Networking

1
Networking

• Access methods
• Protocols including TCP/IP
• Carl Smith - National Certificate(parts adapted
  from HNC lectures by Steve Leggett)

2
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

3
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

4
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

5
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.

6
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.)

7
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

8
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.

9
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.

10
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

11
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.

12
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.
• Todays typical protocols include, TCP/IP, OSI,
  SNA, NETBEUI, IPX/SPX, etc

13
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).

14
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.

15
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.

16
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

17
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

18
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

19
Address Resolution Protocol (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 its local
network, with the IP address of the destination
contained within the broadcasts header.
20
ARP Address Resolution Protocol
That broadcast message was for me. Heres my
Ethernet (MAC) address for you
D
I need the Ethernet address of 192.2.2.4
A
192.2.2.4
192.2.2.1
192.2.2.4 what?
192.2.2.4 Ethernet 0000.8000.1118
192.2.2.2
192.2.2.3
B
C
21
TCP/IP - Background
Its history lies within the US military
One of the main benefits of TCP/IP is that it is
royalty free.
Introduced as a protocol suite in the 1970s 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.
22
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

23
TCP Port numbers

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

24
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
  LANs and the Internet
• Finally, we looked at the format of an IP
  address and TCP port numbers