Chapter 6 Medium Access Control Protocols and Local Area Networks

Chapter 6 Medium Access Control Protocols and Local Area Networks Part I: Medium Access Control Part II: Local Area Networks

Chapter Overview Broadcast Networks All information sent to all users No routing Shared media Radio Cellular telephony Wireless LANs Medium Access Control Local Area Networks Copper & Optical Ethernet LANs Cable Modem To coordinate access to shared medium Data link layer since direct transfer of frames High-speed, low-cost communications between co-located computers Typically based on broadcast networks Simple & cheap Limited number of users

Chapter 6 Medium Access Control Protocols and Local Area Networks Part I: Medium Access Control 6.1 Multiple Access Communications 6.2 Random Access Scheduling Channelization Delay Performance

Chapter 6 Medium Access Control Protocols and Local Area Networks Part II: Local Area Networks 6.6 LAN Protocols 6.7 Ethernet and IEEE 802.3 Token Ring and FDDI 802.11 Wireless LAN 6.11 LAN Bridges

Chapter 6 Medium Access Control Protocols and Local Area Networks 6.1 Multiple Access Communications

Multiple Access Communications Shared media basis for broadcast networks Inexpensive: radio over air; copper or coaxial cable M users communicate by broadcasting into medium Key issue: How to share the medium? 3 1 2 4 Shared multiple access medium 5 M

Approaches to Media Sharing Medium sharing techniques Static channelization Partition medium Dedicated allocation to users Satellite transmission Cellular Telephone Dynamic medium access control Scheduling Random access Polling: take turns Request for slot in transmission schedule Token ring Wireless LANs Loose coordination Send, wait, retry if necessary Aloha Ethernet

Channelization: Satellite Satellite Channel uplink fin downlink fout

Channelization: Cellular uplink f1 ; downlink f2 uplink f3 ; downlink f4

Scheduling: Polling Data from 1 from 2 Data Poll 1 Host computer Inbound line Data to M Poll 2 Outbound line 1 2 M 3 Stations

Scheduling: Token-Passing Ring networks token Data to M token Station that holds token transmits into ring

Random Access Multi-tapped Bus Crash!! Transmit when ready Transmissions can occur; need retransmission strategy

Wireless LAN AdHoc: station-to-station Infrastructure: stations to base station Random access & polling

Selecting a Medium Access Control Applications Scale What type of traffic? Voice streams? Steady traffic, low delay/jitter Data? Short messages? Web page downloads? Enterprise or Consumer market? Reliability, cost How much traffic can be carried? How many users can be supported? Current Examples: Design MAC to provide wireless-DSL-equivalent access to rural communities Design MAC to provide wireless-LAN-equivalent access to mobile users (user in car travelling at 100 km/hr)

Delay-Bandwidth Product Delay-bandwidth Coordination in sharing medium involves using bandwidth (explicitly or implicitly) How many bits are enroute from source to destination? Prop delay * bandwidth Simple product is key parameter two-station example Station with frame to send listens to medium and transmits if medium found idle Station monitors medium to detect collision If collision occurs, station that begin transmitting earlier retransmits

Two-Station MAC Example Two stations are trying to share a common medium A transmits at t 0 Distance d meters tprop d / seconds A B Case 1 A B Case 2 A detects collision at t 2 tprop A B A B B does not transmit before t tprop & A captures channel B transmits before t tprop and detects collision soon thereafter

Efficiency of Two-Station Example Each frame transmission requires 2tprop of quiet time Station B needs to be quiet tprop before and after time when Station A transmits R transmission bit rate L bits/frame L 1 1 Efficiency max L 2t prop R 1 2t prop R / L 1 2a L 1 MaxThrough put Reff R bits/secon d L / R 2t prop 1 2a Normalized Delay-Bandwidth Product a t prop L/R Propagation delay Time to transmit a frame

