Everything you need to know about network troubleshooting can be learned in elementary school. Networking involves hardware and software that allows computers to communicate. No two networks are exactly alike. Basic network components include end stations, applications, and the network itself. The OSI model provides a standard way to understand how data moves through a network via different layers. TCP/IP is the most common network protocol and uses IP for addressing and routing and TCP for reliable data delivery. Gathering basic network statistics is an important part of troubleshooting.

2. Everything you need to
know about Network
Troubleshooting,
You learned in the
first grade!

3. 128 64 32 16 8 4 2 1 = Decimal

4. What is Networking?
A combination of hardware and software that provides a
means for computers in an organization to communicate
with one another.
No two computer networks are exactly the same
No two companies networking needs are exactly the same
Each network is continually evolving. Applications, services
and technology are changing at a very rapid pace
Technology is continually being updated and older technology
phased out
Users and network support personnel need to continually adapt
to the technologies installed at their place of business.

5. Network Types
Local Area Network
Building Backbone
Campus Backbone
Metropolitan Area
Network (MAN)
Wide Area Network
Enterprise Network
Connects users in a department,
company or work group - Token
Ring, Ethernet
Connects LANs together in a
building
ATM, FDDI, Fast Ethernet, Gigabit
Ethernet
Connects building LANs together
ATM, FDDI, Fast Ethernet, Gigabit
Ethernet
Used to connect sites in a large
city
FDDI, ATM, SONET

6. MAN

7. Wide Area Networks (WAN)
Designed to connect geographically distant network sites
together, although they could be in same city or state
Obtained through a service provider (SWBT, SPRINT,
AT&T)
Use either a leased line or switched circuit technology
Usually connect routers together
Can use low, or medium speeds like 56 Kbps to 1.544
Mbps (T-1)
Use leased lines and pay fixed fee regardless of usage
Use circuit switched services where fee is based on usage
Examples include Switched 56 Kbps circuits, ISDN,
Frame Relay, T-1, X.25, DSL and ATM

8. Enterprise Networks
Is a term used to describe the entire network of a company
which has many or all of the following networks connected:
Local Area Networks (LAN)
Building Backbone
Campus Backbone
Metropolitan Area Network (MAN)
Wide Area Network (WAN)

10. Basic Network Components
There are three basic Network Components
The end station
The Applications that run on the end stations
The network which supports traffic flow between end stations
End Stations:
PCs, Servers, Workstations, Laptops, Printers, Scanners
Applications:
File Services, Print Services, Operating Systems, E-
mail, Imaging, Fax
Networks:
Network Operating System (NOS), Ethernet, Token
Ring, FDDI, ATM, Fast Ethernet, Frame Relay, ISDN,
DSL, T-1, Switches, Bridges, Routers, Hubs

11. Network Bandwidth
Networks have a finite bandwidth or capacity that
can be used by the particular network.
Most of the LAN topologies use a shared media
access control protocol
Ethernet - CSMA/CD - Carrier Sense Multiple Access / Collision
Detection
10 Mbps, 100 Mbps, 1Gbps
Token Ring - MAC - Media Access Control
4 Mpbs, 16 Mpbs
FDDI - SMT - Station Management
100 Mbps

12. The Internet
Uses TCP/IP as the common transport protocol
Developed out of the US Government ARPANET
Supports access around the world to both public and
private users (VPN) Virtual Private Networks
Some of the applications that run on the Internet include:
World Wide Web (WWW) - application that allows access to
different types of data including text, audio and video.
File Transfer Protocol (FTP) - application that allows files to be
transferred from one computer to another
Simple Mail Transfer Protocol (SMTP) - application that
provides electronic mail (email) to be sent from user to user.

