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Understanding LANs and the Data Link Layer in Networking

This document provides a comprehensive overview of LANs and the Data Link Layer, focusing on the Physical Layer's role in defining electrical, mechanical, and procedural standards for network communication. It discusses the specifics of MAC addressing, hardware addressing formats, and frame types including unicast, broadcast, and multicast. Additionally, it outlines the functionality of Network Interface Cards (NICs) and the methods of addressing (static, configurable, and dynamic). Practical insights into frame headers, detection of errors, and network analysis tools are also included, enhancing your networking knowledge.

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Understanding LANs and the Data Link Layer in Networking

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  1. CS335 Networking &Network Administration Tuesday, April 13, 2010

  2. LANs and data link layer • Physical layer specifies electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating the physical link • Physical layer specifies voltage levels, data rates, maximum transmission distances, and physical connectors • Data link layer (layer 2) communicates with upper layers through Logical Link Control (LLC) • Uses framing to group the bits of data • Uses MAC Address to identify devices

  3. LAN standards • Media Access Control (MAC) (IEEE802.3) • The MAC sublayer defines how to transmit frames on the physical wire. It handles physical addressing associated with each device, network topology definition, and line discipline • Logical Link Control (LLC) (IEEE802.2) • Logically identifies different protocol types and then encapsulates them. A type code or a service access point (SAP) identifier performs the logical identification. The type of LLC frame used by an end station depends on what identifier the upper layer protocol expects.

  4. Hardware Addressing • Physical address • Hardware address • MAC address (media access control) • When a sender transmits a frame the sender and receiver MAC address are in the frame header • Source address field • Destination address field

  5. NICS • LAN Hardware handles details of sending and receiving frames (NIC) • Independent of the processor

  6. NICS • If a frame is addressed to the NIC, the NIC accepts the frame and passes it on to the CPU • Otherwise it discards the frame • Checks CRC and discards frames with errors • NICs have • The computer therefore is isolated from activity because the NIC isolates the CPU from unnecessary frames

  7. Format of a physical address • Static – hardware manufacturer assigns a unique physical address to all devices • Advantages • Ease of use and permanence • Unique, no conflicts

  8. Format of a physical address • Configurable – customer assigns an address manually (switches or jumpers) or electronically with nonvolatile memory like EPROM • Advantages • Address remains the same • Can be smaller

  9. Format of a physical address • Dynamic – automatically assigns a physical address when the station boots, ex. Current time of day, check to see if that address is taken, different address every time it boots • Advantages • Eliminates need for hardware manufacturers to coordinate assigning addresses • Addresses can be smaller • Disadvantage • Lack of permanence • Potential conflict

  10. Unicast • A single packet is sent from a source to a destination • Uses the MAC address

  11. Broadcasting • Data sent to all devices on the network, not a single destination physical address • Broadcast address is a reserved address of each network • EX. 10.10.10.0 subnet mask 255.255.255.0, the broadcast address is 10.10.10.255 • Broadcast address is usually reserved as all 1’s • When a frame is sent to the broadcast address each computer on the network receives a copy

  12. Multicasting • Broadcasting is inefficient • Computers waste CPU time deciding if a broadcast frame is necessary • Sends a single frame over the network and allows a specific subset of nodes to receive the transmission • The source addresses by using a multicast address

  13. Frame headers and format • Two parts • Frame header has a fixed size • Size of the data area is determined by the type of data being sent

  14. Example frame format • Ethernet frame format • 64 bit preamble contains alternating 0’s and 1’s to allow synchronizing signals • Ethernet uses 48 bit addresses (6 byte) • 16 bit frame type

  15. Frame types

  16. Network Analyzers • NIC is in promiscuous mode so it receives copies of all frames • Can choose what kind of frames to report • Can graph results

  17. Next • http://cs.eou.edu/CSMM/twelch/networkdevices.ppt • This next link covers much of what we are covering, use it to further review the basics and extend your knowledge • http://www.erg.abdn.ac.uk/users/gorry/eg3561/road-map.html

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