Typical MAC Efficiencies Two-Station Example: 1 Efficiency 1 2a CSMA-CD (Ethernet) protocol: 1 Efficiency 1 6.44a Token-ring network If a 1, then efficiency close to 100% As a approaches 1, the efficiency becomes low 1 Efficiency 1 a a latency of the ring (bits)/average frame length

Typical Delay-Bandwidth Products Distance 10 Mbps 100 Mbps 1 m 3.33 x 10-02 3.33 x 10-01 Desk area network 3.33 x 1003 Local area network 10 km 3.33 x 1002 3.33 x 1003 3.33 x 1004 Metropolitan area network 100000 km 3.33 x 1006 3.33 x 1007 3.33 x 100 Network Type 100 m 3.33 x 1001 3.33 x 1002 1000 km 3.33 x 1004 3.33 x 1005 1 Gbps 3.33 x 1006 Wide area network 3.33 x 1008 Global area network Max size Ethernet frame: 1500 bytes 12000 bits Long and/or fat pipes give large a

MAC protocol features Delay-bandwidth product Efficiency Transfer delay Fairness Reliability Capability to carry different types of traffic Quality of service Cost

MAC Delay Performance Frame transfer delay Throughput From first bit of frame arrives at source MAC To last bit of frame delivered at destination MAC Actual transfer rate through the shared medium Measured in frames/sec or bits/sec Parameters R bits/sec & L bits/frame X L/R seconds/frame frames/second average arrival rate Load X, rate at which “work” arrives Maximum throughput (@100% efficiency): R/L fr/sec

Normalized Delay versus Load E[T]/X E[T] average frame transfer delay Transfer delay X average frame transmission time 1 Load max 1 At low arrival rate, only frame transmission time At high arrival rates, increasingly longer waits to access channel Max efficiency typically less than 100%

Dependence on Rtprop/L a a E[T]/X a Transfer Delay a 1 max Load max 1

Chapter 6 Medium Access Control Protocols and Local Area Networks 6.2 Random Access

ALOHA Wireless link to provide data transfer between main campus & remote campuses of University of Hawaii Simplest solution: just do it A station transmits whenever it has data to transmit If more than one frames are transmitted, they interfere with each other (collide) and are lost If ACK not received within timeout, then a station picks random backoff time (to avoid repeated collision) Station retransmits frame after backoff time First transmission t0-X t0 Backoff period B Retransmission t t0 X Vulnerable period t0 X 2tprop Time-out t0 X 2tprop B

ALOHA Model Definitions and assumptions X: frame transmission time (assume constant) S: throughput (average # of successful frame transmissions per X seconds) G: load (average # of transmission attempts per X sec.) P success : probability a frame transmission is successful S GPsuccess X Prior interval X frame transmission Any transmission that begins during vulnerable period leads to collision Success if no arrivals during 2X seconds

Abramson’s Assumption What is probability of no arrivals in vulnerable period? Abramson assumption: Effect of backoff algorithm is that frame arrivals are equally likely to occur at any time interval G is avg. # arrivals per X seconds Divide X into n intervals of duration X/n p probability of arrival in interval, then G n p since there are n intervals in X seconds Psuccess P[0 arrivals in 2X seconds] P[0 arrivals in 2n intervals] G 2n (1 - p) (1 ) e 2G as n n 2n

Throughput of ALOHA S GPsuccess Ge 2G S e-2 0.184 0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 Collisions are means for coordinating access Max throughput is max 1/2e (18.4%) Bimodal behavior: Small G, S G Large G, S 0 G Collisions can snowball and drop throughput to zero

Slotted ALOHA Time is slotted in X seconds slots Stations synchronized to frame times Stations transmit frames in first slot after frame arrival Backoff intervals in multiples of slots Backoff period kX (k 1)X Vulnerabl eperiod t0 X 2tprop B t t0 X 2tprop B Time-out Only frames that arrive during prior X seconds collide

