Local Area Networks/School of Engineering in Computer Science/2009-2010

http://www.redes.upv.es/ralir/en/

1.- LAN basics

Networking basics The Internet TCP/IP

LANs topologiesMedia Access Control (MAC) techniques

Local Area Networks/School of Engineering in Computer Science/2009-2010

http://www.redes.upv.es/ralir/en/

1.- LAN basics

Networking basics

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Goals of computer networks

to provide ubiquitous access to shared resources (e.g., printers, databases, file systems...),

to allow remote users to communicate (e.g., email, IP telephony),

to do transactions (banking, e-commerce, stock trading), and…

… save money: downsizing

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A “nuts and bolts” view of a network

Millions of connected computing devices: hosts, end-systems pc’s workstations, servers PDA’s phones, toasters

running network apps communication links

fiber, copper, radio, satellite routers: forward packets

(chunks) of data thru network protocols: control sending,

receiving of msgs TCP, IP, and HTTP, FTP, PPP,

…

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local ISP

companynetwork

regional ISP

router workstation

servermobile

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A closer look at the network structure

1. The network edge: applications and hosts

2. The network core: routers network of networks

3. The access networks and physical media: communication links

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The network edge

End systems (hosts): run application programs

at the “edge of network” client/server model

client host requests, receives service from server

e.g., WWW client (browser)/ server; email client/server

peer-peer model: host interaction symmetric e.g.: Gnutella, KaZaA

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The network core

Mesh of interconnected routers The fundamental question: how

is data transferred through net? Circuit switching:

dedicated circuit per call: telephone net

Packet switching: data sent through the network in discrete “chunks”

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The network core: Circuit switching

End-end resources reserved for “call”

Characterizing parameters: link bandwidth, switch capacity

dedicated resources: no sharing

circuit-like (guaranteed) performance

call setup required

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The network core: Packet switching

Data traffic divided into packets Each packet contains a header (with address)

Packets travel separately through network Packet forwarding based on the header Network nodes may store packets temporarily

Destination reconstructs the message

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The network core: Packet switching (routing)

Goal: move packets among routers from source to destination

datagram network: destination address determines next hop routes may change during session analogy: driving, asking directions

virtual circuit network: each packet carries tag (virtual circuit ID), tag determines next

hop fixed path determined at call setup time, remains fixed thru call routers maintain per-call state

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The access networks and physical media

How to connect end systems to edge router? Residential access networks Institutional access

networks (school, company) Wireless access networks

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Residential access networks: point to point access

Dialup via modem up to 56Kbps direct access

to router (conceptually) ISDN: integrated services digital

network: 128Kbps all-digital connect to router

ADSL: asymmetric digital subscriber line up to 1 Mbps home-to-

router up to 8 Mbps router-to-

home ADSL deployment:

happening HFC: hybrid fiber coax

asymmetric: up to 10Mbps upstream, 1 Mbps downstream

network of cable and fiber attaches homes to ISP router

shared access to router among home

issues: congestion, dimensioning

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Residential access networks: cable modems

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Diagram: http://www.cabledatacomnews.com/cmic/diagram.html

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Institutional access networks: local area networks

company/univ local area network (LAN) connects end system to edge router

Ethernet: shared or dedicated cable

connects end system and router

10 Mbs, 100Mbps, Gigabit Ethernet

deployment: institutions, home LANs happening now

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Wireless access networks

Shared wireless access network connects end system to router

Wireless LANs: radio spectrum replaces wire e.g., WiFi, Bluetooth, WiMAX

Wireless WANs: GPRS/EDGE over GSM High-Speed Downlink Packet

Access (HSDPA) a 3G (third generation) mobile telephony communications based on Universal Mobile Telecommunications System (UMTS) networks.

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basestation

mobilehosts

router

Local Area Networks/School of Engineering in Computer Science/2009-2010

http://www.redes.upv.es/ralir/en/

1.- LAN basics

Networking basics The Internet

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Internet structure: network of networks

Roughly hierarchical National/international

backbone providers (NBPs) e.g. BBN/GTE, Sprint, AT&T,

IBM, UUNet interconnect (peer) with each

other privately, or at public Network Access Point (NAPs)

A point of presence (POP) is a machine that is connected to the Internet.

