Layer 2 LAN Technologies & Media Access Methods

(II)

Minimum Ethernet Frame Size

•To ensure that no node may completely receive a frame before the transmitting node has finished sending it, Ethernet defines a minimum frame size (i.e. no frame may have less than 46 bytes of payload).

• The minimum frame size is related to the distance which the network spans, the type of media being used and the number of repeaters which the signal may have to pass through to reach the furthest part of the LAN.

• Together these define a value known as the Ethernet Slot Time, corresponding to 512 bit times at 10 Mbps.

Minimum Ethernet Frame Size•The longest time between starting to transmit a frame and receiving the first bit of a jam sequence is twice the propagation delay from one end of the cable to the other.

•This means that a frame must have enough bits to last twice the propagation delay.•The 802.3 CSMA/CD Bus LAN transmits data at the standard rate of r = 10Mbps.•The speed of signal propagation is about v = 2108m/s.

A BPacket starts attime 0

A BPacket at time tp-

A BCollision occurs

at time tp

Jam sequence

A BJam sequence getsback to A at 2tp

Jam sequence

(a)

(c)

(b)

(d)

IEEE 802.3: Minimum Frame Length

In order to calculate the minimum frame length, we must first work out the propagation delay from one end of the cable to the other.

Propagation delay time:

sec2102102

400 68

V

dtprop

The round-trip propagation delay is, of course, twice this. Thus the round trip delay is sec42 propt

With a data rate of MbpsR 10 each bit has

durationsec1.0000,000,10

11 R

tb

Example #1: Cable = 400m, transm. speed = 10 Mbit/sec, propagation speed = 2*108 m/sec

IEEE 802.3: Minimum Frame Length

Example #1 – cont.

The number of bits we can fit into a round-trip propagation delay is

bitst

tn

b

pb 40

1.0

42

The minimum frame length is thus 40 bits (5 bytes).

A margin of error is usually added to this (often to make it a power of 2) so we might use 64 bits (8 bytes).

IEEE 802.3: Minimum Frame Length

Two nodes are communicating using CSMA/CD protocol.Speed transmission is 100 Mbits/sec and frame size is 1500 bytes. The propagation speed is 3*10**8 m/sec.Calculate the distance between the nodes such that the time to transmit the frame = time to recognize that the collision have occurred.

46

102.110100

81500

R

Lt frame

propframetripround ttT 2_

54

1062

102.1

2

frameprop

tt

V

dtprop

kmVtd prop 181018103106 385

Example # 2IEEE 802.3: Minimum Frame Length

• The standard frame length is at least 512 bits (64 bytes) long, which is much longer than our minimum requirement of 64 bits (8 bytes).– We only have to start worrying when the LAN reaches lengths

of more than 2.5km.

• 802.3 CSMA/CD bus LANs longer than 500m are usually composed of multiple segments joined by in-line passive repeaters, which output on one cable the signals received on another cable.– When we work out the minimum frame length for these longer

LANs, we also have to take the delays caused by the passive repeaters (about 2.5sec each) into account as well.

IEEE 802.3: Minimum Frame Length

Shortest Ethernet Frame

64 bytes sent at 10Mbps 51.2sec 500m/segment, 4 repeaters between nodes 2500m 25 sec

propagation delayThe frame should be longer enough for sender to detect the

collision(2x25 or about 50 sec )

Why specify a shortest frame of 64byte?

Node A Node BR1 R2 R3 R4

500m 25 sec propagation delay

IEEE 802.3: Non-Deterministic

• The 802.3 CSMA/CD bus LAN is said to be a non-deterministic network. This means that no host is guaranteed to be able to send its frame within a reasonable time (just a good probability of doing so).– When the network is busy, the number of collisions rises

dramatically and it may become very difficult for any hosts to transmit their frames.

• A real-time computing application (such as an assembly line) will demand that data is transmitted within a specified time period.– Since the 802.3 bus LAN cannot guarantee this, its use for

real-time applications may not only be undesirable but potentially dangerous in some situations.

