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LAN ProtocolsLAN Protocols

CSE 3213, Fall 2010Instructor: N. Vlajic

Required reading:Garcia 6.7, 6.8

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How to allow station to place groups of bits on the medium?Aloha, CSMA, Scheduling Schemes

How to transmit data (bits) over a medium?ASK, FSK, PSK, Line Coding

How to reliably transmit groups of bits(frames) over the medium?Forward Error Correction (e.g. Internet Checksum, CRC)Flow & Error Control (e.g. Go-Back-N, Selective Repeat)

2.12.2

0100101111

3What is LAN?

Local Area Network Local Area Network (LAN)(LAN)

– properties

• private ownershipprivate ownershipfreedom from regulatory constraints of WANs

• low costlow costsingle broadcast mediumrelatively small number of stations ⇒ complex andexpensive switching equipment NOT necessary

• high speedhigh speedshort distance ~ 1 km between computers ⇒ relativelyerror free (high-speed) communication possiblecomplex error control unnecessary

4What is LAN? (cont.)

• computers computers and network devicesnetwork devices (e.g. printers) connected to broadcast cabling systembroadcast cabling system through network interface card (NIC)network interface card (NIC)

• computers connected via a LAN to the Internet need all 5 layers ofthe Internet model

3 upper layers (network, transport, application) are common to all LANsphysical layer can be considerably different

• data link layer is divided into 2 sublayer:

medium access control (MAC) – coordinates access to shared medium;provides connectionless transfer of datagrams – several standards !!!

logical link control (LLC) – may be needed to provide extra flow and errorcontrol to upper layers (in reality, only IP exists, and IP does not need additional flow and error control) – single IEEE 802.2 standard!

Typical LANTypical LANStructureStructure

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EthernetEthernet(IEEE 802.3)(IEEE 802.3)

6MAC Protocols: Ethernet

EthernetEthernetHistoryHistory

– set of protocols at the physical and data link layer (MAC sublayer)

• developed by Robert Metcalfe, at Xerox, in 1970s

• promoted and used by Dec, IBM and Xerox in 1980s

• 10 Mbps Ethernet became an IEEE standard in 1985 – IEEE 802.3IEEE 802.3• high-speed versions:

100 Mbps - Fast Ethernet (1995)1000 Mbps - Gigabit Ethernet (1998)10 Gbps - 10 Gigabit Ethernet (2002)100 Gbps - 100 Gigabit Ethernet (2007 / 2010)

• currently used in about 80-90 % of all LANs

"He chose to base the name on the word 'ether' as a way of describing an essential feature of the system: the physical medium (i.e., a cable) carries bits to all stations, much the same way that the old ‘luminiferous ether‘ was once thought to propagate

electromagnetic waves through space. Thus, Ethernet was born”

7MAC Protocols: Ethernet (cont.)

IEEE 802.3 IEEE 802.3 (10 Mbps)(10 Mbps)MAC FeaturesMAC Features

• backoff: 1-persistent CSMA/CD CSMA/CD with truncated binary exponential backoff algorithm

if medium idle – transmit; if medium busy, wait until idlethen transmit with p=1;

in case of retransmission, re-transmission time is determined by selecting an integer in range: 0 < r < 2k, where k=min(n,10)give up after 16 retransmissions

• frame size: original IEEE 802.3 was designed to operateat 10 Mbps over max distance of 2500 [m] with 4 repeaters(additional delay!!!)

c≈2*108 [m/sec] ⇒ 2*tprop + delays on repeaters ≈ 51.2 μ ⇒512 [bits]

min frame size = 512 bits = 64 bytes = 46 + 18

max frame size = 1518 bytes = 1500 + 18 (prevents one stationfrom monopolizing the medium)

• for the given min frame size, each 10x increase in bit-rate is accompanied with 10x decrease in max distance

min frame time = 2*max segment / c = min frame size / data rate

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9MAC Protocols: Ethernet (cont.)

IEEE 802.3 IEEE 802.3 (10 Mbps)(10 Mbps)MAC FrameMAC Frame

• Preamble – 7 bytes / 56 bits of alternating 0s and 1salerts receiving stations of the coming frame and enables them tosynchronize – 56 bits long , to allow stations to synchronize evenif they miss some bits at the beginningadded at the physical layer, not (formally) part of the frame

• Start-Frame Delimiter – 1 byte (10101011)signals the beginning of a frame; last chance for synchronizationtwo consecutive 1-bits indicate that the next bit is the first bit ofthe destination address

• Destination Address – 6 bytescontains the physical address of the station to receive the frame

• Source Address – 6 bytescontains the physical address of the sending station

64 to 1518 bytes

10MAC Protocols: Ethernet (cont.)

