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More on LANs Module C Copyright 2001 Prentice Hall.

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Presentation on theme: "More on LANs Module C Copyright 2001 Prentice Hall."— Presentation transcript:

1 More on LANs Module C Copyright 2001 Prentice Hall

2 More on LANs More on Ethernet – Base5 and 10Base2 –100Base-TX signaling –Ethernet II versus –802.2 Logical Link Control Layer Frame More on Token-Ring Networks Wireless LANs Bridges versus Switches

3 Ethernet Recap Created by Digital Equipment Corporation, Intel, and Xerox Taken over by the Working Group of the IEEE 802 LAN MAN Standards Committee has since produced many standards –Especially at the physical layer: 10Base5, 10Base2, 10Base-T, 100Base-TX, 1000Base-X –Only one MAC layer standard--the MAC layer standard for framing and CSMA/CD

4 Original Ethernet Versus Base5 Created by Digital, Intel, and Xerox –Popular version was Ethernet II Working Group took over –Created the first Ethernet standard, –Slightly changed the physical layer; became 10Base-5 –Changed the data link layer (later) 10Base5 –10 Mbps, baseband, 500 meters/segment

5 Ethernet 10Base5 Multidrop Layout –Main trunk cable: thick coaxial cable Up to 500 meters long –Drop cable: 15-wire cable Up to 50 meters Also called the attachment unit interface (AUI) Trunk Cable Drop Cable (AUI) Up to 500 meters

6 Ethernet 10Base5 NIC 15-pin AUI Connector Drop Cable (Attachment Unit Interface or AUI) 15 wires 50 m maximum Trunk Cable Coaxial Cable 500 m maximum N-Connector Transceiver (Medium Attachment Unit)

7 Ethernet 10Base5 1. Sending NIC Transmits a bit 2. Transceiver Broadcasts the bit 3. Each Transceiver Sends the bit to Its NIC

8 Ethernet 10Base5 Broadcast (Bus) Topology –Station B transmits –Signal travels down trunk to all other stations ABC

9 Ethernet 10Base-5 Up to 5 segments connected by repeaters –2,500 meters maximum distance between farthest stations –Four repeaters maximum –No loops Repeater Drop cable Segment of Trunk Cable AUI Port

10 Ethernet 10Base2 (802.3a) Cheaper Physical Layer Standard –NICs have BNC connector –T-connector attaches to it –T-connector has BNC connectors for cable runs attaching it to adjacent stations NIC BNC T-connector To next NIC To next NIC

11 Ethernet 10Base2 (802.3a) Segments are thin coaxial cable –Run only between NICs –Daisy chain of NICs is a segment –Terminator at end of each segment –Up to 30 stations per segment –5 segments (4 repeaters) maximum –10Base2: 185 meters/segment NIC Terminator

12 Repeaters can Mix PHY Standards Repeater NIC BNC AUI Connectors RJ-45 UTPDrop Cables 10Base5 Trunk Cable 10Base2 10Base-T Repeaters must have the correct ports

13 Ethernet II vs Frames Ethernet II Frame –Ethertype field (2 bytes) –Tells protocol of message in data field (e.g., IP) –No length field –No LLC layer; Full data link layer protocol PreambleSFDDASAEthertype PreambleSFDDASALengthLLC frameFCS DataFCS Ethernet II Frame Frame

14 Ethernet II vs Frames Why the Differences –Lack of a length field could cause problems –802 would create multiple standards; Giving them all the same connection to the next higher layer (802.2 seemed wise) PreambleSFDDASAEthertype PreambleSFDDASALengthLLC FrameFCS DataFCS Ethernet II Frame Frame

15 Ethernet II vs Frames Telling them Apart –NIC must recognize incoming frames type –If 2 bytes after SA <= 1500, must be because of maximum length limitation in –If 2 bytes after SA >1500, its an Ethertype PreambleSFDDASAEthertype PreambleSFDDASALengthLLC FrameFCS DataFCS Ethernet II Frame Frame

16 802.2 LLC Layer Functions Logical Link Control Layer (LLC) –Links subnet to next-higher-layer (internet) –One LLC standard (802.2) for all 802 LANs –Makes interface to internet layer simple Internet Layer LLC MAC Base-T Internet Layer LLC MAC Base-T Internet Layer LLC MAC Mbps

17 802.2 LLC Layer Functions Logical Link Control –At the MAC layer, the destination NIC discards incorrect MAC layer frames –802.2 standard provides optional error correction to retransmit lost or discarded LLC frames

18 802.2 LLC Frame Fields –Destination Service Access Point (DSAP) designates the next-higher layer protocol on the receiving device (IP, IPX, etc.) –There can be multiple internet layer programs on the destination machine; DSAP designates the specific one to get the frames data field –SSAP: Source Service Access Point DSAPSSAPControlData Field

19 802.2 LLC Frame Fields –Control field contains instructions to the receivers LLC process on the NIC –Data field contains message of the next higher layer (usually internet) DSAPSSAPControlData Field

20 802.2 LLC Frame Subnet Access Protocol (SNAP) Alternative for IP and other TCP/IP internet layer standards –DSAP = AA hex –SSAP = AA hex –Control = 03 hex DSAP =AA SSAP =AA Org Code Data Field Ether- type SNAP message Control =03

21 802.2 LLC Frame Subnet Access Protocol (SNAP) Alternative for IP and other TCP/IP internet layer standards –Org code lists the organization maintaining a list of protocol codes for next higher layer –Ethertype (Ethernet type) code names a specific protocol for the data field DSAP =AA SSAP =AA Org Code Data Field Ether- type SNAP message Control =03

22 802.2 LLC Frame Subnet Access Protocol (SNAP) Alternative for IP and other TCP/IP internet layer standards –Data field has message of that protocol DSAP =AA SSAP =AA Org Code Data Field Ether- type SNAP message Control =03


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