Objectives. Hexadecimal Numbering and Addressing. Ethernet / IEEE LAN Technology. Ethernet

Transcription

1 2007 Cisco Systems, Inc. All rights reserved. Cisco Public Objectives Ethernet Network Fundamentals Chapter 9 ITE PC v4.0 Chapter 1 1 Introduce Hexadecimal number system Describe the features of various Ethernet standards. Describe the function and operation of the media access control method used by Ethernet protocol. Explain the importance of Layer 2 addressing used for data transmission. Compare and contrast the application and benefits of using Ethernet switches in a LAN as apposed to using hubs. Explain the ARP process. 2 Hexadecimal Numbering and Addressing Representing hexadecimal numbers or 0x71 or 71h Converting hexadecimal to binary and decimal Remember each 4 binary digits is one hex digit = = = Layer 2 MAC addresses are displayed in hex E.g E9-42-AC Ethernet / IEEE LAN Technology The success of Ethernet is due to the following factors: Simplicity and ease of maintenance Ability to incorporate new technologies Reliability Low cost of installation and upgrade Ethernet uses UTP copper cables and optical fiber to interconnect network devices via intermediary devices such as hubs and switches. 5 6

2 Ethernet Type 10Base-5 10Base-2 10Base-T 100Base-TX 100Base-FX Bandwidth 10 Mbps 10 Mbps 10 Mbps 100 Mbps 100 Mbps Cable Thick coax Thin coax Cat 3 UTP Cat 5 UTP Multimode Fiber Duplex / Distance 500m 185m 400m Ethernet Bandwidths 10 Mbps Legacy, Classic or Standard Ethernet 100 Mbps Fast Ethernet 1000 Mbps or 1Gbps 1000Base-T 1 Gbps Cat 5e UTP Gigabit Ethernet 1000Base-SX 1000Base-LX 10GBase-T 10GBase-LX 1 Gbps 1 Gbps 10 Gbps 10 Gbps Multimode Fiber Singlemode Fiber Cat 6/7 UTP M Fiber S Fiber 550m 2km 300m 10km 7 10 Gbps Future Working on 40-Gbps, 100-Gbps, or even 160- Gbps standards. 8 Early Ethernet Media and Topology Current Ethernet LANs - Switched Ethernet Logical topology of Ethernet is a multi-access bus Shared bandwidth -duplex. MAC method: CSMA/CD Physical topology: Initially a bus using coaxial media Migrated to a star using hubs and UTP media Switches replace hubs One device per switchport allows link to operate in full-duplex mode Effectively doubles bandwidth with no collisions 100 Mbps on desktops, 1000 Mbps over UTP and fiber media 9 10 Future Ethernet Physical and Data Link Features of Ethernet Initially limited to LANs Then extended to the campus IEEE is implemented in the lower half of the Data Link layer, (the Media Access Control (MAC) sublayer), and the Physical layer only. LAN/WAN divide now blurred Possibility of end-to-end Ethernet across MANs/WANs 11 12

3 IEEE Logic Link Control Makes the connection with the upper layers (software) and the lower layers (NIC hardware) Identifies the Network layer protocol a.k.a. NIC driver Media Access Control MAC Sublayer Frame delimiting, addressing, error detection Control of frame placement on and off the media The Frame Encapsulating the Packet IEEE used on TCP/IP networks 15 Fields in the IEEE Frame Preamble and Start of Frame Asynchronous transmission Receiver gets into synch with sender Detects the beginning of the frame Length/Type If < 0x0600 hex (1536) then length of Data Else code indicates the Layer 3 protocol Frame Check Sequence CRC checksum The frame has a maximum and minimum size Frames outside this range are dropped (runts) 16 Layer 2 addressing Layer 2 and Layer 3 Addressing The Network layer address enables the packet to be forwarded toward its destination. The Data Link layer address enables the packet to be carried by the local media across each segment