Internet Service Providers (ISPs) provide dial-up or direct access to POPs. regional ISPs

connect into NBPs local ISP, company

connect into regional ISPs

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NBP A

NBP B

NAP NAP

regional ISP

regional ISP

localISP

localISP

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Network Access Points (NAPs)

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Source: Boardwatch.com

Note: Peers in this context are commercial backbones.

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MCI/WorldCom/UUNET Global Backbone

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Source: Boardwatch.com

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The situation in Europe

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See: http://www.redes.upv.es/ralir/en/MforS/GEANT2.WMVAlso: http://video.google.com/googleplayer.swf?docId=-4949195951027294198&hl=en-GBMore about technolgies: http://video.google.com/googleplayer.swf?docId=-4634094763983277329&hl=en-GB

Local Area Networks/School of Engineering in Computer Science/2009-2010

http://www.redes.upv.es/ralir/en/

1.- LAN basics

Networking basics TCP/IP

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A simple TCP/IP Example

A user on host argon.tcpip-lab.edu (“Argon”) makes a web access to URL

http://neon.tcpip-lab.edu/index.html.

What actually happens in the network?

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argon.tcpip-lab.edu("Argon")

neon.tcpip-lab.edu("Neon")

Web request

Web page

Web client Web server

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HTTP Request and HTTP response

Web browser runs an HTTP client program Web server runs an HTTP server program HTTP client sends an HTTP request to HTTP server HTTP server responds with HTTP response

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HTTP client

Argon

HTTP server

Neon

HTTP request

HTTP response

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HTTP Request

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GET /index.html HTTP/1.1

Accept: image/gif, */*

Accept-Language: en-us

Accept-Encoding: gzip, deflate

User-Agent: Mozilla/4.0

Host: neon.tcpip-lab.edu

Connection: Keep-Alive

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HTTP Response

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HTTP/1.1 200 OKDate: Sat, 25 May 2002 21:10:32 GMTServer: Apache/1.3.19 (Unix) Last-Modified: Sat, 25 May 2002 20:51:33 GMTETag: "56497-51-3ceff955"Accept-Ranges: bytesContent-Length: 81Keep-Alive: timeout=15, max=100Connection: Keep-AliveContent-Type: text/html <HTML><BODY><H1>Internet Lab</H1>Click <a href="http://www.tcpip-lab.net/index.html">here</a> for the Internet Lab webpage.</BODY></HTML>

• How does the HTTP request get from Argon to Neon ?

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From HTTP to TCP

To send a request, the HTTP client program establishes an TCP connection to the HTTP server at Neon.

The HTTP server at Neon has a TCP server running

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HTTP client

TCP client

Argon

HTTP server

TCP server

Neon

HTTP request / HTTP response

TCP connection

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Resolving hostnames and port numbers

Since TCP does not work with hostnames and also does not know how to find the HTTP server program at Neon, two things must happen:

1. The name “neon.tcpip-lab.edu” must be translated into a 32-bit IP address.

2. The HTTP server at Neon must be identified by a 16-bit port number.

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Translating a hostname into an IP address

The translation of the hostname neon.tcpip-lab.edu into an IP address is done via a database lookup

The distributed database used is called the Domain Name System (DNS)

All machines on the Internet have an IP address:argon.tcpip-lab.edu

128.143.137.144neon.tcpip-lab.edu 128.143.71.21

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HTTP client DNS Server

argon.tcpip-lab.edu 128.143.136.15

neon.tcpip-lab.edu

128.143.71.21

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Finding the port number

Note: Most services on the Internet are reachable via well-known ports. E.g. All HTTP servers on the Internet can be reached at port number “80”.

So: Argon simply knows the port number of the HTTP server at a remote machine.

