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Data communication and computer network- chapter 04.pdf
- 1. Data Communications andComputer Networks CH-04 Connecting Devices Baessa K. Mettu University College of Engineering and Technology Department of Information Technology March 15, 2022 Baessa K. (Mettu University) 04 Data link layer March 15, 2022 1 / 18
- 2. Lecture Topics :# 1 Connecting Devices Hubs Link-Layer Switches Routers Baessa K. (Mettu University) 04 Data link layer March 15, 2022 2 / 18
- 3. Connecting Devices Outline 1 ConnectingDevices Hubs Link-Layer Switches Routers Baessa K. (Mettu University) 04 Data link layer March 15, 2022 3 / 18
- 4. Connecting Devices Introduction • Hostsand networks do not normally operate in isolation. • Connecting devices are used to connect hosts together to make a network or to connect networks together to make an internet. • Connecting devices can operate in different layers of the Internet model. • three kinds of connecting devices: Baessa K. (Mettu University) 04 Data link layer March 15, 2022 4 / 18
- 5. Connecting Devices Introduction • Hostsand networks do not normally operate in isolation. • Connecting devices are used to connect hosts together to make a network or to connect networks together to make an internet. • Connecting devices can operate in different layers of the Internet model. • three kinds of connecting devices: Baessa K. (Mettu University) 04 Data link layer March 15, 2022 4 / 18
- 6. Connecting Devices Hubs Outline 1Connecting Devices Hubs Link-Layer Switches Routers Baessa K. (Mettu University) 04 Data link layer March 15, 2022 5 / 18
- 7. Connecting Devices Hubs Hubs •A hub is a device that operates only in the physical layer. • Signals that carry information within a network can travel a fixed distance before attenuation endangers the integrity of the data. • A repeater receives a signal and, before it becomes too weak or corrupted, regenerates and retimes the original bit pattern. • The repeater then sends the refreshed signal. • In the past, when Ethernet LANs were using bus topology, • …a repeater was used to connect two segments of a LAN to overcome the length restriction of the coaxial cable. • Today, however, Ethernet LANs use star topology. • In a star topology, a repeater is a multiport device, often called a hub, • …that can be used to serve as the connecting point and • …at the same time function as a repeater. Baessa K. (Mettu University) 04 Data link layer March 15, 2022 6 / 18
- 8. Connecting Devices Hubs Hubs •A hub is a device that operates only in the physical layer. • Signals that carry information within a network can travel a fixed distance before attenuation endangers the integrity of the data. • A repeater receives a signal and, before it becomes too weak or corrupted, regenerates and retimes the original bit pattern. • The repeater then sends the refreshed signal. • In the past, when Ethernet LANs were using bus topology, • …a repeater was used to connect two segments of a LAN to overcome the length restriction of the coaxial cable. • Today, however, Ethernet LANs use star topology. • In a star topology, a repeater is a multiport device, often called a hub, • …that can be used to serve as the connecting point and • …at the same time function as a repeater. Baessa K. (Mettu University) 04 Data link layer March 15, 2022 6 / 18
- 9. Connecting Devices Hubs Hubs •A hub is a device that operates only in the physical layer. • Signals that carry information within a network can travel a fixed distance before attenuation endangers the integrity of the data. • A repeater receives a signal and, before it becomes too weak or corrupted, regenerates and retimes the original bit pattern. • The repeater then sends the refreshed signal. • In the past, when Ethernet LANs were using bus topology, • …a repeater was used to connect two segments of a LAN to overcome the length restriction of the coaxial cable. • Today, however, Ethernet LANs use star topology. • In a star topology, a repeater is a multiport device, often called a hub, • …that can be used to serve as the connecting point and • …at the same time function as a repeater. Baessa K. (Mettu University) 04 Data link layer March 15, 2022 6 / 18
- 10. Connecting Devices Hubs Hubs… • hub forwards the packet from all outgoing ports except the one from which the signal was received. • …the frame is broadcast. • All stations in the LAN receive the frame, but only station B keeps it. • The rest of the stations discard it • hub does not have a filtering capability • it does not have the intelligence to find from which port the frame should be sent out. Baessa K. (Mettu University) 04 Data link layer March 15, 2022 7 / 18
- 11. Connecting Devices Hubs Hubs… • A hub or a repeater is a physical-layer device. • They do not have a link-layer address and they do not check the link-layer address of the received frame. • They just regenerate the corrupted bits and send them out from every port. • There are two types of hubs 1 Passive Hubs • it does nothing except provide a pathway for the electrical signals to travel along. 2 Active Hubs • provide a pathway PLUS regenerates the signal before it forwards • A Hub does not perform any processing on the data that it forwards, nor does it perform any error checking. Baessa K. (Mettu University) 04 Data link layer March 15, 2022 8 / 18
- 12. Connecting Devices Hubs Hubs… • A hub or a repeater is a physical-layer device. • They do not have a link-layer address and they do not check the link-layer address of the received frame. • They just regenerate the corrupted bits and send them out from every port. • There are two types of hubs 1 Passive Hubs • it does nothing except provide a pathway for the electrical signals to travel along. 2 Active Hubs • provide a pathway PLUS regenerates the signal before it forwards • A Hub does not perform any processing on the data that it forwards, nor does it perform any error checking. Baessa K. (Mettu University) 04 Data link layer March 15, 2022 8 / 18
