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Network Architectures

ITEC 370

George Vaughan

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Sources for Slides

• Material in these slides comes primarily from course text, Guide to Networking Essentials,Tomsho, Tittel, Johnson (2007).

• Other sources are cited in line and listed in reference section.

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TCP/IP and OSI ModelsTCP/IP and OSI Models (OSI-Model, n.d.) and (Tomsho, 2007)

TCP/IP Layers

PDU OSI Layers Function Devices - Apps Standards

7 Application Network process to application, Initiates or accepts a request to transfer data

Browsers, servers, Gateways

HTTP, SNMP, FTP, Telnet

6 Presentation Adds formatting, display, and encryption of information

Gateways ASCII, MPEG

Application Data

5 Session Adds communication session control information, Login/Logout

DNS, Gateways

NetBIOS

Transport Segments 4 Transport Adds End-to-end connections and reliability, re-sequencing, flow control

Gateways TCP, UDP

Network Packets 3 Network Path determination and logical addressing (IP), translates MAC address to logical address

Routers IP, ICMP, ARP, NetBEUI

LLC Frames 2 Data Link

MAC

Adds error checking and physical addressing (MAC & LLC)

Switches, Bridges, NICs

802.3, 802.11, FDDI

Link

Bits 1 Physical Media, signal and binary transmission, sends data as a bit stream

Hubs, Repeaters

10Base-T, T1, E1

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Major Access Methods

• Channel access is handled at the MAC sublayer of the Data Link layer in the OSI model

• Five major types of channel access– Contention– Switching– Token passing– Demand priority– Polling

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Contention

• Carrier Sense Multiple Access Collision Detection (CSMA/CD)

– Used by Ethernet– Computer listens – if quiet then transmit– If collision occurs, wait and then retry– Server doesn’t have priority over workstation

• Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

– Used by wireless LANs because not all wireless devices can hear each other

– When network is quiet, device sends ‘intent-to-transmit’ signal– Slower than CSMA/CD

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Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

Tomsho, Tittel, Johnson (2007)

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Switching

• Contention only occurs when 2 or more senders try to access the same receiver

• More fair than contention based technologies• Supports multiple, simultaneous transmissions.• Priorities can be assigned for Quality of

Service (QoS)• Different ports can operate at different speeds.• Can emulate all common topologies

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Token Passing

• All devices have equal guaranteed access

• Good for time sensitive applications

• If only computer is sending it must share token periodically with other non-sending devices

• Requires more expensive hardware

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Token PassingTomsho, Tittel, Johnson (2007)

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Demand Priority

• Used only by 100VG-AnyLAN (IEEE 802.12)

• Smart hubs use ‘round-robin’ to control access.

• Not common, therefore expensive

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Polling

• One of oldest access methods

• Uses centralized controller to control access

• Supports priorities

• Not efficient use of network media

• Used in IBM’s Systems Network Architecture (SNA)

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PollingTomsho, Tittel, Johnson (2007)

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Ethernet Architecture

• Most popular network architecture• Ranges from 10Mbps – 10 Gbps• Uses NIC’s MAC address to address frames.• Access handled at MAC sub-layer of Data Link

layer (layer 2) • MAC – Media Access Control address

– 48 bits in length– Unique physical address defined in NIC

• What differs is cable/speed – Frame format is common – backward compatibility.

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Ethernet Architecture

• Best effort transmission – no guarantee of frame delivery.

• Upper layers must ensure reliable delivery.• Access method is

– CSMA/CD (logical bus – Half Duplex)– Switching

• Relies on CRC at frame for error detection.• Bad frames are just dropped at destination• Collisions can occur in Half-Duplex• In Switched topology (Full Duplex), collisions do not

occur.

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Ethernet Architecture (History)

• 1960s and 1970s: many organizations worked on methods to connect computers and share data

– E.g., the ALOHA network at the University of Hawaii

– 1972: Robert Metcalf and David Boggs, from Xerox’s PARC, developed an early version of Ethernet

• 1975: PARC released first commercial version (3 Mbps, up to 100 computers, max. 1 km of total cable)

• DIX developed standard based on Xerox’s Ethernet (10 Mbps)

• 1990: IEEE defined the 802.3 specification

– Defines how Ethernet networks operate at layers 1-2

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Accessing Network Media

• Ethernet uses CSMA/CD in a shared-media environment (a logical bus)– Ethernet device listens for a signal or carrier