13. Accessing the Internet
The Internet is a computer network that connects
many networks together and is built upon a tiered
architecture
Tier 1 - National Access Points (NAP) - Regional
interchange facility which connects networks in an
area using routers
Tier 2 - National Service Providers (NSP) - Connect
National Access Points together
Tier 3 - Internet Service Providers (ISP) - Provide
Internet access to commercial and private users

15. The OSI Reference Model
Application
Provides
Services
Connects
processes
Moves
Data
Allows users to transfer files, send mail, etc.
Only layer that users can communicate with directly
Key features are ease of use and functionality
Standardized data encoding and decoding
Data compression
Data encryption and decryption
Manages user sessions
Reports upper-layer errors
Supports Remote Procedure Call activities
Connection management
Error and flow control
Provides reliable, efficient service
Inter-network packet routing
Minimizes subnet congestion
Resolves differences between subnets
Network access control
Packet framing
Error and flow control
Moves bits across a physical medium
Interface between network medium and interface devices
Defines electrical and mechanical characteristics of LAN
7
6
5
4
3
2
1 Physical
Data Link
Network
Transport
Session
Presentation
Application
Presentation
Session
Transport
Network
Data Link
Physical
Host A Host B

16. Frame Headers
• Frames headers revolve around the OSI model.
• The protocol headers are used to move the data.
– How many bytes is the DLC Header
– How about the IP header
– Good, now how about the TCP header
• *The number of headers in a frame is protocol-
dependent.
• There are many good reference books and classes
specific to protocols, a good reference site is
www.protocols.com
DLC * LLC Network Transport Application

17. Network Communication
Each layer of the OSI model provides a specific
communications function and each layer should be
independent of its adjacent layers
A header, created by each layer, implements the
function for that layer. The combination of header and
data is called a Protocol Data Unit (PDU)
When the PDU created by the Network layer reaches
the Data Link layer, both a header and trailer are added
and this creates a frame
When the frame from Host A reaches Host B, each
header that was added by a layer in Host A determines
the action taken by the peer layer on Host B

18. Protocols
Computers on networks use protocols to communicate
and these protocols define the procedures that each
system involved in the communications process will use.
Protocols are a set of procedures that are agreed upon and
then followed by each communicator
Protocols are combined into what is called a
communication architecture or protocol stack
Each protocol in the architecture provides a function that
is necessary to make data communications possible
Generally, there are two types of protocols:
Connectionless Protocols
Connection Oriented Protocols

19. Connectionless Protocols
Simply send data from a source address to a destination address
No verification is performed to determine if the destination address
is available.
Connectionless protocols are generally referred to as Datagram
Service
Much like sending a postcard via the US Mail
Datagram protocols usually do not support error recovery or any
type of acknowledgement routine classifying them as an unreliable
transmission service - (not to be confused with the USPS)
Connectionless protocols are typically used because of efficiency
and because the data being sent does not justify the extra overhead
necessary to perform the error recovery or acknowledgment
routine.
Example: TCP/IP’s - UDP (User Datagram Protocol), SNMP
(Simple Network Management Protocol)

20. Connection Oriented Protocols
A Connection Process or handshake is implemented between two
stations before the transmission of data occurs
Connections are referred to as sessions, virtual circuits, or logical
connections
Most connection oriented protocols require some form of
acknowledgement as data is transmitted, this is the mechanism
that provides reliable data transmission over the network
When data is found to be in error, the sending side is asked to re-
transmit the frame, or if the sender has not received an
Acknowledgement, it will re-transmit after a specific amount of
time
When the connection is no longer needed, there is a defined
disconnect process
Example: TCP/IP’s - TCP (Transport Control Protocol), SPX
(Sequenced Packet Exchange), SDLC

22. What Does IP Do?
Delivers packets across a network.
Delivery is based on IP address.
Route frames from one network to another.
IP does not provide flow control or error control, but
can provide Type of Service (Quality of Service).
IP fragments and re-assembles frames for traversal
across networks that require small frames (X.25)
based on the Maximum Transmission Unit (MTU)
size.

