Networking

Access methods & Protocols including TCP/IP

Carl Smith - National Certificate(parts adapted from HNC lectures by Steve Leggett)

Access Methods

On a busy network protocols have been devised to control access to the medium

These include Token passing for Token ring & bus networks

CSMS/CD for Ethernet based networks

AppleTalk (MAC) Networks use CSMA/CA

Collisions

All methods have one aim in mind… To avoid collisions between packets of

data using the network Collisions still occur, which slow down

the network, but the protocols discussed are in place to manage them

If collisions occur the packet(s) are corrupted and have to be re-transmitted

CSMA/CD

We have discussed Token Passing as an access method

CSMA/CD is used on Ethernet networks – IEEE 802.3

It is an Acronym for “Carrier Sense with Multiple Access and Collision Detection”

It operates at Layer 2 of the OSI model

The Dinner Party Analogy

The CSMA/CD protocol functions somewhat like a dinner party in a dark room, no one interrupts each other.

If two computers start talking at the same time, they both stop, wait a bit and then one of them starts talking again.

CSMA/CD - Dinner Party - 2

That is, everyone around the table must listen for a period of quiet before speaking (Carrier Sense).

Once a space occurs everyone has an equal chance to say something (Multiple Access).

If two people start talking at the same instant they detect that fact, and quit speaking (Collision Detection.)

CSMA/CD – Carrier Sense

To translate this into Ethernet terms, each interface must wait until there is no signal on the channel, then it can begin transmitting.

If some other interface is transmitting there will be a signal on the channel, which is called the carrier.

All other interfaces must wait until carrier ceases before trying to transmit, and this process is called Carrier Sense

CSMA/CD – Multiple Access

All Ethernet interfaces are equal in their ability to send frames onto the network.

No one gets a higher priority than anyone else, and democracy reigns.

This is what is meant by Multiple Access.

CSMA/CD – Collisions

Since signals take a finite time to travel from one end of an Ethernet system to the other, the first bits of a transmitted frame do not reach all parts of the network simultaneously.

Therefore, it's possible for two interfaces to sense that the network is idle and to start transmitting their frames simultaneously.

CSMA/CD – Collision Detect

When this happens, the Ethernet system has a way to sense the "collision" of signals and to stop the transmission and resend the frames.

This is called Collision Detect

CSMA/CD - Summary

The CSMA/CD protocol is designed to provide fair access to the shared channel so that all stations get a chance to use the network. After every packet transmission all stations use the CSMA/CD protocol to determine which station gets to use the Ethernet channel next.

CSMA/CD is a type of contention protocol which is a competition for resources.

The term is in networking to describe the situation where two or more nodes attempt to transmit a message across the same wire at the same time.

Higher Level Protocols

However, the use of protocol suites at the higher levels of the seven layer model has gathered pace over the years, as networking has become more prevalent.

Today’s typical protocols include, TCP/IP, OSI, SNA, NETBEUI, IPX/SPX, etc

TCP/IP v IPX

TCP/IP was developed as the protocol suite for the Internet in America as part of the DARPA project, (Defence Advanced Research Projects Agency), with the USA having supplied funding into communications research for quite some time now.

Novell went for a proprietary version (SPX/IPX) (Sequenced Packet Exchange / Internetworked Packet Exchange) of a Xerox protocol (XNS).

Evolving Network Protocols

TCP/IP is generally a heavyweight protocol suite, as it is able to handle any network, whereas the Novell protocols suites were generally considered to be lightweight in so much as they were efficient and quick.

Apple went their own way and developed (Apple Transfer Protocol).

These days, the trend tends to be towards TCP/IP, and indeed is offered now by Novell, as well as IPX/SPX.

Protocols and the OSI Model

Whilst each protocol has its own approach to communications, they must all cover the ISO layer 4 for functionality, as well as layer 3 for addressing.

With TCP/IP this is split off into two parts with the first part, layer 3 (the Internet Protocol (IP)), dealing with addressing issues, and the Transmission Control Protocol (TCP) part dealing with functionality - layer 4.

Transport Control Protocol & IP (Internet Protocol)

IP – moves the packet of data from node to node

TCP – verifies the accurate delivery of data from client to server. TCP can detect errors or lost data and controls re-transmission if the data is not accurate or entirely received

IP Addressing

Transmission of messages between two machines within the same network is different between each technology E.g. MAC addresses are used on an Ethernet LAN

TCP/IP assigns a singular “IP” number to each existing node in addition to the local end network address

Packets and Frames

The high-level protocol packets are carried between computers in the data field of Ethernet frames (MAC address)

The system of high-level protocols carrying application data and the Ethernet system are independent entities that cooperate to deliver data between computers

The protocol that provides that cooperation is called “ARP”

ARP is used to help a host to locate a hardware (MAC) address for the destination target in a given transmission, when the destination IP address is known.

ARP accomplishes this by transmitting broadcast messages on it’s local network, with the IP address of the destination contained within the broadcast’s header.

Address Resolution Protocol (ARP)

ARP – Address Resolution Protocol…

A

B C

D

192.2.2.1

192.2.2.2

192.2.2.4

192.2.2.3

I need the Ethernet

address of 192.2.2.4

That broadcast message was for me. Here’s my Ethernet (MAC) address for you

192.2.2.4 – Ethernet = 0000.8000.1118

192.2.2.4 = what?

TCP/IP - Background

Introduced as a protocol suite in the 1970’s to support ARPAnet (American Research Projects Agency), who are the US defence network.

After installing TCP/IP on ARPANET in 1983 all devices were configured to use the new protocol. Using this protocol a hybrid network known as the Internet was born.

One of the main benefits of TCP/IP is that it is royalty free.

It’s history lies within the US

military

The format of an IP address

IP addresses are 32 bits long They consist of two parts :–

The Network number and Host number The 4 byte value or IP number converts

each byte into a decimal number (1-255) and separates each byte with a periode.g. 192.100.75.123

They are divided into classesCLASS A – 1 to 126 CLASS B – 128 to 191CLASS C – 192 to 223

TCP Port numbers

“Port” numbers are used by TCP to advance information to the upper layers of the OSI model

Application layer

FTP Telnet SMTP DNS TFTP HTTP SNMP

Port Number

21 23 25 53 69 80 161

Summary There are many protocols involved in

networking CSMS/CD is one “access method” protocol

discussed operating at layer 2 High level protocols to transfer data operate at

layer 3 and 4 of the OSI Model, ARP is used to match these to the MAC address at layer 2

TCP/IP is the de-facto protocol now used for LAN’s and the Internet

Finally, we looked at the format of an “IP” address and TCP port numbers