IEEE 802.3 IEEE 802.3 MAC FrameMAC Frame(cont.)(cont.)

• Length – 2 bytes

indicates the number of bytes in ‘data’ (information) field

min allowable frame size 64 bytes, with 18 bytes of overhead⇒ min data length = 46 bytes

max allowable frame size 1518 bytes, with 18 bytes of overhead⇒ max data length = 1500 bytes

• Data – 46 to 1500 bytesdata from upper-layer protocols

• Padding –ensures that the frame size is always at least 64 bytes

• CRC – 4 bytesCCITT 32-bit CRC check that covers addresses, length and data

64 to 1518 bytes

11MAC Protocols: Ethernet (cont.)

IEEE 802.3IEEE 802.310 Mbps10 MbpsPhysical LayerPhysical Layer

• 10 Mbps Mthernet uses Manchester signaling – additional bandwidthto achieve better synchronization, not a big issue

• thick (10 mm) coaxial cable Ethernet – awkward to handle and install

• thin (5 mm) coaxial cable Ethernet – cheaper and easier to handle,but the length of each segment cannot exceed 200 m, due to highlevel of attenuation in thin coaxial cable

• unshielded twisted pair Ethernet – low-cost and prevalent in offices,but due to poor transmission qualities of twisted pair the length ofindividual links is limited to 100 m

hub – multiport repeater

baseband

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Ethernet vs. Telephone Cable

MAC Protocols: Ethernet (cont.)

Ethernet Coaxial CableEthernet Coaxial Cable

Ethernet Twisted PairEthernet Twisted Pair

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IEEE 802.3IEEE 802.3100 Mbps100 MbpsPhysical LayerPhysical Layer

MAC Protocols: Ethernet (cont.)

• 100 Mbps Ethernet uses a combination of 4B/5B block codingMLT-3 linear coding (instead of Manchester coding) to minimize the demand on bandwidth

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Token RingToken Ring(IEEE 802.5)(IEEE 802.5)

15MAC Protocols: Token Ring

HistoryHistory – set of protocols at the physical and data link layer (MAC sublayer)

• developed by IBM in 1980s

• IEEE 802.5IEEE 802.5 standard modeled after IBM Token Ring in 1990s

• IBM and IEEE specifications differ in minor ways:IBM’s Token Ring specifies a star; IEEE 802.5 does not specify atopology, but most IEEE 802.5 implementations are based on a star

IBM’s Token Ring uses twisted-pair wire; IEEE 802.5 does notspecify a media type

• speed: 4 Mbps and 16 Mbps

• signalling: Differential Manchester

• size: max 250 stations

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Wiring Center

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B

C D

E

MAC Protocols: Token Ring (cont.)

Token Ring Token Ring –– AdvantagesAdvantages • fairness and stability

Token Ring Token Ring –– DisadvantagesDisadvantages • entire network fails if any link, station ormechanism for token passing fails

Token Ring withToken Ring withStar TopologyStar Topology

– stations connected to a hub (Multistation Access Unit) in‘star’ fashion

• physical star, logical ring

• hub performs token passing management and network diagnostics

• advantage: failed wires or stations can be easily bypassed

• disadvantage: hub – single point of failure

17MAC Protocols: Token Ring (cont.)

IEEE 802.5 IEEE 802.5 MAC FrameMAC Frame

– two basic frame formats supported:• data / control frame format: 21-byte overhead + data• token frame format: 3-byte

• Start Delimiter – alerts arrival of a token / data / control frame

• Access Control – determines the frame type – token or data/control

• Frame Control – indicates whether frame contains data or control info

• Frame Check Sequence – CCITT-32 CRC checksum

• End Delimiter – signals the end of token / data frame

• Frame Status – tells the sending device whether the destination device is on thering and, if it is, whether it copied the frame – primitive ACK

Max frame size = 4500 bytes.

Token frame format

SD FCAC Destinationaddress

Source address

Information FCS1 4

ED6 61 11

FS1

Data frame format

SD AC ED

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(1) CostCost: Ethernet is generally less expensive and easier to install than TokenRing.

(2) StabilityStability: Token Ring is generally more secure and more stable than Ethernet.

(3) ScalabilityScalability: It is usually more difficult to add more computers on a Token RingLAN than it is to an Ethernet LAN. However, as additional computersare added, performance degradation will be less pronounced on theToken Ring LAN than it will be on the Ethernet LAN.

(4) QoSQoS: Ethernet uses CSMA/CD media access control and Token Ring usestoken passing. This makes Ethernet better suited in a situation wherethere are a large number of computers sending fewer, larger dataframes. Token Ring is better suited for small to medium size LANs sending many, smaller data frames.

Performance Comparison

Ethernet vs. Token RingEthernet vs. Token Ring

Wiring Center

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B

C D

E