On most Unix systems, the well-known ports are listed in a file with name /etc/services. The well-known port numbers of some of the most popular services are:

ftp 21 finger 79telnet 23 http 80smtp 25 nntp 119

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Requesting a TCP Connection

The HTTP client at argon.tcpip-lab.edu requests the TCP client to establish a connection to port 80 of the machine with address 128.141.71.21

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HTTP client

TCP client

argon.tcpip-lab.edu

Establish a TCP connectionto port 80 of 128.143.71.21

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Invoking the IP Protocol

The TCP client at Argon sends a request to establish a connection to port 80 at Neon

This is done by asking its local IP module to send an IP datagram to 128.143.71.21

(The data portion of the IP datagram contains the request to open a connection)

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TCP client

argon.tcpip-lab.edu

IP

Send an IP datagram to128.143.71.21

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Sending the IP datagram to an IP router

Argon (128.143.137.144) can deliver the IP datagram directly to Neon (128.143.71.21), only if it is on the same IP network (sometimes called “subnet”).

But Argon and Neon are not on the same IP network (Q: How does Argon know this?)

So, Argon sends the IP datagram to its default gateway The default gateway is an IP router The default gateway for Argon is Router137.tcpip-

lab.edu (128.143.137.1).

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The route from Argon to Neon

Note that the gateway has a different name for each of its interfaces.

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neon.tcpip-lab.edu"Neon"

128.143.71.21

argon.tcpip-lab.edu"Argon"128.143.137.144

router137.tcpip-lab.edu"Router137"

128.143.137.1

router71.tcpip-lab.edu"Router71"128.143.71.1

Ethernet NetworkEthernet Network

Router

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Finding the MAC address of the gateway

To send an IP datagram to Router137, Argon puts the IP datagram in an Ethernet frame, and transmits the frame.

However, Ethernet uses different addresses, so-called Media Access Control (MAC) addresses (also called: physical address, hardware address)

Therefore, Argon must first translate the IP address 128.143.137.1 into a MAC address.

The translation of addressed is performed via the Address Resolution Protocol (ARP)

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Address resolution with ARP

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argon.tcpip-lab.edu128.143.137.14400:a0:24:71:e4:44

ARP message: What is the MACaddress of 128.143.137.1?

ARP message: IP address 128.143.137.1belongs to MAC address 00:e0:f9:23:a8:20

router137.tcpip-lab.edu128.143.137.100:e0:f9:23:a8:20

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Invoking the device driver

The IP module at Argon, tells its Ethernet device driver to send an Ethernet frame to address 00:e0:f9:23:a8:20

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argon.tcpip-lab.edu

IP module

Ethernet

Send an Ethernet frameto 00:e0:f9:23:a8:20

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Sending an Ethernet frame

The Ethernet device driver of Argon sends the Ethernet frame to the Ethernet network interface card (NIC)

The NIC sends the frame onto the wire

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argon.tcpip-lab.edu128.143.137.14400:a0:24:71:e4:44

IP Datagram for Neon

router137.tcpip-lab.edu128.143.137.100:e0:f9:23:a8:20

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Forwarding the IP datagram

The IP router receives the Ethernet frame at interface 128.143.137.1, recovers the IP datagram and determines that the IP datagram should be forwarded to the interface with name 128.143.71.1

The IP router determines that it can deliver the IP datagram directly

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neon.tcpip-lab.edu"Neon"

128.143.71.21

argon.tcpip-lab.edu"Argon"128.143.137.144

router137.tcpip-lab.edu"Router137"

128.143.137.1

router71.tcpip-lab.edu"Router71"128.143.71.1

Ethernet NetworkEthernet Network

Router

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Another lookup of a MAC address

The router needs to find the MAC address of Neon. Again, ARP is invoked, to translate the IP address of

Neon (128.143.71.21) into the MAC address of neon (00:20:af:03:98:28).

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ARP message: What is the MACaddress of 128.143.71.21?

