Topic 7 Local Area Networks (LAN) Overview of LANs LAN Bridges LAN Standards Reference A. Leon-Garcia and I. Widjaja, Communication Networks, pp. 421-479. (Reserved in the DC library. Call No. TK5105. L46 2004.)

What is a LAN? LAN Topology LAN Protocol Architecture 7.1 Overview of LANs What is a LAN? LAN Topology LAN Protocol Architecture

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

LAN Topology Bus Ring Star A number of nodes (computers and network devices) are interconnected by a shared transmission medium Nodes are connected to the cabling system through a network interface card (NIC) or LAN adapter card Nodes can be arranged in bus, ring, or star topology Bus Ring Star

LAN Protocol Architecture Data link layer 802.3 CSMA-CD 802.5 Token Ring 802.2 Logical link control Physical MAC LLC 802.11 Wireless LAN Network layer OSI IEEE 802 Fiber, twisted pairs, coax, wireless Other LANs

Medium Access Control Coordinate access to medium Connectionless frame transfer service Machines identified by MAC/physical address Broadcast frames with MAC addresses

7.2 LAN Bridges

Interconnecting Networks Types of devices Repeater (physical layer): Signal regeneration - extend the range Bridge (data link layer): MAC address filtering - reduce LAN saturation Routers (network layer): Internet routing Gateway (higher layers): Protocol conversion and security ? Hub Station Two Twisted Pairs Hub Station Two Twisted Pairs

Bridges Operation at data link level However, need deal with Network Physical LLC MAC Operation at data link level Implies capability to work with multiple network layers However, need deal with Difference in MAC formats, data rates, maximum frame length Security Broadcast storm Types: transparent, source routing

Transparent Bridges Prevents loops in the topology Bridge S1 S2 S4 S3 Interconnection of IEEE LANs with complete transparency Use table lookup, and discard frame, if source & destination in same LAN forward frame, if source & destination in different LAN use flooding, if destination unknown Use backward learning to build table observe source address of arriving LANs handle topology changes by removing old entries Prevents loops in the topology Bridge S1 S2 S4 S3 S5 S6 LAN1 LAN2 10

Address Port Address Port S1 S2 S3 S4 S5 LAN1 LAN2 LAN3 B1 B2 Port 1 11

S1→S5 S1 to S5 S1 to S5 S1 to S5 S1 to S5 Address Port Address Port S1 LAN1 LAN2 LAN3 B1 B2 Port 1 Port 2 Port 1 Port 2 Address Port Address Port S1 1 S1 1 12

S3→S2 S3S2 S3S2 S3S2 S3S2 S3S2 Address Port Address Port S1 1 S1 LAN1 LAN2 LAN3 B1 B2 Port 1 Port 2 Port 1 Port 2 Address Port Address Port S1 1 S1 1 S3 2 S3 1 13

S4S3 S4S3 S4S3 S4S3 S1 S2 S3 S4 S5 S4 S3 LAN1 LAN2 LAN3 B1 B2 Port 1 Port 2 Port 1 Port 2 Address Port Address Port S1 1 S1 1 S3 2 S3 1 S4 2 S4 2 14

S2S1 S2S1 S2S1 S1 S2 S3 S4 S5 LAN1 LAN2 LAN3 B1 B2 Port 1 Port 2 Address Port S1 1 S3 S4 2 Address Port S1 1 S3 2 S4 2 S2 1 15

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

Spanning Tree Algorithm Select a root bridge among all the bridges. root bridge = the lowest bridge ID. Determine the root port for each bridge except the root bridge root port = port with the least-cost path to the root bridge Select a designated bridge for each LAN 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) (3) B3 LAN2 (1) (2) B4 (2) LAN3 (1) B5 19

Bridge 1 selected as root bridge LAN1 (1) (1) Bridge 1 selected as root bridge B1 B2 (1) (2) (2) (3) B3 LAN2 (1) (2) B4 (2) LAN3 (1) B5 (2) LAN4 20

Root port selected for every bridge except root port LAN1 (1) R (1) Root port selected for every bridge except root port B1 B2 R (1) (2) (2) (3) B3 LAN2 R (1) (2) B4 (2) LAN3 R (1) B5 (2) LAN4 21

Select designated bridge for each LAN (1) R (1) Select designated bridge for each LAN B1 B2 R (1) (2) (2) D (3) B3 LAN2 R (1) (2) D D B4 (2) LAN3 R (1) B5 (2) LAN4 22

All root ports & designated ports put in forwarding state LAN1 D (1) R (1) All root ports & designated ports put in forwarding state B1 B2 R (1) (2) (2) D (3) B3 LAN2 R (1) (2) D D B4 (2) LAN3 R (1) B5 (2) LAN4 23