- 13. Connecting Devices Hubs Hubs… • A hub or a repeater is a physical-layer device. • They do not have a link-layer address and they do not check the link-layer address of the received frame. • They just regenerate the corrupted bits and send them out from every port. • There are two types of hubs 1 Passive Hubs • it does nothing except provide a pathway for the electrical signals to travel along. 2 Active Hubs • provide a pathway PLUS regenerates the signal before it forwards • A Hub does not perform any processing on the data that it forwards, nor does it perform any error checking. Baessa K. (Mettu University) 04 Data link layer March 15, 2022 8 / 18
- 14. Connecting Devices Link-LayerSwitches Outline 1 Connecting Devices Hubs Link-Layer Switches Routers Baessa K. (Mettu University) 04 Data link layer March 15, 2022 9 / 18
- 15. Connecting Devices Link-LayerSwitches Link-Layer Switches • Operates in both the physical and the data-link layers. • As a physical-layer device, • …it regenerates the signal it receives. • As a link-layer device, • …the link-layer switch can check the MAC addresses (source and destination) contained in the frame. Baessa K. (Mettu University) 04 Data link layer March 15, 2022 10 / 18
- 16. Connecting Devices Link-LayerSwitches Link-Layer Switches … Filtering • A link-layer switch has filtering capability. • It can check the destination address of a frame and can decide from which outgoing port the frame should be sent. • A link-layer switch has a table used in filtering decisions. Baessa K. (Mettu University) 04 Data link layer March 15, 2022 11 / 18
- 17. Connecting Devices Link-LayerSwitches Link-Layer Switches … • Advantages of Switches • A link-layer switch has several advantages over a hub. • Collision Elimination • by creating a direct path between two devices and controlling their communication, • …it can greatly reduce the number of collisions on the network. • the lack of collisions enables switches to communicate with devices in full-duplex mode. • Connecting Heterogenous Devices • A link-layer switch can connect devices that use different protocols at the physical layer (data rates) and different transmission media. Baessa K. (Mettu University) 04 Data link layer March 15, 2022 12 / 18
- 18. Connecting Devices Link-LayerSwitches Link-Layer Switches … • There are three kinds of switching methods: 1 Cut-through • packet begins to be forwarded as soon as it is received (fast, but error prone) • the switch starts the frame processing as soon as it reads the destination MAC address of the frame. • To start the frame processing, a switch only needs the destination MAC address. • Just after reading the destination address, it checks the CAM table and based on the match it starts the forwarding of the frame 2 Store-and-forward • entire packet is received and error checked before being forwarded (error-free, but slow) • After receiving the entire frame, switch verifies whether the received frame is in the good state or not. • If the received frame is in the good state, only then the switch processes it. • If the received frame is not in the good state, the switch discards it. Baessa K. (Mettu University) 04 Data link layer March 15, 2022 13 / 18
- 19. Connecting Devices Link-LayerSwitches Link-Layer Switches … 3 Fragment-Free • enough of the packet is read to check errors (Takes advantage of both). • the switch starts the frame processing after reading the first 64 bytes of the frame. • 64 bytes is the minimum legal size of an Ethernet frame Baessa K. (Mettu University) 04 Data link layer March 15, 2022 14 / 18
- 20. Connecting Devices Link-LayerSwitches Link-Layer Switches … Internal frame processing • A switch has multiple ports. • These ports are used to connect various devices with it. • To know which device is connected to which port, the switch stores the MAC addresses of all connected devices with their related ports in a table, known as the CAM table. • When a switch receives a frame on any of its port, it reads the destination address from the frame and finds that destination address in the CAM table. • If the CAM table has an entry for that destination address, • …the switch forwards that frame from the port on which the destination address is connected. • If the CAM table does not have an entry for that destination address, the switch forwards that frame from all of its ports. Baessa K. (Mettu University) 04 Data link layer March 15, 2022 15 / 18
- 21. Connecting Devices Routers Outline 1Connecting Devices Hubs Link-Layer Switches Routers Baessa K. (Mettu University) 04 Data link layer March 15, 2022 16 / 18
- 22. Connecting Devices Routers Routers •A router is a three-layer device; it operates in the physical, data-link, and network layers • As a physical-layer device, it regenerates the signal it receives. • As a link-layer device,the router checks the physical addresses (source and destination) contained in the packet • As a network-layer device, a router checks the network-layer addresses. • A router can connect networks. • a router is an internetworking device; • …it connects independent networks to form an internetwork. • …two networks connected by a router become an internetwork or an internet. Baessa K. (Mettu University) 04 Data link layer March 15, 2022 17 / 18
- 23. Connecting Devices Routers Router… • There are three major differences between a router and a repeater or a switch 1 A router has a physical and logical (IP) address for each of its interfaces. 2 A router acts only on those packets in which the link-layer destination address matches the address of the interface at which the packet arrives. 3 A router changes the link-layer address of the packet (both source and destination) when it forwards the packet. Baessa K. (Mettu University) 04 Data link layer March 15, 2022 18 / 18