(carrier sense) on the medium first– If no signal is present, no other device is

using the medium, so a frame can be sent– Ethernet devices have circuitry that detects

collisions and automatically resends the frame that was involved in the collision

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Collisions and Collision DomainsTomsho, Tittel, Johnson (2007)

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Ethernet Error Handling

• Collisions are the only type of error for which Ethernet automatically attempts to resend the data

• Errors can occur when data is altered in medium – Usually caused by noise or faulty media connections– When the destination computer receives a frame, the

CRC is recalculated and compared against the CRC value in the FCS

– If values match, the data is assumed to be okay– If values don’t match, the data was corrupted

• Destination computer discards the frame• No notice is given to the sender

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Half-Duplex Versus Full-Duplex Communications

• When half-duplex communication is used with Ethernet, CSMA/CD must also be used

• When using a switched topology, a computer can send and receive data simultaneously (full-duplex communication)– The collision detection circuitry is turned off because

collisions aren’t possible– Results in a considerable performance advantage

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Ethernet FrameEthernet Frame Styles (Odom, 2006) DIX (Ethernet II) – Used in TCP/IP

Preamble Destination Source Type Data FCS 8 6 6 2 46-1500 4

IEEE 802.3 (Original) Preamble SFD Destination Source Length Data FCS

7 1 6 6 2 46-1500 4 IEEE 802.3 (Revised 1997) Preamble SFD Destination Source Length

xor Type Data FCS

7 1 6 6 2 46-1500 4

Field Length (Bytes)

Purpose

Preamble 7 Synchronization Start Frame Delimiter (SFD) 1 Signifies that next byte begins Destination MAC field Destination MAC address 6 Identifies recipient Source MAC Address 6 Sender Length 2 Length of data field Type 2 Type of protocol of encapsulated data Data and Pad 46-1500 Holds data from higher layer Frame Check Sequence (FCS) 4 Info for receiving NIC to check for errors

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TCPDUMP

• Unix/Linux command line protocol analyzer (packet sniffer) used for: – Debugging networks– Debugging applications that depend on networking.– Monitoring traffic

• Available for Windows• Supports user defined filters• Command Line syntax:

tcpdump -v –e

• Check Man page for other options

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TCPDUMP (Cont.)

• Example: Ping (Internet Control Message Protocol)

– 16:23:57.892354 00:15:f2:4d:52:19 > 00:20:ed:73:b7:1d, ethertype IPv4 (0x0800), length 74: IP (tos 0x0, ttl 128, id 8475, offset 0, flags [none], proto 1, length: 60) 192.168.1.101 > 192.168.1.12: icmp 40: echo request seq 11520

– Timestamp = 16:23:57.892354– Source (MAC Address) = 00:15:f2:4d:52:19– Destination (MAC Address) = 00:20:ed:73:b7:1d– Source IP = 192.168.1.101– Destination IP = 192.168.1.12– Protocol = icmp

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TCPDUMP (Cont.)

• Example: arp (Address Resolution Protocol)

– 16:22:37.497442 00:15:f2:4d:52:19 > Broadcast, ethertype ARP (0x0806), length 60: arp who-has 192.168.1.112 tell 192.168.1.101

– Timestamp = 16:22:37.497442– Source (MAC Address) = 00:15:f2:4d:52:19– Destination (MAC Address) = Broadcast– Protocol = arp who-has 192.168.1.112 tell

192.168.1.101

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TCPDUMP (Cont.)

• example: Web Request

– 16:22:43.383893 00:20:ed:73:b7:1d > 00:16:b6:21:71:d1, ethertype IPv4 (0x0800), length 74: IP (tos 0x0, ttl 64, id 42626, offset 0, flags [DF], proto 6, length: 60) 192.168.1.12.56478 > www8.cnn.com.http: S [tcp sum ok] 970586877:970586877(0) win 5840 <mss 1460,sackOK,timestamp 5790847 0,nop,wscale 2>

– Timestamp = 16:22:43.383893– Source (MAC Address) = 00:20:ed:73:b7:1d– Destination (MAC Address) = 00:16:b6:21:71:d1– Source IP = 192. 92.168.1.12– Destination IP = www8.cnn.com.http

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Ethereal

• GUI based protocol analyzer

• Available for Unix, Linux, Windows

• Open Source application

• www.ethereal.com

• Can be used to analyze raw data files from TCPDUMP tool.

• Supports user-defined filters.

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Ethereal (Cont.)