Throughput of Slotted ALOHA S GPsuccess GP[no arrivals in X seconds] GP[no arrivals in n intervals] G n G (1 p ) G (1 ) Ge G n n 0.4 0.368 0.35 0.3 Ge-G 0.25 0.184 0.2 0.15 0.1 Ge-2G 0.05 G 8 4 2 1 0.5 0.25 0.125 0.0625 0.03125 0 0.01563 S Max throughput is max e (36.8%)

Application of Slotted Aloha cycle . . Reservation mini-slots X-second slot Reservation protocol allows a large number of stations with infrequent traffic to reserve slots to transmit their frames in future cycles Each cycle has mini-slots allocated for making reservations Stations use slotted Aloha during mini-slots to request slots

Carrier Sensing Multiple Access (CSMA) A station senses the channel before it starts transmission If busy, either wait or schedule backoff (different options) If idle, start transmission Vulnerable period is reduced to tprop (due to channel capture effect) When collisions occur they involve entire frame transmission times If tprop X (or if a 1), no gain compared to ALOHA or slotted ALOHA Station A begins transmission at t 0 Station A captures channel at t tprop A A

CSMA Options Transmitter behavior when busy channel is sensed 1-persistent CSMA (most greedy) Start transmission as soon as the channel becomes idle Low delay and low efficiency Non-persistent CSMA (least greedy) Wait a backoff period, then sense carrier again High delay and high efficiency p-persistent CSMA (adjustable greedy) Wait till channel becomes idle, transmit with prob. p; or wait one mini-slot time & re-sense with probability 1-p Delay and efficiency can be balanced Sensing

1-Persistent CSMA Throughput 0.6 0.53 0.5 0.4 a 0.01 0.45 0.3 0.2 0.16 a 1 64 32 16 8 4 2 1 0.5 0.25 0.13 0.06 0 0.03 a 0.1 0.1 0.02 S G Better than Aloha & slotted Aloha for small a Worse than Aloha for a 1

Non-Persistent CSMA Throughput a 0.01 0.81 0.9 0.8 0.7 0.6 0.51 0.5 0.4 a 0.1 0.3 0.2 0.14 0.1 a 1 64 32 16 8 4 2 1 0.5 0.25 0.13 0.06 0.03 0 0.02 S G Higher maximum throughput than 1-persistent for small a Worse than Aloha for a 1

CSMA with Collision Detection (CSMA/CD) Monitor for collisions & abort transmission Stations with frames to send, first do carrier sensing After beginning transmissions, stations continue listening to the medium to detect collisions If collisions detected, all stations involved stop transmission, reschedule random backoff times, and try again at scheduled times In CSMA, collisions result in wastage of X seconds spent transmitting an entire frame CSMA-CD reduces wastage to time to detect collision and abort transmission

CSMA/CD reaction time A begins to transmit at A t 0 B A B A detects collision at A t 2 tprop- B B begins to transmit at t tprop- ; B detects collision at t tprop It takes 2 tprop to find out if channel has been captured

CSMA-CD Model Assumptions Collisions can be detected and resolved in 2t prop Time slotted in 2t prop slots during contention periods Assume n busy stations, and each may transmit with probability p in each contention time slot Once the contention period is over (a station successfully occupies the channel), it takes X seconds for a frame to be transmitted It takes tprop before the next contention period starts. (a) Busy Contention Busy Idle Contention Busy Time

Contention Resolution How long does it take to resolve contention? Contention is resolved (“success’) if exactly 1 station transmits in a slot: Psuccess np(1 p) n 1 By taking derivative of Psuccess we find max occurs at p 1/n max success P 1 1 n 1 1 n 1 1 n (1 ) (1 ) n n n e On average, 1/Pmax e 2.718 time slots to resolve contention Average Contention Period 2t prop e seconds

CSMA/CD Throughput Busy Busy Contention Busy Contention Busy Time At maximum throughput, systems alternates between contention periods and frame transmission times max Contention X 1 1 X t prop 2et prop 1 2e 1 a 1 2e 1 Rd / L where: R bits/sec, L bits/frame, X L/R seconds/frame a tprop/X meters/sec. speed of light in medium d meters is diameter of system 2e 1 6.44

