CIT 384: Network Administration Slide #1

CIT 384: Network Administration

LANs and WANs

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Topics

1. Ethernet

2. Hubs

3. UTP Cabling

4. Switches

5. Ethernet Addresses

6. Ethernet Frames

7. WAN Protocols

8. Frame Relay

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Ethernet

Most LANs use Ethernet.– Historically token ring, ATM, FDDI, etc.– Consists of a broad range of protocols.

Cheap and ubiquitous– Most motherboards have gigabit Ethernet now.– Ethernet cabling is cheaply available.

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Ethernet MediaCommon Name Speed Name IEEE

StandardCable Type

Ethernet 10Mbps 10BASE-T 802.3i Copper

10Mbps 10BASE-F 802.3j Fiber

Fast Ethernet 100Mbps 100BASE-TX 802.3u Copper

100Mbps 100BASE-FX 802.3u Fiber

Gigabit 1000Mbps 1000BASE-T 802.3ab Copper

1000Mbps 1000BASE-LX,

1000BASE-SX

802.3z Fiber

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History of Ethernet1973: Invented by Robert Metcalfe at Xerox. (2.94 Mbps)

1980: DEC-Intel-Xerox publish 10Mbps Ethernet standard.1985: IEEE published 802.3 standard (thicknet: 10BASE5)1985: Thinnet 10BASE2 standard published as 802.3a.1990: 10BASE-T twisted pair std published.1995: 100BASE-T fast Ethernet.1998: 1000BASE-X gigabit over fiber.1999: 1000BASE-T gigabit over twisted pair.2005: 10 gigabit Ethernet over fiber and Infiniband.2007: Working group for 100 gigabit project auth request.

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How 10BASE2 Ethernet Works

• Single electrical circuit (bus) shared by all computers on LAN.

• Transmitted signal received by all computers on the bus.

• If two transmit at once, a collision results.

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CSMA/CD

Carrier sense multiple access/Collision detection1. Device listens until Ethernet LAN is quiet.

2. When quiet, device begins sending frame.

3. Device listens for collisions while sending.

4. If collision occurs, the sending devices each send a jamming signal to ensure all devices recognize collision.

5. After jamming, each sending device waits a random time then tries again.

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Hubs

Advantages of 10BASE-T– In 10BASE2, a single cable failure takes down

the entire LAN.– Twisted-pair cabling instead of coax 10BASE2.

Hubs– Repeat transmitted signal on each port.– Increases max distance.– Star topology.

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UTP Cabling

Unshielded Twisted Pair– 2 or 4 twisted pairs– RJ-45 connectors– Cat 3: 10 Mbps– Cat 5: 100Mbps– Cat 5e: 100+1000

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UTP Cabling Pinouts

Colors• green• orange• blue• brown

Stripes• green/white• orange/white• blue/white• brown/white

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Straight-through Cable

• PC transmits on 1,2

• Hub receives on 1,2

• Hub transmits on 3,6

• PC receives on 3,6

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Crossover Cable• Swaps transmit/receive wire pairs.• Used for two PC network without a hub.• Used for switch/switch communications.• Some switches can work with either cable.

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Hubs

1. NIC sends a frame.

2. Hub receives signal on one port.

3. Hub interprets signal as bits.

4. Hub sends bits out all other ports.

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Switches

1. NIC sends a frame.2. Switch receives

frame on one port.3. Switch looks up

destination MAC address.

4. Switch forwards frame on port where that destination can be reached.

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Switches

• Switch interprets Ethernet header.

• Maintains table of address/port mappings.

• Buffers frames and sends one at a time.

• Reduces collisions– Forwards frames to single port using table.– Buffered frames are sent one at a time.

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Switches vs. Hubs

Hubs

• Layer 1

• Frequent collisions.

• Bandwidth per hub, i.e. 100Mbps 12-port hub has 100 Mbps total.

Switches

• Layer 2

• Infrequent collisions.

• Bandwidth per port, i.e. a 100 Mbps 12-port switch has 1200 Mbps.

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Half Duplex

Receiving (RX) line monitored. If frames seen, no frames sent until clear. If frame received while transmitted on TX, a collision

occurs.

Hubs can only work in half-duplex mode.

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Full Duplex

RX, TX lines always available. Transfer in both directions simultaneously. No collisions possible.

Misleadingly advertised as 200 Mbps Ethernet.

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Auto-Negotation

Protocol where both sides of link decide on Speed Duplex

If only one side set to auto-negotatiate, it fails. Both switch and NIC must support to succeed.

