William Stallings Data and Computer Communications Chapter 13

William Stallings Data and Computer Communications Chapter 13 Local Area Network Technology

LAN (Local Area Networks) z A LAN consists of y. Shared transmission medium yset of hardware and software for the interfacing devices yregulations for orderly access to the medium

LAN Applications (1) z Personal computer LANs ymostly in office environment yto share data and resources (e. g. laser printer) z Back end networks and storage area networks y. Interconnecting large systems (mainframes and large storage devices) x. High data rate x. Limited distance x. Limited number of devices

LAN Applications (2) z High speed office networks y. Desktop image processing y. High capacity local storage z Backbone LANs y. Interconnect low speed local LANs with a higher capacity LAN

LAN Architecture z Layered protocol architecture y. Physical (topologies) y. Media access control y. Logical Link Control

Protocol Architecture z Lower layers of OSI model z IEEE 802 reference model z Physical z Logical link control (LLC) z Media access control (MAC)

IEEE 802 v OSI

802 Layers - Physical z Signal encoding/decoding z Preamble generation/removal (for synchronization) z Bit transmission/reception z Transmission medium and topology ybelow the physical layer

802 Layers - Medium Access Control & Logical Link Control z OSI layer 2 (Data Link) is divided into two in 802 y. Logical Link Control (LLC) layer y. Medium Access Control (MAC) layer z MAC layer y. Assembly of data into frame with address and error detection fields (for transmission) y. Disassembly of frame (on reception) x. Address recognition x. Error detection y. Govern access to transmission medium x. Not found in traditional layer 2 data link control

802 Layers - Medium Access Control & Logical Link Control z LLC layer y. Interface to higher levels yflow control

LAN Protocols in Context

LAN Topologies z Bus z Ring z Star

Bus Topology z Multipoint medium z Heard by all stations y. Need to identify target station x. Each station has unique address z Full duplex connection between station and tap y. Allows for transmission and reception z Terminator absorbs frames at end of medium z Need to regulate transmission y. To avoid collisions y. To avoid continuous transmission from a single station y. Solution: Data in small blocks - frames

Frame Transmission - Bus LAN

Ring Topology z Repeaters joined by pointto-point links in closed loop y. Receive data on one link and retransmit on another y. Links unidirectional y. Stations attach to repeaters z Data transmitted in frames y. Circulate past all stations y. Destination recognizes address and copies frame y. Frame circulates back to source where it is removed z Medium access control determines when station can insert frame

Frame Transmission Ring LAN

Star Topology z Each station connected directly to central node y. Usually via two point-to-point links z Central node can broadcast y. Physical star, logically bus y. Only one station can transmit at a time z Central node can act as frame switch yretransmits only to destination

Medium Access Control z Why? yorder yefficient use of medium z Synchronous yeveryone knows when to transmit xlike TDM z Asynchronous ydynamic y. Three categories x. Round robin x. Reservation x. Contention

Medium Access Control z Round robin yeach station has a turn to transmit xdeclines or transmits (to a certain limit) xoverhead of passing the turn in either case y. Good if many stations have data to transmit over extended period z Reservation y. Stations reserve slots to transmit y. Good for stream traffic ycentralized/distributed reservations

Medium Access Control z Contention y. No control to determine whose turn is it y. All stations contend for time/slot to transmit y. Good for bursty traffic y. Efficient under light or moderate load y. Performance is bad under heavy load

MAC Frame Format z Actual format differs from protocol to protocol z MAC layer receives data from LLC layer z MAC layer detects errors and discards frames z LLC optionally retransmits unsuccessful frames

Bus LANs z Signal balancing y. Signal must be strong enough to meet receiver’s minimum signal strength requirements y. Give adequate signal to noise ratio y. Not so strong that it overloads transmitter and creates distortions y. Simultaneous satisfaction for all stations on bus y. Solution is to divide network into small segments y. Segments are connected with repeaters

Transmission Media z Baseband coaxial cable y. Used by Ethernet and IEEE 802. 3 ynot a preferred option nowadays z Broadband coaxial cable y. Included in 802. 3 specification but no longer made z Optical fiber y. Expensive y. Not used

Baseband Coaxial Cable z Uses digital signaling z Manchester or Differential Manchester encoding z Bi-directional z Few kilometer range ydue to attenuation z Ethernet (basis for 802. 3) at 10 Mbps z 50 ohm cable

10 Base 5 z Thick coax z Ethernet and 802. 3 originally used 0. 4 inch diameter cable at 10 Mbps z Max cable length 500 m z Distance between taps a multiple of 2. 5 m y. Ensures that reflections from taps do not add in phase z Max 100 taps z Vampire taps, twisted pair transceiver cable z 10 Base 5