CSMA-CD Application: Ethernet First 1-persistent Carrier Sensing R 10 Mbps tprop 51.2 microseconds Ethernet LAN standard used CSMA-CD 512 bits 64 byte slot accommodates up to 2.5 km using 4 repeaters Uses Binary Exponential Backoff After nth collision, select backoff from {0, 1, , 2 k – 1}, where k min(n, 10)

Chapter 6 Medium Access Control Protocols and Local Area Networks Part II: Local Area Networks 6.6 LAN Protocols 6.7 Ethernet and IEEE 802.3 Token Ring and FDDI 802.11 Wireless LAN 6.11 LAN Bridges

Chapter 6 Medium Access Control Protocols and Local Area Networks Overview of LANs

What is a LAN? Local area means: Private ownership Short distance ( 1km) between computers low cost very high-speed, relatively error-free communication complex error control unnecessary Machines are constantly moved freedom from regulatory constraints of WANs Keeping track of location of computers a chore Simply give each machine a unique address Broadcast all messages to all machines in the LAN Needs a medium access control protocol

Typical LAN Structure Ethernet Processor RAM ROM RAM Transmission Medium Network Interface Card (NIC) Unique MAC “physical” address

Medium Access Control Sublayer 1. In IEEE 802, Data Link Layer divided into: Medium Access Control Sublayer 2. Coordinate access to medium Connectionless frame transfer service Machines identified by MAC/physical address Broadcast frames with MAC addresses Logical Link Control Sublayer Between Network layer & MAC sublayer

MAC Sub-layer OSI IEEE 802 Network layer LLC Network layer 802.2 Logical link control Data link layer 802.11 802.3 802.5 MAC CSMA-CD Token Ring Wireless LAN Physical layer Various physical layers Other LANs Physical layer

Logical Link Control Layer IEEE 802.2: LLC enhances service provided by MAC C A A Unreliable Datagram Service Reliable frame service C LLC LLC LLC MAC MAC MAC MAC MAC MAC PHY PHY PHY PHY PHY PHY

Logical Link Control Services Type 1: Unacknowledged connectionless service Unnumbered frame mode of HDLC Type 2: Reliable connection-oriented service Asynchronous balanced mode of HDLC Type 3: Acknowledged connectionless service Additional addressing A workstation has a single MAC physical address Can handle several logical connections, distinguished by their SAP (service access points)

LLC PDU Structure 1 1 byte 1 Source SAP Address Destination SAP Address 1 or 2 bytes Control Source SAP Address Destination SAP Address C/R I/G 1 Information 7 bits I/G Individual or group address C/R Command or response frame 1 7 bits Examples of SAP Addresses: 06 IP packet E0 Novell IPX FE OSI packet AA SubNetwork Access protocol (SNAP)

Encapsulation of MAC frames IP Packet LLC LLC PDU Header MAC Header IP Data FCS

Chapter 6 Medium Access Control Protocols and Local Area Networks Ethernet and IEEE 802.3

A bit of history 1970 ALOHAnet radio network deployed in Hawaiian islands 1973 Metcalf and Boggs invent Ethernet, random access in wired net 1979 DIX Ethernet II Standard 1985 IEEE 802.3 LAN Standard (10 Mbps) 1995 Fast Ethernet (100 Mbps) 1998 Gigabit Ethernet 2002 10 Gigabit Ethernet Ethernet is the dominant LAN standard today Metcalf’s Sketch

IEEE 802.3 MAC: Ethernet MAC Protocol: CSMA/CD Slot Time is the critical system parameter upper bound on time to detect collision upper bound on time to acquire channel upper bound on length of frame generated by collision quantum for retransmission scheduling max{round-trip propagation, MAC jam time} binary exponential backoff for retransmission n: 0 r 2k, where k min(n,10) Give up after 16 retransmissions

