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Data Link LayerData Link LayerLecture 22Lecture 22

Imran AhmedImran AhmedUniversity of Management & TechnologyUniversity of Management & Technology

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Agenda

• Introduction & services

• Error detection and correction

• Multiple access protocols

• LAN addresses and ARP

• Ethernet

• Hubs, bridges and switches

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Ethernet• IEEE divides the baseband category into five different

standards:-

– 10Base5

– 10Base2

– 10Base-T

– 1Base5

– 100Base-T

• IEEE defines only one specification for the broadband category:-

– 10Base36

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Ethernet• Access Method (CSMA/CD):

– When a station wants to transmit, it checks the cable. If the cable is busy, the station waits until it goes idle; otherwise it transmits immediately.

– If two or more stations simultaneously begin transmitting on an idle cable, they will collide.

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Ethernet• Addressing: Each station on an Ethernet

network has its own NIC. Which has a unique address of 48-bits.

• Signaling: The baseband system uses Manchester digital encoding and 10Base36 system uses Differential PSK.

• Data Rate: Ethernet LANs can support data rates between 1 to 100 Mbps.

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Ethernet• Frame Format: IEEE 802.3 specifies one

type of frame containing seven fields:-– Preamble– Start Frame Delimiter (SFD)– Destination Address (DA)– Source Address (SA)– 802.2 Frame– CRC– Length of PDU

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MAC Frame

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Ethernet• Preamble:

– It contains 7-bytes of alternating 0s and 1s that alert the receiving system to the coming frame and enable it to synchronize its input timing.

– HDLC combined the alert, timing and start synchronization into a single field: the flag.

– IEEE 802.3 divides these three functions between the preamble and the second field (SFD).

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Ethernet• Start Frame Delimiter (SFD): The second field

(1-byte) of the 802.3 frame signals the beginning of the frame. The SFD tells the receiver that everything that follows is data, starting with the addresses.

• Destination Address (DA): The DA field is allotted 6-bytes and contains the physical address of the packet’s next destination.

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Ethernet• Source Address (SA): The source address field

is allotted 6-bytes and contains the physical address of the last device to forward the packet.

• PDU: The next 2-bytes indicate the number of bytes in the coming PDU.

• 802.2 Frame: This field contains the entire data.

• CRC: The last field in the 802.3 frame contains the error detection mechanism.

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Ethernet (10Base5)

• 10base5 (thicknet) has a bus topology LAN that uses baseband signaling and has a maximum segment length of 500 meters.

• The physical connectors and cables utilized by 10base5 include coaxial cable, NIC, transceivers and attachment unit interface (AUI) cables– RG-8 Cable

– Each station is linked to its corresponding transceiver by an AUI, also called transceiver cable.

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10BASE5

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Ethernet Segments

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Ethernet (10Base2)

• 10Base2 (thinnet) provides an inexpensive alternative to 10base5 Ethernet with the same data rate.– The disadvantage is shorter range (185 meters).– It uses RG-58 coaxial cable and BNC-T type

connectors.

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10BASE2

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Ethernet (10Base-T)

• 10Base-T (twisted-pair), a star topology LAN using UTP cable instead of coaxial cable.

• It supports a data rate of 10 Mbps and has a maximum length (hub to station) of 100 meters.

• Instead of individual transceivers, 10base-T Ethernet places all of its networking operations in an intelligent hub with a port of each station.

• Stations are linked into the hub by four-pair RJ-45 cable.

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10BASET

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Ethernet (1Base5)

• 1Base5 (Star LAN) is an AT&T product used infrequently today because of its slow speed (only 1Mbps).

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Switched Ethernet• Switched Ethernet is an attempt to improve the

performance of 10Base-T Ethernet.• The 10Base-T Ethernet is a shared media network, which

means that the entire media is involved in each transmission.

• This is because of topology, though physically a star, is logically a bus.

• For example: Station A is sending a frame to station E. The frame is received by the hub and is sent to every station (broadcast). All of the cabling in the system is involved in this transmission.

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Switched Ethernet• However, if we replace the hub with a switch (a device

that can recognize the destination address and can route the frame to the port to which the destination station is connected), the rest of the media are not involved in the transmission process.

• This means that the switch can receive another frame from another station at the same time and can route this frame to its own final destination.

• For example: When station A is sending a frame to station E, station B can also send a frame to station D without any collision.

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Hubs &

Switches

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Gigabit Ethernet• Gigabit Ethernet operates at 1000 Mbps or 1 Gbps

approx.• In Gigabit Ethernet, the MAC layer and the access

method remain the same, but the collision domain is reduced.

• Gigabit is mainly designed to use optical fiber, although the protocol does not eliminate the use of twisted pair cables.

• Gigabit Ethernet usually serves as a backbone to connect Fast Ethernet networks.

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Gigabit Ethernet

Feature 1000Base-SX 1000Base-LX 1000base-CX

1000Base-T

Medium Optical Fiber Optical Fiber STP UTP

Signal Short-wavelaser

Long-waveLaser

Electrical Electrical

Max.Distance

550 m 550 m (multi-mode)5000 m(single-mode)

25 m 25 m