Fundamentals of Network Media Local Area Networks Ursula Holmström Goals Learn the basic concepts related to LAN technologies, for example use of shared media medium access control topologies Know the most common LAN standards and what their properties are Topics What is a LAN how does it differ from other networks? Approaches to sharing the same media Random access ALOHA CSMA Scheduling LAN standards Ethernet Token-ring FDDI wireless LAN More topics Multiple access communication All satations share exactly the same media The media is broadcast type a message sent to the media is received by all channels If two or more stations transmit at the same time there will be a collision interference & messed up transmission Multiple access solutions Channelization each station is assigned an own part of the media for collision-free transmission steady stream of data MAC schemes stations send when they need to and collisions are detected & avoided according to some scheme bursty traffic
Examples Satellite (figure 6.3) Multidrop telephone line (figure 6.4) Ring and bus (figure 6.5) Wireless LAN (figure 6.6) Network topologies bus, ring, star,... ad hoc Recuirements Requirements for a MAC protocol Transfer delay Fairness Reliability Support for different types of traffic Quality of service Scalability Cost Delay-Bandwidth product The product of the bitrate and the delay that elapsese before an action can take place A property of the transmission channel how much data is in the pipe before we have a chance to react on what is happening on the channel Delay-Bandwidth product Important factor in choosing what kind of flow control to use Example: Ethernet frame 1500 bytes (12000 bits) TCP packet ca. 65000 bytes (520000 bits) nicely fills the delay-bandwidth of a typical local network (desctop... LAN) LAN Standards IEEE 802 LAN standards Most standards developed by IEEE 802 committee Physical layer cabling, connectors, distances, modulation, line code Link Layer logical link control, medium access control 802.3 CSMA-CD Netwok layer 802.2 Logical link control 802.5 Token ring 802.11 Wireless LAN Various physical layers Other LANs LLC MAC
Network Interface Card physical connection to the network unique physical address firmware that implement MAC protocols parallell communication with the computer and serial to the network buffering MAC sublayer Connectionless transfer of datagrams usually no error control (just detection) Adds source and destination MAC adresses to the frame as well as a frame check sequence MAC address specifies the physical connection Main task: transmitting the frame on the shared medium LLC sublayer Enhance the services provided by the MAC protocol offer the network layer standard services independently of the underlying network connection-orientied, reliable transfer Supplement the address several on going data exchanges Random access No prior scheduling to who can transmit when access at random whenever you have something to transmit Some means of detecting collisions Some algorithm for avoiding recollision in re-transmission ALOHA Originates in Hawaii Radio links between main campus and local campuses on different islands Messages are sent as they become available Collisions are treated as transmission errors and handled by retransmission ALOHA (2) Packet collisions cause retransmissions that cause more traffic and more collisions random wait before retransmission Transmission in two modes everything works fine thing start colliding badly
ALOHA (3) When does a collision occure? First transmission Retransmission t 0 -X t 0 t 0 +X t 0 +X+2t prop t 0 +X+2t prop +B Vunerability period Backoff period ALOHA (4) Throughput S arrival rate of packets G total arrival rate (including retransm.) in packets per X seconds where X is the time it takes to transmit one packet After some assumptions and calculations we get G/S = e 2G attempts per packet Slotted ALOHA Reduce the probability of collisions by constraining the transmissions to timeslots shorter vunerability period G/S = e G attempts per packet See figure 6.12 CSMA Carrier Sensing Multiple Access Avoid obvious collisions by sensing the medium for ongoing transmissions Vunerability time is t prop that is the one way propagation time over the whole media after this a station at one end has effectively captured the media for its transmission CSMA (2) Different flavours 1-persistent: send as soon as you can non-persistent: if busy reschedule p-persistent: if channel is idle, with probability p send, othervise don t Influence the overall delay in transmission depends on amount of traffic CSMA-CD Carrier Sensing Multiple Access with Collision Detection avoid wasting bandwidth by stopping a collided transmission How long is the reaction time? When can we be sure there is no collision?