IEEE 802.3 Original Parameters Transmission Rate: 10 Mbps Min Frame: 512 bits 64 bytes Slot time: 512 bits/10 Mbps 51.2 sec 51.2 sec x 2x105 km/sec 10.24 km, 1 way 5.12 km round trip distance Max Length: 2500 meters using 4 repeaters Each x10 increase in bit rate, must be accompanied by x10 decrease in distance

IEEE 802.3 MAC Frame 802.3 MAC Frame 7 1 Preamble SD Synch Start frame 6 Destination address 6 Source address 2 Length Information Pad 4 FCS 64 - 1518 bytes Every frame transmission begins “from scratch” Preamble helps receivers synchronize their clocks to transmitter clock 7 bytes of 10101010 generate a square wave Start frame byte changes to 10101011 Receivers look for change in 10 pattern

IEEE 802.3 MAC Frame 802.3 MAC Frame 7 1 Preamble SD Synch 6 Destination address Start frame 0 Single address 1 Group address 0 Local address 1 Global address 6 Source address 2 Length Information Pad 4 FCS 64 - 1518 bytes Destination address single address group address broadcast 111.111 Addresses local or global Global addresses first 24 bits assigned to manufacturer; next 24 bits assigned by manufacturer Cisco 00-00-0C 3COM 02-60-8C

IEEE 802.3 MAC Frame 802.3 MAC Frame 7 1 Preamble SD Synch Start frame 6 Destination address 6 Source address 2 Length Information Pad 4 FCS 64 - 1518 bytes Length: # bytes in information field - Max frame 1518 bytes, excluding preamble & SD - Max information 1500 bytes: 05DC Pad: ensures minimum frame of 64 bytes FCS: CCITT-32 CRC, covers addresses, length, information, pad fields - NIC discards frames with improper lengths or failed CRC

IEEE 802.3 Physical Layer Table 6.2 IEEE 802.3 10 Mbps medium alternatives Medium Max. Segment Length Topology (a) 10base5 10base2 10baseT 10baseFX Thick coax Thin coax Twisted pair Optical fiber 500 m 200 m 100 m 2 km Bus Bus Star Point-topoint link transceivers Thick Coax: Stiff, hard to work with (b) Hubs & Switches! T connectors flaky

Ethernet Hubs & Switches Single collision domain (a) (b) High-Speed backplane or interconnection fabric Twisted Pair Cheap Easy to work with Reliable Star-topology CSMA-CD Twisted Pair Cheap Bridging increases scalability Separate collision domains Full duplex operation

Ethernet Scalability CSMA-CD 30 25 20 15 10 5 0.96 0.9 0.84 0.78 0.72 0.66 0.6 0.54 0.48 0.42 0.36 0.3 0.24 0.18 0.12 0.06 0 0 Avg. Transfer Delay a .2 a .01 a .1 Load CSMA-CD maximum throughput depends on normalized delaybandwidth product a tprop/X x10 increase in bit rate x10 decrease in X To keep a constant need to either: decrease tprop (distance) by x10; or increase frame length x10

Fast Ethernet Table 6.4 IEEE 802.3 100 Mbps Ethernet medium alternatives Medium Max. Segment Length Topology 100baseT4 100baseT 100baseFX Twisted pair category 3 UTP 4 pairs Twisted pair category 5 UTP 2 pairs Optical fiber multimode Two strands 100 m 100 m 2 km Star Star Star To preserve compatibility with 10 Mbps Ethernet: Same frame format, same interfaces, same protocols Hub topology only with twisted pair & fiber Bus topology & coaxial cable abandoned Category 3 twisted pair (ordinary telephone grade) requires 4 pairs Category 5 twisted pair requires 2 pairs (most popular) Most prevalent LAN today