4 The Ethernet MAC Address Physical address, Layer 2 address A 48-bit number that uniquely identifies an interface connected to the media The Ethernet MAC Address Example: E9-42-AC-28 The IEEE assigns the vendor a 3-byte code, called the Organizationally Unique Identifier (OUI). Burned-in address (BIA) Display the Ethernet address with Window command prompt: ipconfig /all Linux shell command: ifconfig Ethernet Unicast, Multicast and Broadcast Different Layer 2 MAC addresses are used for these: Unicast the burned-in address Broadcast FF-FF-FF-FF-FF-FF Multicast begins with E Ethernet Media Access Control MAC control access to the media so that only one device transmits at a time Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Contention-based Non-deterministic cannot guarantee how long a device will have to wait before obtaining access to the media Works well (few collisions) for most networks Excessive traffic causes network to fall-over Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Carrier Sense Multiple Access with Collision Detection (CSMA/CD) 23 Carrier Sense Listens before transmitting Multi-Access Collisions may occur Collision Detection Illegal signal levels When a collision is detected: Send 32-bit Jam Signal Wait a Random Backoff time-interval Attempt to send the frame again Maximum 16 attempts before reporting network error to upper layers 24

5 Hubs and Repeaters Legacy Ethernet Using Hubs Hubs widely used with Legacy Ethernet Operate at the physical layer Repeaters regenerate the signal Hubs also called Multi-port repeaters or concentrators basically connection devices Interconnected hubs form physical topology called an extended star. Hubs flood frames Capacity of media is shared between devices Collision Domains Switches More devices are being connected to the network. Devices access the network media more frequently. Distances between devices are increasing Hubs increase collision domains Reduced throughput in network Replaced by switches in later networks 27 Layer 2 devices work with frames High port-density, low latency (like hubs) By connecting each device directly to a switchport: Dedicated bandwidth to each port Collision-free environment -duplex operation Each switchport is a separate collision domain Increases network throughput 28 Ethernet Switch Operation Ethernet LAN switches use five basic operations: Learning - MAC table must be populated with MAC addresses dynamic mappings Aging - timestamp is used to remove old entries Flooding - If the switch does not know which port to send a frame because the destination MAC address is not in the MAC table, the switch floods the frame arrived. Selective Forwarding - examining a frame's destination MAC address and forwarding it out the appropriate port. Filtering dropping a frame - corrupt frames security

6 Use of Ethernet Switches versus Hubs in a LAN. Ethernet Latency An Ethernet frame takes a measurable time to travel from the sender to the receiver. The electrical signal that is transmitted takes a certain amount of time (latency) to propagate (travel) down the cable (wire speed) Each intermediate device contributes to the overall latency. The higher the device is in the OSI model, the higher the latency: Hubs/repeaters < Switches < Bridges < Routers Ethernet Timing Slot Time The actual calculated slot time is just longer than the theoretical amount of time required to travel between the furthest points of the collision domain, collide with another transmission at the last possible instant, and then have the collision fragments return to the sending station and be detected. Slot time establishes the minimum size of an Ethernet frame as 64 bytes Any frame less than 64 bytes in length is considered a "collision fragment" or "runt frame" and is automatically discarded by receiving stations. 33 Ethernet Timing Constraints A maximum cable length and maximum number of repeaters between the two furthest hosts on the network A maximum number of nodes that can be connected to a segment If these timing constraints are not met, a node transmitting a frame may not detect a collision before finishing transmitting, and the frame will not be resent Interframe spacing - minimum spacing between two non-colliding frames - time for the devices to process the frame. 96 bit times 34 Address Resolution Protocol (ARP) The ARP protocol provides two basic functions: Resolving IPv4 addresses to MAC addresses Maintaining a cache of mappings There are two ways a device can gather MAC addresses: Monitor the traffic that occurs on the local network segment Broadcast an ARP request. ARP entries are timestamped and aged If no device responds to the ARP request, the frame Address Resolution Protocol (ARP) process. The ARP request is a broadcast The ARP reply is a unicast cannot be delivered and is dropped 35 36

7 ARP Cache ARP Broadcasts Issues Using the arp command, the ARP cache can be displayed, entries deleted, static entries added, etc. C:\>arp -a Interface: x2 Internet Address Physical Address Type d dynamic C:\> Summary After studying this chapter you should be able to: Identify the basic characteristics of network media used in Ethernet Describe the Physical and Data Link layer features of Ethernet Describe the media access control method used by Ethernet Explain the importance of Layer 2 addressing Contrast the benefits of using Ethernet switches compared to hubs Describe the ARP process 39