Ethernet Token Ring FDDI 802.11 Wireless LANs 7.3 LAN Standards Ethernet Token Ring FDDI 802.11 Wireless LANs

Ethernet: 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 Metcalf’s Sketch

IEEE 802.3 MAC Protocol CSMA/CD Minislot 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 segment generated by collision quantum for retransmission scheduling max{round-trip propagation, MAC jam time} Truncated binary exponential backoff algorthm For retransmission n, select an integer r equally likely btw 0 and 2k -1 (0 <= r <= 2k -1), where k=min(n,10) Retransmission time = (minislots time)x(r) Give up after 16 retransmissions

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

Ethernet Repeaters & Bridges       Single collision domain (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

Optical fiber multimode Fast Ethernet IEEE 802.3 100 Mbps Ethernet medium alternatives 100baseT4 100baseTX 100baseFX Medium Twisted pair category 3 UTP 4 pairs Twisted pair category 5 UTP two pairs Optical fiber multimode Two strands Max. Segment Length 100 m 2 km Topology 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 IEEE 802.3 1 Gbps Fast Ethernet medium alternatives 1000baseSX 1000baseLX 1000baseCX 1000baseT Medium Optical fiber multimode Two strands single mode Shielded copper cable Twisted pair category 5 UTP Max. Segment Length 550 m 5 km 25 m 100 m Topology Star Slot time increased to 512 bytes Small frames need to be extended to 512 B 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 IEEE 802.3 10 Gbps Ethernet medium alternatives 10GbaseSR 10GBaseLR 10GbaseEW 10GbaseLX4 Medium Two optical fibers Multimode at 850 nm 64B66B code Single-mode at 1310 nm 64B66B Single-mode at 1550 nm SONET compatibility Two optical fibers multimode/single-mode with four wavelengths at 1310 nm band 8B10B code Max. Segment Length 300 m 10 km 40 km 300 m – 10 km Frame structure preserved CSMA-CD protocol officially abandoned LAN PHY for local network applications WAN PHY for wide area interconnection using SONET OC-192c Extensive deployment in metro networks anticipated

Token Ring LAN: IEEE 802.5 Unidirectional ring network 4 Mbps and 16 Mbps on twisted pair Token passing protocol provides access Fairness Access priorities Breaks in ring bring entire network down Reliability can be improved by using star topology Relatively low speed

Star Topology Ring LAN Stations connected in star fashion to wiring closet Use existing telephone wiring Ring implemented inside equipment box Relays can bypass failed links or stations Wiring Center A B C D E

Fiber Distributed Data Interface (FDDI) Token ring protocol for LAN/MAN 100 Mbps on optical fiber Up to 200 km diameter, up to 500 stations FDDI has option to operate in multitoken mode Counter-rotating dual ring topology A E D C B X Dual ring becomes a single ring

Wireless LAN (+) Easy, low-cost deployment Mobility & roaming: Access information anywhere Supports personal devices PDAs, laptops, data-cell-phones Supports communicating devices Cameras, location devices, wireless identification (-) Signal strength varies in space & time Signal can be captured by snoopers Spectrum is limited & usually regulated

Wireless LAN Communications Ad hoc mode B D C A

Basic Service Set (BSS) A Infrastructure mode A2 B2 B1 A1 AP1 AP2 Distribution System Server Gateway to the Internet Portal Basic Service Set (BSS) A BSS B

IEEE 802.11 Wireless LAN Stimulated by availability of unlicensed spectrum U.S. Industrial, Scientific, Medical (ISM) bands 902-928 MHz, 2.400-2.4835 GHz, 5.725-5.850 GHz Ad Hoc & Infrastructure networks Based on CSMA with Collision Avoidance (CA) Why not CSMA/CD? Cost: requires a full duplex radio Wireless media: not all stations hear each other

IEEE 802.11 MAC MAC sublayer responsibilities Channel access PDU addressing, formatting, error checking Fragmentation & reassembly of MAC SDUs MAC security service options Authentication & privacy MAC management services Power management

MAC Services Contention Service: Best effort Contention-Free Service: time-bounded transfer Physical Distribution coordination function (CSMA-CA) Point coordination function Contention-free service Contention service MAC MSDUs

IEEE 802.11 Physical Layer Options Frequency Band Bit Rate Modulation Scheme 802.11 2.4 GHz 1-2 Mbps Frequency-Hopping Spread Spectrum, Direct Sequence Spread Spectrum 802.11b 11 Mbps Complementary Code Keying & QPSK 802.11g 54 Mbps Orthogonal Frequency Division Multiplexing (OFDM) & CCK for backward compatibility with 802.11b 802.11a 5-6 GHz Orthogonal Frequency Division Multiplexing 802.11n 2.4GHz 5-6GHz 600+Mbps OFDM, Multiple input mulitple output (MIMO)