Gigabit Ethernet Table 6.3 IEEE 802.3 1 Gbps Ethernet (GE) medium alternatives Medium Max. Segment Length Topology 1000baseSX 1000baseLX 1000baseCX 1000baseT Optical fiber multimode Two strands Optical fiber single mode Two strands Shielded copper cable Twisted pair category 5 UTP 550 m 5 km 25 m 100 m Star Star Star Star Slot time increased to 512 bytes Small frames need to be extended to 512 bytes (by padding) Frame bursting to allow stations to transmit burst of short frames Frame structure preserved but CSMA-CD essentially abandoned Extensive deployment in backbone of enterprise data networks and in server farms

10 Gigabit Ethernet Table 6.5 IEEE 802.3 10 Gbps Ethernet medium alternatives 10GbaseSR Two optical fibers Multimode at Medium 850 nm 10GBaseLR Two optical fibers Two optical fibers Single-mode at 1310 nm Single-mode at 1550 nm SONET compatibility 64B66B code 64B66B Max. Segment Length 300 m 10GbaseEW 10 km 40 km 10GbaseLX4 Two optical fibers multimode/single -mode with four wavelengths at 1310 nm band 8B10B code 300 m – 10 km Frame structure preserved LAN PHY for local network applications WAN PHY for wide area interconnection using SONET OC-192c Extensive deployment in metro networks anticipated

Typical Ethernet Deployment Server farm Server Switch/router Server Ethernet switch 100 Mbps links Hub 10 Mbps links Department A Server Server Gigabit Ethernet links Gigabit Ethernet links Ethernet switch 100 Mbps links Server Hub 10 Mbps links Department B Switch/router Ethernet switch 100 Mbps links Server Hub 10 Mbps links Department C

Chapter 6 Medium Access Control Protocols and Local Area Networks LAN Bridges

Hubs, Bridges & Routers Hub: Active central element in a star topology Twisted Pair: inexpensive, easy to install Simple repeater in Ethernet LANs “Intelligent hub”: fault isolation, net configuration, statistics User community grows, need to interconnect hubs ? Hub Two Twisted Pairs Two Twisted Pairs Station Hub Station Station Station Station Station

Hubs, Bridges & Routers Interconnecting Hubs Repeater: Signal regeneration All traffic appears in both LANs Bridge: MAC address filtering (layer 2) Local traffic stays in its own LAN Routers: Internet routing (layer 3) Based on IP addresses Higher Scalability ? Hub Hub Two Twisted Pairs Station Two Twisted Pairs Station Station Station Station Station

General Bridge Issues Network Network LLC LLC MAC 802.3 802.3 802.5 802.5 MAC PHY 802.3 802.3 802.5 802.5 PHY 802.3 CSMA/CD 802.5 Token Ring Operation at data link level implies capability to work with multiple network types However, must deal with Difference in MAC formats Difference in data rates, buffering, timers Difference in maximum frame lengths

Bridges of Same Type Network Network Bridge LLC LLC MAC MAC MAC MAC Physical Physical Physical Physical Common case involves LANs of same type Bridging is done at MAC level

Transparent Bridges Interconnection of LANs with complete transparency Use table lookup, and discard frame, if source & destination in same LAN forward frame, if source & destination in different LANs use flooding, if destination unknown Use backward learning to build table observe source address of arriving LANs handle topology changes by removing old entries S1 S2 S3 LAN1 Bridge LAN2 S4 S5 S6

S1 S2 S3 LAN1 LAN2 Port 1 B1 Port 2 Address Port S5 S4 LAN3 Port 1 B2 Port 2 Address Port

S1 S5 S1 S2 S3 S1 to S5 S1 to S5 S1 to S5 LAN1 S1 to S5 LAN2 Port 1 B1 Port 2 Address Port S1 1 S5 S4 LAN3 Port 1 B2 Port 2 Address Port S1 1

S3 S2 S1 S2 S3 S3 S2 S3 S2 S3 S2 S3 S2 S3 S2 LAN1 LAN2 Port 1 B1 Port 2 Address Port S1 S3 1 2 S5 S4 LAN3 Port 1 B2 Port 2 Address Port S1 S3 1 1