10 Base 2 z Thin coax z 0. 5 cm cable y. More flexible y. Easier to bring to workstation z Cheaper electronics z T-connectors yeasier maintanence z Greater attenuation, lower noise resistance y. Fewer taps (30) y. Shorter distance (200 m)

Repeaters z Transmits in both directions z Joins two segments of cable z No buffering z No logical isolation of segments z If two stations on different segments send at the same time, packets will collide z Only one path of segments and repeaters between any two stations

Baseband Configuration

Star LANs z Use unshielded twisted pair wire in star wiring z Attach to a central active hub z Two links y. Transmit and receive z Hub repeats incoming signal on all outgoing lines z Link lengths limited to about 100 m z Logical bus - with collisions

Two Level Star Topology All stations are logically on the same bus

Shared Medium Hub z Central hub z Hub retransmits incoming signal to all outgoing lines z Only one station can transmit at a time z With a 10 Mbps LAN, total capacity is 10 Mbps

Switched Hubs z Hub acts as switch z Incoming frame switches to appropriate outgoing line y. Unused lines can also be used to switch other traffic y. Each device has dedicated capacity equal to the LAN capacity

Switched Hubs z No change to software or hardware of devices z Attachment is not logically different from the attached device point of view z Store and forward switch y. Accept input, buffer it briefly, then output z Cut through switch y. Take advantage of the destination address being at the start of the frame y. Begin repeating incoming frame onto output line as soon as address recognized y. May propagate some bad frames

Bridges z Need to expand beyond single LAN z Interconnection to other LANs/WANs z Use Bridge or router z Bridge is simpler y. Connects similar LANs y. Identical protocols for physical and link layers y. Minimal processing z Router more general purpose y. Interconnect various LANs and WANs

Functions of a Bridge z Read all frames transmitted on one LAN and accept those address to any station on the other LAN z Using MAC protocol for second LAN, retransmit each frame z Do the same the other way round

Bridge Operation

Bridge Design Aspects z No modification to content or format of frame z No additional header z Exact bitwise copy of frame from one LAN to another ythat is why two LANs must be identical z Enough buffering to meet peak demand z Routing and addressing intelligence y. Must know the addresses on each LAN to be able to tell which frames to pass y. May be more than one bridge to reach the destination z May connect more than two LANs

Bridge Protocol Architecture z IEEE 802. 1 D z operates at MAC level y. Station address is at this level y. Bridge does not need LLC layer

Routing in Bridges z Bridge must have routing capability y. Bridge must decide whether to forward frame y. Bridge must decide which LAN to forward frame on z Alternate routes

Fixed Routing z Routing selected for each source-destination pair of LANs y. Usually least hop route y. Only changed when topology changes y. Manual routing data entry needed z Similar to fixed routing in packet switched networks ycentral routing table shows the first bridge in the route for each source-destination pair ylocal routing tables can be derived from the central table

Example Fixed Routing LAN E to LAN F • LAN E • Bridge 107 • LAN A • Bridge 102 • LAN C • Bridge 105 • LAN F

Spanning Tree Routing z Bridge automatically develops routing tables z Automatically updates in response to changes z Station locations are learned from packets yforwarding databases are constructed for each port of bridges z If the location of a station is known, packet is forwarded accordingly z Otherwise packet is forwarded to all ports yto guarantee delivery yto allow learning

Address Learning z When frame arrives at port X, it has come from the LAN attached to port X z Use the source address to update forwarding database for port X to include that address z Example: when Bridge 102 receives a packet from LAN A port with source address 2, it learns that station 2 is on LAN A port z Examples of port databases y Bridge 102 A port x Stations 1, 2, 3, 4 and 5 y Bridge 102 C port x Stations 6 and 7

Other Considerations z Address learning works for tree layout yi. e. no closed loops ya spanning tree must be found in case of a loop z Response to topology changes is required ydatabase entries are not permanent xi. e. learning algorithm is repeated periodically

Loop of Bridges

William Stallings Data and Computer Communications Chapter 14 LAN Systems (CSMA/CD Ethernet, Ring Systems)

Ethernet (CSMA/CD) z Carriers Sense Multiple Access with Collision Detection y. Xerox - Ethernet y. IEEE 802. 3 z Random Access (ALOHA) y Stations access medium randomly z Contention y. Stations contend for time on medium

ALOHA z Packet Radio (applicable to any shared medium) z When station has frame, it sends y collisions may occur z Station listens for max round trip time z If no collision, fine. If collision, retransmit after a random waiting time z Max utilization 18% - very bad