ARP message: IP address 128.143.71.21belongs to MAC address 00:20:af:03:98:28

neon.tcpip-lab.edu128.143.71.21

00:20:af:03:98:28

router71.tcpip-lab.edu128.143.71.1

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Invoking the device driver at the router

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The IP protocol at Router71, tells its Ethernet device driver to send an Ethernet frame to address 00:20:af:03:98:28

router71.tcpip-lab.edu

IP module

Ethernet

Send a frame to00:20:af:03:98:28

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Sending another Ethernet frame

The Ethernet device driver of Router71 sends the Ethernet frame to the Ethernet adapter, which transmits the frame onto the wire.

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IP Datagram for Neon

neon.tcpip-lab.edu128.143.71.21

00:20:af:03:98:28

router71.tcpip-lab.edu128.143.71.1

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Data has arrived at Neon

Neon receives the Ethernet frame The payload of the Ethernet frame is

an IP datagram which is passed to the IP protocol.

The payload of the IP datagram is a TCP segment, which is passed to the TCP server

Note: Since the TCP segment is a connection request (SYN), the TCP protocol does not pass data to the HTTP program for this packet. Instead, the TCP protocol at neon will respond with a SYN segment to Argon.

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HTTP server

Neon.cerf.edu

TCP server

IP module

Ethernet

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Wrapping-up the example

So far, Neon has only obtained a single packet Much more work is required to establish an actual TCP

connection and the transfer of the HTTP Request

The example was simplified in several ways: No transmission errors The route between Argon and Neon is short (only one IP router) Argon knew how to contact the DNS server (without routing or address resolution) ….

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Local Area Networks/School of Engineering in Computer Science/2009-2010

http://www.redes.upv.es/ralir/en/

1.- LAN basics

LANs topologies

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LAN basics

A local area network is a communication network that interconnects a variety of data devices within a small geographic area and broadcasts data at high data transfer rates with very low error rates.

They are typically private Since the local area network first appeared in the 1970s, its

use has become widespread in commercial and academic environments.

Functions of a LAN: a few examples File server - A large storage disk drive that acts as a central storage

repository. Print server - Provides the authorization to access a particular printer,

accept and queue print jobs, and provides a user access to the print queue to perform administrative duties.

Interconnection - A LAN can provide an interconnection to other LANs and to wide area networks

Manufacturing support - LANs can support manufacturing and industrial environments.

Distributed processing - LANs can support network operating systems which perform the operations of distributed processing.

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LAN Selection Criteria

Cost For most of us, cost is an overriding constraint, and you must

choose the best solution within your budget. Usually, cost is the most inflexible constraint under which you must operate, and in the final analysis the LAN must be a cost-effective solution to your problem.

Number of Workstations Each LAN is physically capable of supporting some maximum

number of workstations. If you exceed that maximum number, you must make some provision for extending the maximum number.

Number of Concurrent Users / type of use As the number of concurrent users goes up, so does the LAN

workload. As the LAN workload increases, you have two basic choices: You can allow system responsiveness to decrease, or you can increase the work potential of the system.

Many concurrent users may increase the LAN workload.

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LAN Selection Criteria (cont.)

Distance and Medium Attaining high speed over long distances can be very expensive.

Thus, each LAN has a maximum distance it can cover.

Speed It is important to you select a LAN capable of meeting your

performance goals. Available LAN speeds are 10, 100, and 1,000 Mbps, and the trend is for increasing speeds.

Device connectivity Some organizations need to attach special devices to the LAN, for

example, a plotter or scanner. LAN interfaces for such devices may not be available on some LANs or on some LAN file servers.

Connectivity to Other Networks A variety of connection capabilities exist, but a given LAN may not

support all of them.

Adherence to Established Standards There are several standards for LAN implementation. Some LANs

conform to these standards whereas others do not.