S4 S3 S1 S2 S3 S4 LAN1 S4 S3 Port 1 B1 Port 2 Address Port S1 S3 S4 1 2 2 LAN2 S5 S4 S4 S3 S4 S3 Port 1 LAN3 B2 Port 2 Address Port S1 S3 S4 S3 1 1 2

S2 S1 S1 S2 S3 S5 S4 S2 S1 LAN1 S2 S1 Port 1 LAN2 B1 Port 2 Address Port S1 S3 S4 S2 1 2 2 1 LAN3 Port 1 B2 Port 2 Address Port S1 S3 S4 1 1 2

Adaptive Learning In a static network, tables eventually store all addresses & learning stops In practice, stations are added & moved all the time Introduce timer (minutes) to age each entry & force it to be relearned periodically If frame arrives on port that differs from frame address & port in table, update immediately

Avoiding Loops LAN1 (1) (1) B1 B2 (2) B3 LAN2 B4 LAN3 B5 LAN4

Spanning Tree Algorithm 1. Select a root bridge among all the bridges 2. Determine the root port for each bridge except the root bridge 3. root port port with the least-cost path to the root bridge Select a designated bridge for each LAN 4. root bridge the lowest bridge ID designated bridge bridge has least-cost path from the LAN to the root bridge designated port connects the LAN and the designated bridge All root ports and all designated ports are placed into a “forwarding” state. These are the only ports that are allowed to forward frames. The other ports are placed into a “blocking” state

LAN1 (1) (1) B1 B2 (1) (2) (2) LAN2 B3 (2) (1) B4 (2) LAN3 (1) B5 (2) LAN4 (3)

LAN1 (1) (1) B1 B2 (1) (2) (2) LAN2 B3 (2) (1) B4 (2) LAN3 1. Bridge 1 selected as root bridge (1) B5 (2) LAN4 (3)

LAN1 (1) R (1) B1 B2 (2) (2) LAN2 R B3 R (1) (2) B4 (2) LAN3 (1) R (1) B5 (2) LAN4 (3) 2. Root port selected for every bridge except root port

LAN1 D (1) R (1) B1 B2 (2) D (2) LAN2 R D (2) B4 (2) LAN3 (1) B3 R (1) R (1) B5 (2) LAN4 3. Select designated bridge for each LAN (3) D

LAN1 D (1) R (1) B1 B2 (2) D (2) LAN2 R D (2) B4 (2) LAN3 (1) B3 R (1) R (1) B5 (2) LAN4 4. All root ports & designated ports put in forwarding state (3) D

VLAN Group of devices on one or more LANs that are configured so that they can communicate as if they were attached to the same wire, when in fact they are located on a number of different LAN segments Benefits of VLAN Increased performance Improved manageability Network tuning and simplification of software configurations Physical topology independence Increased security options

Virtual LAN VLAN 1 S3 VLAN 2 S6 VLAN 3 S9 Floor n 1 Physical S2 S5 S8 partition Floor n 1 2 3 4 5 6 Bridge or switch 7 S1 S4 S7 8 9 Floor n – 1 Logical partition

Per-Port VLANs VLAN 1 S3 VLAN 2 S6 VLAN 3 S9 Floor n 1 S2 S5 S8 Floor n 1 2 3 4 5 6 Bridge 7 or 8 switch 9 S1 S4 S7 Floor n – 1 Logical partition Bridge only forwards frames to outgoing ports associated with same VLAN

Tagged VLANs More flexible than Port-based VLANs Insert VLAN tag after source MAC address in each frame VLAN protocol ID tag VLAN-aware bridge forwards frames to outgoing ports according to VLAN ID VLAN ID can be associated with a port statically through configuration or dynamically through bridge learning IEEE 802.1q Visit http://en.wikipedia.org/wiki/VLAN for more details

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