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Data Link Protocols (Tomsho, 2007)

Type Standard Advantages Dis-advantages

Min Cable

Physical Topology

Access Max Segment Length

Max Segments

Max Devices

per Segment

Max Devices

per Network

Trans Speed

10BaseT 802.3 Cheap Small segments

Cat 3 Star CSMA/CD 100m 1024 2 1024 10 Mbps

100BaseT (Fast Ethernet)

802.3u Fast, Easy Cost, distance

Cat 5 Star CSMA/CD 100m 1024 1 1024 100 Mbps

1000BaseT 802.3ab Fast, Full duplex

Cost, distance

Cat 5 Star CSMA/CD or Switching

100m 1024 2 1024 1000 Mbps

1000BaseLX 802.3z Fast, Full duplex

High cost, complex

Fiber Star Switching 5000m 1024 2 1024 1000 Mbps

1000BaseSX 802.3z Fast, Full duplex

Cost, complex

Fiber Star Switching 550m 1024 2 1024 1000 Mbps

Token Ring 802.5 Fast, Reliable

Cost and debugging

Cable - fiber

Star Token Passing

45m 33 hubs Depends on hub

72 UTP 260 STP

4 and 16 Mbps

FDDI X3T9.1 Very fast, reliable, distance

Cost, complex

Fiber Ring Token Passing

100 km 500 100 Mbps

ATM/Sonet X-fast, Qos Fiber Dedicated Circuit

>100 Gps

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The Token Ring ArchitectureTomsho, Tittel, Johnson (2007)

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Token Ring FunctionTomsho, Tittel, Johnson (2007)

• A token passes around the ring– If an “in use” token is received from the Nearest Active Up-

stream Neighbor (NAUN), and the computer has data to send, it attaches its data to the token and sends it to its Nearest Active Down-stream Neighbor (NADN)

– If received token is in use, NIC verifies if it is the destination station

• If not, the computer re-creates the token and the data exactly and sends them to its NADN

• If it is, data is sent to the upper-layer protocols– Two bits in data packet are toggled and token is

sent to NADN; when original sender receives it, it frees the token and then passes it along

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BeaconingTomsho, Tittel, Johnson (2007)

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Hardware ComponentsTomsho, Tittel, Johnson (2007)

• A hub can be a multistation access unit (MSAU) or smart multistation access unit (SMAU)

• IBM’s token ring implementation is the most popular adaptation of the IEEE 802.5 standard– Minor variations but very similar to IEEE specs

• IBM equipment is most often used– 8228 MSAU has 10 connection ports, eight of which

can be used for connecting computers– The RO port on one hub connects to RI port on the

next hub, and so on, to form a ring among the hubs• IBM allows connecting 33 hubs

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The Fiber Distributed Data Interface (FDDI) Architecture Tomsho, Tittel, Johnson (2007)

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The Fiber Distributed Data Interface (FDDI) Architecture (continued)Tomsho, Tittel, Johnson (2007)

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Networking Alternatives

• Many other network architectures are available• Some are good for specialized applications, and others

are emerging as new standards• Topics

– Broadband technologies (cable modem and DSL)– Broadcast technologies– ATM– ATM and SONET Signaling Rates– High Performance Parallel Interface (HIPPI)

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Broadband TechnologiesTomsho, Tittel, Johnson (2007)

• 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 TechnologyTomsho, Tittel, Johnson (2007)

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Digital Subscriber Line (DSL)Tomsho, Tittel, Johnson (2007)

• 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

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Asynchronous Transfer Mode (ATM)Tomsho, Tittel, Johnson (2007)

• High-speed network technology for LANs and WANs

– Connection-oriented switches

• Dedicated circuits are set up before communicating

– Data travels in fixed-size 53-byte cells (5 byte-header)

• Enables ATM to work at extremely high speeds

– Quick switching

– Predictable traffic flow

• Enables ATM to guarantee QoS

– Used quite heavily for the backbone and infrastructure in large communications companies

– LAN emulation (LANE) required for LAN applications

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ATM and SONET Signaling RatesTomsho, Tittel, Johnson (2007)

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References

Tomsho, Tittel, Johnson (2007). Guide to Networking Essentials. Boston: Thompson Course Technology.

Odom, Knott (2006). Networking Basics: CCNA 1 Companion Guide. Indianapolis: Cisco Press

Wikipedia (n.d.). OSI Model. Retrieved 09/12/2006 from http://en.wikipedia.org/wiki/OSI_Model