Slotted ALOHA z Divide the time into discrete intervals (slots) yequal to frame transmission time yneed central clock (or other sync mechanism) ytransmission begins at slot boundary z Collided frames will do so totally or will not collide z Max utilization 37%

CSMA z First listen for clear medium (carrier sense) z If medium idle, transmit z If busy, continuously check the channel until it is idle and then transmit z If collision occurs y Wait random time and retransmit z Collision probability depend on the propagation time z Longer propagation delay, worse the utilization z Collision occurs even if the propagation time is zero. WHY? z 1 -persistent CSMA z Better utilization than ALOHA

Nonpersistent CSMA z Patient CSMA z If channel idle, send z If not, do not continuously seize the channel yinstead wait a random period of time z Better utilization, longer delay

CSMA/CD z With CSMA, collision occupies medium for duration of transmission z Stations listen while transmitting z If medium idle, transmit z If busy, listen for idle, then transmit z If collision detected, cease transmission and wait random time then start again y. Binary exponential back off xrandom waiting period but consecutive collusions increase the mean waiting time xlow delay with small amount of waiting stations xlarge delay with large amount of waiting stations

CSMA/CD Operation

Collision Detection z On baseband bus, collision produces much higher signal voltage than signal z Collision detected if cable signal greater than single station signal z Signal attenuated over distance z Limit distance to 500 m (10 Base 5) or 200 m (10 Base 2)

IEEE 802. 3 Frame Format >= SFD is 10101011 FCS excludes Preamble and SFD >=

10 Mbps Specification (Ethernet) z 10 Base 5 and 10 Base 2 y Coax (thick/thin) y Bus topology y 500/200 meters max segment length y 100/30 max stations z 10 Base. T y Twisted pair (regular telephone wiring) y Star topology with central hub or switch y Point to point with cross cables y max 100 meters segment length z 10 Base. F y Optical fiber y star topology or point to point y on/off manchester encoding y too expensive for 10 Mbps

100 Mbps (Fast Ethernet) z 100 Base. T 4 y to use voice grade cat 3 cables y 3 pairs in each direction (with 33. 3 Mbps on each) x total 4 pairs (2 of them bidirectional) y cat 3 cables carry 25 MHz signal x Manchester encoding does not work with 3 pairs x How many pairs? x A different coding is used (8 B 6 T = 8 bits map to 6 trits) y Can be used with cat 5 cables (but waste of resources) z 100 Base-TX y STP or cat 5 UTP only (one pair in each direction) y at 125 Mhz with special encoding that has 20% overhead x 4 bits are encoded using 5 -bit time z 100 Base-FX y Optical fiber (one at each direction) y Similar encoding

Gigabit Ethernet Configuration

Gigabit Ethernet z Backward compatible y. CSMA/CD protocol ysame frame format z Carrier extension ya mechanism to make shortest frame 512 bytes z Frame bursting yability to send multiple short frames consecutively

Gigabit Ethernet - Physical z 1000 Base-SX, LX y. Optical fiber options z 1000 Base-CX y. A special STP (<25 m) xone for each direction z 1000 Base-T y 4 pairs, cat 5 UTP (bidirectional)

Token Ring (802. 5) z MAC protocol y. Small frame (token) circulates when idle y. Station waits for token y. Changes one bit in token to make it start-of-frame sequence for data frame y. Append rest of data frame y. Other stations must wait since there is no token circulating

Token Ring (802. 5) z MAC protocol (cont’d) y. Frame makes round trip and is absorbed by transmitting station y. Station then inserts new token when xtransmission has finished and xleading edge of returning frame arrives y. Under light loads, some inefficiency y. Under heavy loads xround robin xfair

Token Ring Operation

Token Ring z Characteristics ymaintenance needed xloss tokens xmultiple tokens ypriorities and reservations possible z Variations y. Early token release xsender does not wait the leading edge of the frame to release the token y. Dedicated Token Ring (DTR) xcentral hub acts as switch (concentrator) xdedicated connection between stations and concentrator z 4/16/1000 Mbps

FDDI z 100 Mbps y. Optical fiber or cat 5 UTP or STP z LAN and MAN applications z Token Ring

FDDI MAC Protocol z As for 802. 5 except: y. Station seizes token by aborting token transmission y. Once token captured, one or more data frames transmitted y. New token released as soon as transmission finished (early token release in 802. 5)

FDDI Operation

Token Bus z IEEE 802. 4 yold standard (1990) yinactive working group z Physical bus, logical ring ypreferred by manufacturers with linear production lines y. Complex MAC protocol