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Simple LAN Topologies

Physical topology: Physical layout of a network Bus topology consists of a single cable—called a bus—

connecting all nodes on a network without intervening connectivity devices

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Simple LAN Topologies

Ring topology Each node is connected to the two nearest nodes so the entire

network forms a circle Active topology

Each workstation transmits data Each workstation functions as a repeater

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Simple LAN Topologies

Star topology Every node on the network is connected through a central device

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Hybrid LAN Topologies

Hybrid topology Complex combination of the simple physical topologies

Star-wired ring Star-wired topologies use physical layout of a star in conjunction

with token ring-passing data transmission method

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Hybrid LAN Topologies

Star-wired bus In a star-wired bus topology, groups of workstations are star-

connected to hubs and then networked via a single bus

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Hybrid LAN Topologies

Daisy-Chained Daisy chain is linked series of devices

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Hybrid LAN Topologies

Hierarchical Uses layers to separate devices by their priority or function

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The UPV extended LAN

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Local Area Networks/School of Engineering in Computer Science/2009-2010

http://www.redes.upv.es/ralir/en/

1.- LAN basics

Media Access Control (MAC) techniques

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Media Access Control (MAC)

single shared communication channel two or more simultaneous transmissions by nodes:

interference only one node can send successfully at a time

Media Access Control: distributed algorithm that determines how stations share channel,

i.e., determine when a station can transmit communication about channel sharing must use channel itself! Takes also care of:

Assembly of data into frame with address and error detection fields Disassembly of frame

Address recognition Error detection

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Media Access Control (MAC)

For the same LLC, several MAC options may be available

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MAC Protocols: a taxonomy

Three broad classes: Channel Partitioning

divide channel into smaller “pieces” (time slots, frequency) allocate piece to node for exclusive use

Random Access allow collisions “recover” from collisions

“Taking turns” tightly coordinate shared access to avoid collisions

Goal: efficient, fair, simple, decentralized

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Channel Partitioning MAC protocols TDMA

TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = pkt trans

time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle

inefficient with low duty cycle users and at light load.

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Channel Partitioning MAC protocolsFDMA

FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1,3,4 have pkt, frequency bands

2,5,6 idle

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Random Access MAC protocols

When node has packet to send transmit at full channel data rate R. no a priori coordination among nodes

two or more transmitting nodes -> “collision”, random access MAC protocol specifies:

how to detect collisions how to recover from collisions (e.g., via delayed retransmissions)

Examples of random access MAC protocols: pure ALOHA slotted ALOHA CSMA and CSMA/CD

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Random Access MAC protocols Pure (unslotted) ALOHA

unslotted Aloha: simpler, no synchronization pkt needs transmission:

send without awaiting for beginning of slot

collision probability increases: pkt sent at t0 collide with other pkts sent in [t0 -1, t0 +1]

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Random Access MAC protocols Slotted Aloha

time is divided into equal size slots (= pkt trans. time) node with new arriving pkt: transmit at beginning of next

slot if collision: retransmit pkt in future slots with probability

p, until successful.

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Success (S), Collision (C), Empty (E) slots

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Random Access MAC protocols CSMA: Carrier Sense Multiple Access

CSMA: listen before transmit: If channel sensed idle: transmit entire pkt If channel sensed busy, defer transmission

Persistent CSMA: retry immediately with probability p when channel becomes idle (may cause instability)

Non-persistent CSMA: retry after random interval human analogy: don’t interrupt others!

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Random Access MAC protocols CSMA collisions

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collisions can occur:propagation delay means two nodes may not hear each other’s transmission

collision:entire packet transmission time wasted

spatial layout of nodes along ethernet

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“Taking Turns” MAC protocols

“taking turns” protocols look for best of both worlds, because: Channel partitioning MAC protocols:

share channel efficiently at high load inefficient at low load: delay in channel access, 1/N bandwidth

allocated even if only 1 active node! Random access MAC protocols

efficient at low load: single node can fully utilize channelhigh load: collision overhead

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“Taking Turns” MAC protocols

Polling: master node “invites” slave

nodes to transmit in turn Request to Send, Clear to Send

msgs concerns:

polling overhead latency single point of failure (master)

Token passing: control token passed from one

node to next sequentially. token message concerns:

token overhead latency single point of failure (token)

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