CSMA-CD (2) Scheduling approaces A A A B B B Random access does not use the channel efficiently at high traffic rates Scheduling provides a more organized way to transmit reservation polling token passing Reservation A frame consists of a reservation interval and dataintervals (packets) In the reservation interval a station can indicate that it has packets to transmit Improved time-multiplexing no wasted slots if a station is idle Can be modified in several ways... Polling Stations take turns in using the medium the right to transmit is passed from one station to another With or without central control Polling causes overhead - walktime Token passing Distributed polling in a ring Point to point connections in a ring each NIC listens to the stream and picks up own packets and repeats other Token passing and ring latency multitoken, single token and single packet operation Token reinsertion Multitoken Single token Single packet Busy token Free token
Keeping the token The station with the token can keep the token for as long as it wants to for a certain amount of time for a certain amount of packets for just one packet Comparison of MACs Random Access Uncoordinated Small delays if there is enough bandwidth Scheduling Medium Access Control Orderly access Known delays, less variation Both Use channel bandwidth for signaling Throughput depends on chosen scheme and channel properties such as transfer delay Channelization For continuous data stream a channel is needed Channelization vs. multiplexing dividing of media in the same way allocation of channels different FDMA, TDMA, CDMA LAN standard Ethernet, Token ring, FDDI, WLAN MAC algorithm frame structure physical layer options IEEE 802.3 - Ethernet Coaxial bus, CSMA-CD minislot time > 2t prop to ensure detection of collisions 10 Mbps over 2500 m gives 512 bits or 64 bytes for minimum frame length exponential backoff IEEE 802.3 - Frame structure 7 1 2 or 6 2 or 6 2 4 Destination Source Preamble SD Address Address Length Information Pad FCS 0 Single address 1 Group address 0 Local address Total 64 to 1518 Bytes 0 Global address
IEEE 802.3 - Address Unicast permanent NIC card address Multicast group of recipients Broadcast all 1s, all stations IEEE 802.3 - Physical layer 10base5 thick coax, 500 m, bus 10base2 thin coax, 200 m, bus 10baseT twisted pair, 100 m, star 10baseF Optical fiber, 2 km, point-to-point link IEEE 802.3 - Physical layer A hub (10baseT) forms a collision domain all stations share the same media limited number of stations Ethernet switch separate collision domains possible to connect more stations IEEE 802.3u - Fast Ethernet Same frame sizes and procedures 100BaseT4 TPcat3 four pairs, 100m, star 100BaseT TPcat5 two pairs, 100m, star 100BaseF multimode fiber two strands, 2 km, star IEEE 802.3z - Gigabit Ethernet Again same frames & procedures but faster physical medium too fast for CSMA-CD minimum frame size increased to 512 bytes packet bursting Mainly switched mode Optical fiber(5km), UTP cat5 (100m) IEEE 802.5 - Token ring Point-to-point transmission links in a ring topology ring implemented as a star Token passing 4Mbps and 16 Mbps twisted pair cables, manchester line coding max 250 stationsin the ring
IEEE 802.5 - Frame structure IEEE 802.5 - Frame structure 1 1 1 2 or 6 2 or 6 2 4 SD AC FC Destination Source Address Address Length Information FCS 1 1 ED FS SD: starting delimiter line coding violations AC: access control priority, token, monitor, reservation FC: frame control ED: ending delimiter intermediate, final frame FS: frame status FDDI FDDI - Timed-token Fiber Distributed Data Interface Ring topology, up to 500 stations Optical fiber links at 100 Mbps, up to 200 km Used e.g. as a campus backbone connecting different Ethernet LAN subnetworks Synchronous and asynchronous traffic Target token rotation time (TTRT) token rotation timer (TRT) token hold timer (THT) adjusts how long a station can keep the token (THT = TTRT - TRT) adjusts transmission according to congestion (synch / asynch) WLAN Why not just ame as LAN with different physical layer? noise and interference unpredictable propagation uncontrolled spreading other users on the same frequencies users want to enjoy mobility WLAN infrastructure BSS basic Service Set A number of wireless station within rang of eachother Independent ad hoc network or stations around an access point (AP) ESS extended service set BSSs interconnected by a distribution system, each BSS through an AP Portal for connection to the wired network
WLAN functionalities Association a station associates with a AP to join a BSS reassociation: moving to a new AP dissociation: terminating the previous association Authentication stations may authenticate eachother Privacy protection of message content WLAN vs. (wired) LAN WLANs are dynamic by nature LAN MAC address = physical location WLAN MAC address = station but not location WLAN MAC interface to LLC should be the same as other LANs mobility is handled within the MAC layer (but not included in this course)