CS335 Networking & Network Administration Tuesday, April 13, 2010

LANs and data link layer Physical layer specifies electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating the physical link Physical layer 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) Data link layer Uses framing to group the bits of data Uses MAC Address to identify devicesMAC Address

LAN standards Media Access Control (MAC) (IEEE802.3) Media Access Control 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) Logical Link Control 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.

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

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

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

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

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

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

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

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 subnet mask , the broadcast address is 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

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

Frame headers 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

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

Frame types

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

Next es.ppt es.ppt This next link covers much of what we are covering, use it to further review the basics and extend your knowledge 1/road-map.html 1/road-map.html