Networking Protocols and the OSI Model

OSI Model Overview• The Open Systems

Interconnection (OSI) reference model is an industry standard framework that is used to divide the functions of networking into seven distinct layers.

• Each layer provides specific services to the layers above and below it in order for the network to work effectively.

OSI Model Overview• A message begins at the top

application layer and moves down the OSI layers to the bottom physical layer.

• As the message descends, each successive OSI model layer adds a header to it.

• A header is layer-specific information that basically explains what functions the layer carried out.

• Conversely, at the receiving end, headers are striped from the message as it travels up the corresponding layers.

What is a Protocol?• Protocol is a controlled

sequence of messages that is exchanged between two or more systems to accomplish a given task.

• Protocol specifications define this sequence together with the format or layout of the messages that are exchanged.

Transmission Control Protocol/Internet Protocol

• The Transmission Control Protocol/Internet Protocol (TCP/IP) suite of protocols has become the dominant standard for inter-networking.

• TCP/IP represents a set of public standards that specify how packets of information are exchanged between computers over one or more networks. 

Transmission Control Protocol/Internet Protocol

Internetwork Packet Exchange/Sequenced Packet

Exchange• Internetwork Packet Exchange/Sequenced Packet Exchange (IPX/SPX) is the protocol suite employed originally by Novell®.

• It delivers functions similar to those included in TCP/IP.

NetBEUI• NetBIOS Extended User Interface (NetBEUI) is a

protocol used primarily on small Windows NT networks.

• NetBEUI is a simple protocol that lacks many of the features that enable protocol suites such as TCP/IP to be used on networks of almost any size.

AppleTalk• AppleTalk is comprised of

a e set of protocols that span the seven layers of the OSI reference model.

• AppleTalk protocols were designed to run over the major LAN types, notably Ethernet and Token Ring, and also Apple's own LAN physical topology, LocalTalk.

TCP/IP Utilities

Overview• TCP/IP is a complex

collection of protocols.• Most vendors implement

the suite to include a variety of utilities for viewing configuration information and troubleshooting problems.

Ping• Ping works by sending an ICMP echo request to the

destination computer. • The receiving computer then sends back an ICMP

echo reply message • It is also possible to use Ping to find the IP address of

a host when the name is known.

ARP, RARP, NSLOOKUP• Address Resolution Protocol

(ARP) is the means by which networked computers map Internet Protocol (IP) addresses to physical hardware (MAC) addresses that are recognized in a local network.

• Machines that do not know their IP addresses use Reverse Address Resolution Protocol (RARP).

• It is used to obtain IP address information based on the physical or MAC address.

ARP, RARP, NSLOOKUP• Nslookup returns the IP address for a given

hostname.

• It will also do the reverse and find the host name for a specified IP address.

Netstat/tpcon• The netstat command is used in Windows and UNIX/Linux

to display TCP/IP connection and protocol information.

• The netstat command provides a list of connections that are currently active.

• Netstat statistics can be useful in troubleshooting TCP/IP connectivity problems.

Nbtstat• The Microsoft TCP/IP

stacks included in Windows operating systems provide the nbtstat utility, which is used to display NetBIOS information.

Ipconfig, winipcfg, config, and ifconfig

• TCP/IP configuration information can be displayed using different utilities

• Ipconfig – Windows NT and Windows 2000 (command-line)

• Winipcfg –- Windows 95, 98, and ME (graphical interface)

• Ifconfig – UNIX and Linux (command-line

Tracert, iptrace, and traceroute• It is often useful to trace the

route a packet takes on its journey from source computer to destination host.

• TCP/IP stacks include a route tracing utility that enables users to identify the routers through which the message passes.

• The options depend on the operating system: – Tracert– Iptrace– Traceroute

Connecting to the Internet

Synchronous and Asynchronous Serial lines

• Synchronous serial transmission – Data bits are sent together with a synchronizing clock pulse. Built-in timing mechanism coordinates the clocks of the sending and receiving devices.

• Asynchronous serial transmission – Data bits are sent without a synchronizing clock pulse. Uses a start bit at the beginning of each message. When the receiving device gets the start bit, it can synchronize its internal clock with the sender clock.

Modems• The modem is an electronic

device that is used for computer communications through telephone lines.

• It allows data transfer between one computer and another.

• There are four main types of modems:– Expansion cards – PCMCIA – External modems– Built-in modems

Dial-Up Networking, Modem Standards, AT Commands

• When computers use the public telephone system or network to communicate, it is called Dial-Up Networking (DUN).

• All modems require software to control the communication session.

• The set of commands that most modem software uses are known as the Hayes-compatible command set. The Hayes command set is based on a group of instructions that always begins with a set of attention characters (AT).

ISPs and Internet Backbone Providers

• Services of an Internet Service Provider (ISP) are required to surf the Internet.

• An ISP is a company that connects computers to the Internet and World Wide Web.

• The actual connection to the Internet is tiered.

• The ISP may link to a larger regional ISP, which in turn might connect to one of a number of nationwide computer centers.

ISPs and Internet Backbone Providers

• The current U.S. Internet infrastructure consists of a commercial backbone and a high-speed service known as the Very High-Speed Backbone Network Service (vBNS).

• The vBNS connects five supercomputer networks across the country:– UUNET - a division of WorldCom – Cable & Wireless USA – Sprint – AT&T – BBN Planet

ISPs and Internet Backbone Providers

• The ISP that cannot connect directly to the national backbone is charged a fee to connect to a regional provider that links to the national backbone through a Network Access Point (NAP).

• Not all the Internet traffic goes through NAPs.

• Some ISPs that are in the same geographic area make their own interconnections and peering agreements.

Digital Subscriber Line (DSL)• Digital Subscriber Line (DSL) is an always-on

technology. This means there is no need to dial up each time to connect to the Internet.

• DSL comes in several varieties:

– Asymmetric DSL (ADSL)

– High Data Rate DSL (HDSL)

– Symmetric DSL (SDSL)

– Very High Data Rate DSL (VDSL)

Cable Modems• A cable modem acts like a

LAN interface by connecting a computer to the Internet.

• The cable modem connects a computer to the cable company network through the same coaxial cabling that feeds cable TV (CATV) signals to a television set.

Cable Modem versus DSL Internet Technologies

• When it comes to comparing cable modem and DSL Internet technologies, both have their pros and cons.

ISDN

• Another alternative to using analog telephones lines to establish a connection is ISDN.

• Speed is one advantage ISDN has over telephone line connections.

• ISDN uses a pair of 64Kbps digital lines to connect, which provides a total of 128Kbps throughput.

• A telephone line connects at a maximum speed of 56Kbps, and in some areas, doesn’t even reach that.

Satellite

• Satellite is an option for users in rural areas or with no other access to high speed Internet service.

• Satellite Internet does not require a phone line or cable. Two‑way communication, for upload and download, is achieved with the use of a satellite dish.

• Download speed is up to 500 kbps while the upload speed is one‑tenth of that of that.