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Ethernet evolution through four generations

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100 Mbps IEEE Standards

• The most widely accepted Ethernet standard today is 100BaseT, which is also called fast Ethernet– The current IEEE standard for 100BaseT is 802.3u

• Subcategories:– 100BaseTX: Two-pair Category 5 or higher UTP– 100BaseT4: Four-pair Category 3 or higher UTP– 100BaseFX: Two-strand fiber-optic cable

– Because of its widespread use, the cable and equipment in fast Ethernet are inexpensive

– Architecture of choice for all but heavily used servers and multimedia applications

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100BaseTX

• 100BaseTX is the standard that’s usually in mind when discussing 100 Mbps Ethernet

• Requires two of the four pairs bundled in a Category 5 twisted-pair cable

• Although three cable types are available for 100BaseT, 100BaseTX is the most widely accepted– Generally called fast Ethernet

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100BaseT4

• 100BaseT4 Ethernet uses all four pairs of wires bundled in a UTP cable

• Advantage: capability to run over Category 3 cable– One of the biggest expenses of building a network is

cable installation, so many organizations with Category 3 cabling chose to get the higher speed with the existing cable plant by using 100BaseT4 instead of 100BaseTX

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100BaseFX

• 100BaseFX uses two strands of fiber-optic cable – Advantages:

• Impervious to electrical noise and electronic eavesdropping

• Can span much greater distances between devices– Disadvantage: far more expensive than twisted-pair– Rarely used as a complete 100BaseTX replacement

• Used as backbone cabling between hubs or switches and to connect wiring closets between floors or buildings

• Connect client or server computers to the network when immunity to noise and eavesdropping is required

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100BaseT Design Considerations

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100BaseT Design Considerations

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10 Mbps IEEE Standards

• Four major implementations of 10 Mbps Ethernet– 10Base5: Ethernet using thicknet coaxial cable– 10Base2: Ethernet using thinnet coaxial cable– 10BaseT: Ethernet over UTP cable– 10BaseF: Ethernet over fiber-optic cable

• Of these 10 Mbps standards, only 10BaseT and 10BaseF are seen today

• 10Base2 and 10Base5 are essentially obsolete

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10BaseT

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10BaseF

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Gigabit Ethernet: IEEE 802.3ab and 802.3z Standards

• Gigabit Ethernet implementations– 802.3z-1998 covers 1000BaseX specifications,

including the L (long wavelength laser/fiber-optic), S (short wavelength laser/fiber-optic), and C (copper jumper cables)

– 802.3ab-1999 covers 1000BaseT specifications, which require four pairs of 100 ohm Category 5 or higher cable

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1000BaseT

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1000BaseLX

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1000BaseSX

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1000BaseCX

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10 Gigabit Ethernet: 10 Gbps IEEE 802.3ae Standard

• Defined to run only on fiber-optic cabling, both SMF and MMF, on a maximum distance of 40 km– Provides bandwidth that can transform how WAN

speeds are thought of

• Runs in full-duplex mode only– CSMA/CD is not necessary

• Primary use: as network backbone– It also has its place in storage area networks (SANs) – Will be the interface for enterprise-level servers

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10 Gigabit Ethernet: 10 Gbps IEEE 802.3ae Standard

• Standards– 10GBASE-SR: Runs over short lengths (between 26

and 82 meters) over MMF• For high-speed servers, SANs, etc.

– 10GBASE-LR: Runs up to 10 km on SMF• For campus backbones and MANs

– 10GBASE-ER: Runs up to 40 km over SMF• Primary applications are for MANs

– 10GBASE-SW: Uses MMF for distances up to 300 m– 10GBASE-LW: Uses SMF for distances up to 10 km– 10GBASE-EW: Uses SMF for distances up to 40 km

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Wireless Ethernet: IEEE 802.11b, a, and g

• AP serves as the center of a star topology network

• Stations can’t send and receive at the same time– CSMA/CA is used instead of CSMA/CD

• 802.11b/a/g use handshaking before transmission– Station sends AP an RTS and it responds with CTS

• Standards define a maximum transmission rate, but speeds might be dropped to increase reliability

• No fixed segment length– Maximum of 300 feet without obstructions

• Can be extended with large, high-quality antennas

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Broadband Technologies

• Baseband systems use a digital encoding scheme at a single fixed frequency

• Broadband systems use analog techniques to encode information across a continuous range of values– Signals move across the medium in the form of

continuous electromagnetic or optical waves– Data flows one way only, so two channels are

necessary for computers to send and receive data– E.g., cable TV

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Cable Modem Technology

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Digital Subscriber Line (DSL)

• Competes with cable modem for Internet access– Broadband technology that uses existing phone lines

to carry voice and data simultaneously– Most prominent variation for home Internet access is

Asymmetric DSL (ADSL)• Splits phone line in two ranges: Frequencies below 4

KHz are used for voice transmission, and frequencies above 4 KHz are used to transmit data

• Typical connection speeds for downloading data range from 256 Kbps to 8 Mbps; upload speeds are in the range of 16 Kbps to 640 Kbps