Network Technologies

Chapter 2

Approaches to Network Communication• Connection-Oriented

– Circuit-switched– A dedicated connection (circuit) is formed between two

points– Guaranteed capacity at what cost?

• Connectionless– Packet-switched– Data is divided into small packets, each packet carries

information about destination– Flexible at what cost?

• Connectionless networks are very popular– Multiple computers share the network bandwidth

– Fewer connections are required

• For the rest of the book, networks generally refer to connectionless networks

• Wide area networks– called long-haul networks– connected with packet switch computers and lines– to extend, just add another switch

• Local area networks– operate at faster speeds, with lower delays– and do not span great distances– each computer has a network interface card connecting it to

the network

• Tradeoffs: speed and distance

Network Hardware Addresses

• Each computer attached to a network has a unique address

• When a sender sends a message, the address of the destination is included in the packet

Ethernet Technology

• Packet-switched LAN technology standardized in 1978 by Xerox, Intel and DEC

• IEEE standardized in 802.3• See Figure 2.1 showing 10Base5 using coax cable• To connect, a transceiver connects to the cable and

a host interface card plugs into the computer• See Figures 2.3 and 2.4• Problems: thick wire doesn’t bend well, transceivers

are with cable, not computer

Thinnet

• 10Base2• Thinner, cheaper, more flexible• Does not provide as much protection from

interference, covers shorter distance• Transceiver hardware is replaced with digital circuits

as shown in Figure 2.5• Works well in a small space• Easy to connect• Uses BNC connectors

Twisted Pair

• Uses unshielded copper wires like telephone wires• 10Base-T• A set of 8 wires (4 pair) connects each computer

to an Ethernet hub as in Figure 2.6• A connection to a hub is similar to a connection to

a transceiver• Cable is called category 5 cable (CATV)

Fast Ethernet

• 100Base-T• Also uses CATV cable• Allows more stations and more total traffic

Fiber Optics

• 10Base-F• Good between buildings• Expensive• Good noise immunity

Collision Detection

• When a station sends a message, the signal travels at ~ 70% the speed of light

• Transceivers sense that a signal is present and will not send from their connected station

• If two signals are on the line at once, neither signal will make sense and a collision occurs

• When a collision is detected, the host interface aborts transmission and waits (how long?) for the line to be idle before it sends again; CSMA/CD

Ethernet Hardware Addresses

• Each computer attached to an Ethernet network has a 48-bit address – called Ethernet address, hardware address, physical

address, MAC address (media access), or layer 2 address

• The host interface receives a copy of every packet that passes through a hub– it can recognize when a packet has this address as the

destination address, it ignores all others

• Unicast, Broadcast, Multicast

Ethernet Frame Format

• This is at the link level• See Figure 2.7• Octet is 8-bits (byte is sometimes character size)• Format

– Preamble for synchronization

– Destination address

– Source address

– Frame type indicates protocol (IP, IPX, TCP…)

– Data and CRC

Repeaters and Bridges

• Both extend a network• A repeater relays electrical signals from one cable

to another– Can be used to double the length of a cable– At most two can be placed between two

machines. Why?• A bridge receives a complete frame, and then

sends it on

FDDI

• Provides a data rate comparable to Fast Ethernet

• Uses fiber

• Data is encoded in pulses of light

• Uses token ring technology; a station waits for the token before trying to send– Uses two rings connected as in Figure 2.9 to recover from

failure– If a station fails, network will bypass the failure as in Figure

2.10

• See Frame format in Figure 2.11

ATM

• Connection-oriented network technology• Used in LANs and WANs• Very high data switching speeds• Network consists of high-speed switches and fiber• Uses fixed-size frames called cells of 53 octets• The destination host is first contacted to see if it

will establish a connection; if so, cells are sent with a connection identifier; connection terminates

Example Networks

• ARPANET

• NSF Networks: see Figures 2.12, 2.13, 2.14 and 2.15

• vBNS– Replaced ANSNET in 1995

• X25NET

• Point-To-Point Networks

• Dial-Up

• Wireless

For Next Time

• Read Chapters 3 and 4