If auto-negotation fails, parallel detection used Parallel detection can only determine speed. Not duplex. Assumes half-duplex for 10/100 Mbps. Assumes full duplex for 1000 Mbps and faster.

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Auto-Negotiation FailureCommon failure case

One side hard coded to 100/full. Other side set to auto-negotiate.

Result: one side half-duplex, other full duplex Full-duplex side sends frames w/o checking RX line. Half-duplex side sees many collisions, won’t send until

RX clear.

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Ethernet MAC Addresses

• OUI assigned by IEEE.

• Burned in address (BIA) stored in ROM.

• Can be replaced with a local address.

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Address Types

Unicast addresses represent a single device.

Multicast addresses represent a subset of devices on the LAN. They begin with 0100.5E

The broadcast address represents all devices on the LAN. It is

FFFF.FFFF.FFFF

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Ethernet FramesThree header formats.

In combined length/type field: If value in range 0..1536, it is a length. If value > 1536, it is a type (ex: IP is 2048).

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Ethernet FieldsField Length Description

Preamble 7 Synchronization; 7 octets of 01010101.

Start Frame Delimeter 1 1 octet of 11010101

Destination Address 6 Identifies recipient.

Source MAC Address 6 Identifies sender (who to reply to.)

Length 2 Length of data field.

Type 2 Type of protocol encapsulated in frame.

Data and Pad 46 -1500

Encapsulated data from higher protocol.

Frame Check Sequence 4 CRC checksum to detect errors.

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IP over Ethernet

To create a Type field for frames that use Length field, 1 or 2 headers added after Ethernet header and before data. 802.2 Logical Link Control (LLC) header Subnetwork Access Protocol (SNAP) header.

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Error Detection

CRC used to detect transmission errors. Frames with bad checksums are discarded. There is no provision for retransmission. Transport layer protocols can handle that.

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Why WANs?

The obvious answer: distance– Ethernet can’t travel over many miles.– You don’t have the rights-of-way to run a cable

over many miles between sites.

Solution: point-to-point leased line.

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WAN Components

CSU: Channel service unit.

demarc: division between customer/telco responsibility.

CPE: customer premises equipment.

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WAN Serial Cabling

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Synchronization

• CSU, WAN switch, and router must synchronize clock rates to communicate.

• DCE: data communications equipment, device that provides clock signal.

• DTE: data termination equipment, device that receives clock signal, typically router.

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Link Speeds

Name of Line Link Speed

DS0 64 kbps (digital signal audio channel)

DS1 (T1) 1.544 Mbps (24 DS0s + 8kbps overhead)

DS3 (T3) 44.736 Mbps (28 DS1s + overhead)

OC-1 51.84 Mbps (optical carrier)

OC-3 155.52 Mbps (3 * OC-1)

OC-24 1244.16 Mbps

OC-48 2488.32 Mbps

OC-96 4976.64 Mbps

OC-192 9953.28 Mbps

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Layer 2 Protocols

HDLC: High-level Data Link Control– Address field no longer used.

PPP: Point to Point Protocol– Framing identical to Cisco HDLC.

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Frame Relay

Leased lines don’t scale well.– To add a 10th site to network, need to add 10 new

leased lines, one to each site.– Need routers that support 10 lines each too.

Frame relay– Only need one line per site.– Packet switching provided within telco network.– Given a virtual circuit, not a physical one.

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Frame Relay

Frame Relay contains address field– Data Link Connection Identifier (DLCI)– Telco network switches packets based on DLCI.

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Virtual Circuits

Virtual circuit is a logical path through telco network. Acts like a point-to-point circuit. Provider will guarantee minimum bandwidth.

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Frame Relay Example

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References1. Cisco, Cisco Connection Documentation,

http://www.cisco.com/univercd/home/home.htm2. Cisco, Internetworking Basics,

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introint.htm

3. Gary A. Donahue, Network Warrior, O’Reilly, 2007.4. IEEE 802.3 Ethernet Working Group, http://www.ieee802.org/3/5. Wendell Odom, CCNA Official Exam Certification Library, 3rd

edition, Cisco Press, 2007.6. Priscilla Oppenheimer and Joseph Bardwell, Troubleshooting

Campus Networks, Addison-Wesley, 2002.7. Charles E Spurgeon, Ethernet: The Definitive Guide, O’Reilly, 2000.8. W. Richard Stevens, TCP/IP Illustrated, Addison-Wesley, 1994.