple wireless networks in the same vicinity. – Password – Required from the wireless client to authenticate to the AP. Sometimes called the security key. – Network mode – Refers to the 802.11a/b/g/n/ac/ad WLAN standards. APs and wireless routers can operate in a mixed mode; i.e., it can simultaneously use multiple standards. – Security mode – Refers to the security parameter settings, such as WEP, WPA, or WPA2. – Channel settings – Refers to the frequency bands used to transmit wireless data. Wireless routers and AP can choose the channel setting or it can be manually set. 6.8 Wireless LAN (WLAN) WLAN Operation WiFi Channels 1- WiFi Channels in 2.4GHz Band 6.8 Wireless LAN (WLAN) WLAN Operation WiFi Channels 2- WiFi Channels in 5GHz Band Compliant with the 802.11 standard and supported frequency bands: 802.11ax (Wi-Fi 6), 802.11ac (Wi-Fi 5), 802.11n (Wi-Fi 4), 802.11a, 802.11b/g.