23. IP Functionality
Client
Frames get sent to the MAC address of the Router. The Router
strips the DLC and builds new frame to put on other subnet.
Router also decrements the Time to Live (TTL) field, calculates
the new checksum.
Router
APP
TCP
IP
MAC
IP
APP
TCP
IP
MAC
Server
MAC
MAC
Subnet 1 Subnet 2

24. IP Fragmentation and Reassembly

26. The 4 Layer Internet Model
1 Local Network
Network
2
Transport
3
Application
4
Frames
Packets
Segments
Message

27. Addressing in TCP/IP
Specifies a Host Process
Process I.D.
Port Address
IP Address
LLC Address
Mac (DLC) Address
Application
Transport
Network
Data Link
Physical
“Messages”
“Segments”
“Packets &
Datagrams”
“Frames”
Bits
Specifies a TCP Port
Specifies a Logical
Network Device
Specifies a DLC
Network Device
Specifies a SAP
Socket

28. What Does TCP Offer?
Reliable Packet Delivery
Sequence Number + TCP Length = Acknowledgment or
Sequence Number + TCP Length = Next Sequence Number
Retransmission of Data (Lost Frame, Dropped Frame, CRC
error in frame)
Flow Control
Using Sliding Windows
Positive Acknowledgement with Retransmission
Multiplexing of Conversations or Connections
Error Control (TCP Header Checksum)

29. Virtual Network Circuit
When did you learn to dial a
phone?
Collect Call from ABC, do you accept the charges?
Yes = (3 way handshake) (SYN)
No = Reset Connection (RST)

30. Server
Workstation
What is a Port?
Virtual Circuit Identifier
“Client’s” use an
“ephemeral” port
(Short Lived, for the
duration of the
connection). Client
ports are typically
above 1023.
“Servers” use the
“well-known” port
or listening port.
Usually these ports
are defined between
0 – 1023.
See www.iana.org for list of ports

31. Server
Workstation
What is a Connection?
The Virtual Circuit
The IP address and TCP port number of one application process
(sometimes called a “socket”) is associated with the “socket” of
another application process, creating a pair of addresses used to refer
to the connection.
All data transfers are tracked through the socket pairing, which
defines the virtual circuit being used during the duration of the
connection.
198.54.18.23
Port 1518 Port 23
198.54.18.3

32. TCP Connection
SYN
SYN
ACK
SYN
RST

33. Connection Establishment
“Three-Way Handshake”
1
2
3
Client Server
Send SYN (SEQ = X)
WIN = size
Max Segment Size = 1024 Receive SYN
Send SYN (SEQ = Y)/ACK (X + 1)
WIN = size
Max Segment Size = 1024
Receive SYN/ACK Segment
Send ACK (Y + 1)
Receive ACK Segment

34. Reliability

35. Reliability

36. Server
Workstation
Reliability
Positive Acknowledgment with Retransmission (PAR)
Packet
Acknowledgement
“Client” must receive ACK
before the retransmission
timer expires. Otherwise,
packet will be re-transmitted.
Sliding Window
Seq = 1 + Len=1024
Acknowledgement = 2049
Seq = 1025 + Len=1024

37. Header Data
Header Data
Header Data
Segment Size affects
Performance
Large Segments
Small Segments
Header over 4x Data;
therefore, only 1/4 of
Network Bandwidth
is being used.
Header Data
54B 12B
Header Data
54B 1.5KB 54B 1.5KB 54B 1.5KB
54B 12 K Bytes
Increases response time because of
fragmentation time.
Header Data
Maximum Frame Size = 1518

38. Putting it all together!

39. Simple Information to Gather!
Network Statistics
Average Network Utilization
Total Frames
Total Bytes
Average Frame Size
Packet Drops
Interface Errors
Top Talkers
#1 Station % Usage
#2 Station % Usage
#3 Station % Usage
Top Protocols
Protocol #1 %
Protocol #2 %
Protocol #3 %
Response Times
NCP / SMB Create File Cmd/Resp
NCP / SMB File Read Cmd/Resp
NFS Create File Cmd/Resp
TCP Session Establishment
DNS Request/Reply
DHCP initialization time
Telnet Cmd/Echo/Ack
Gather this information from each subnet,
building or geographic location.