1. Chapter 1 Overview Review Overview of demonstration network
OSI reference model
TCP/IP (or Internet) reference model
Compare OSI and TCP/IP models
Format of Ethernet frames
Function of various network devices
Overview of demonstration network
Chapter 1  Networking Components

2. OSI Reference Model Application (7) Presentation (6) Session (5)
Application: supporting network applications: FTP, SMTP, HTTP, etc.
Presentation: handle different data representations (e.g., encryption)
Session: connections between apps
Transport: host-host: TCP, UDP
Network: routing of datagrams from source to dest: IP, routing protocols
Link: data transfer between adjacent network elements: PPP, Ethernet
Physical: bits “on the wire”
Application (7)
Presentation (6)
Session (5)
Transport (4)
Network (3)
Data link (2)
Physical (1)
Chapter 1  Networking Components

3. OSI Reference Model Each layer provides service to the layer above
Link layer relies on physical layer
Network layer relies on link layer, etc.
Protocol “stack” is conceptual
Layering is supposed to simplify networking
Believe it or not!
Chapter 1  Networking Components

4. Example Network Hosts live at edge of network Routers are at the core
Computers, servers, etc.
Routers are at the core
This network has
WAN
Connected by satellite
2 LANs
2 segments on LAN 1
Connected by bridge
Chapter 1  Networking Components

5. Physical Layer Bits on the “wire” We don’t care much about this
Repeaters and hubs are physical layer devices
Chapter 1  Networking Components

6. Data Link Layer 2 sublayers: LLC sublayer MAC sublayer
Logical Link Control
MAC sublayer
Medium Access Control
CSMA/CD (Ethernet)
Chapter 1  Networking Components

7. Data Link Layer Link layer data unit is a frame
Recall that a bridge operates at the link layer
Bridge looks at MAC address to decide which interface(s) to forward frame
Bridges are self-learning
Bridges separate segments (separate collision domains)
Chapter 1  Networking Components

8. Frame Structure Preamble --- allows NIC to detect frame
Start Frame Delimiter (SFD) --- marks start of frame
Dest. Address --- hardware address of dest
Source Address --- hardware address of source
Chapter 1  Networking Components

9. Frame Structure Length --- number of bytes in packet
Logical Link Control (LLC) --- For reliable communication on the same LAN
Packet --- “data” from link layer perspective
PAD --- pad frame if less than 64 bytes
Frame Check Sequence (FCS) --- error checking
Chapter 1  Networking Components

10. Network Layer Route packets over the network
From source to dest
Data unit is a packet or datagram
Routers live at the network layer
“Core” of the network
Chapter 1  Networking Components

11. Transport Layer Logical host-to-host communication TCP is reliable
UDP is unreliable
Note that SNMP uses UDP
Why?
Transport layer operates in hosts
Routers don’t care about transport layer
Chapter 1  Networking Components

12. Session Layer Handles multiple logical processes on hosts
For example, you can browse the Web, send , FTP and telnet simultaneously
Must keep these processes separate
Is this really worth an entire “layer”?
Chapter 1  Networking Components

13. Presentation Layer
Compression, encryption, encoding, etc., are the job of presentation layer
Also deals with
Abstract Syntax Notation One (ASN.1) which is discussed in Appendix B
Basic Encoding Rules (BER) which is covered in Appendix D
Is this really worth an entire “layer”?
Chapter 1  Networking Components

14. Application Layer
Like an API: transparently provides access to lower layer services
No formal layer above, but application layer provides service to applications
System Management Application Entity (SMAE) provides support to network management applications
Remote Operations Service Element (ROSE) is contained in SMAE
Chapter 1  Networking Components

15. Application Layer
Common Management Information Protocol (CMIP) makes use of ROSE
Note that we’ll be interested in SNMP
SNMP is not part of the OSI model
Instead, it’s part of the TCP/IP model
CMIP is the analog of SNMP in the OSI model
Chapter 1  Networking Components

16. OSI Reference Model: The Bottom Line
OSI model has everything and the kitchen sink
As a result, it’s complex
Over-engineered?
OSI model took a long time to develop
So people started using a simpler model: TCP/IP (or Internet) model
TCP/IP model applies to the Internet
Chapter 1  Networking Components

17. TCP/IP Protocol Stack
application: includes OSI application, presentation and session layers
transport: same as OSI
network: same as OSI
link: same as OSI
Author calls this “Network Interface” layer with MAC implementation
physical: same as OSI
application
transport
network
link
physical
Chapter 1  Networking Components

18. Network Interface Layer
Network interface layer implements Network Device Interface Specification (NDIS)
NDIS is interfaces between higher layers and NIC
Drivers for Ethernet, FDDI, etc.
NDIS allows higher layers to be independent of media access method
Chapter 1  Networking Components

19. Network Interface Layer
Chapter 1  Networking Components

20. OSI vs TCP/IP Stacks Link layer is slightly different
OSI designed with reliable link layer
OSI model “has it all”
But more complex
TCP/IP is “good enough”
Simpler
Chapter 1  Networking Components

21. Demo Network
LAN used in book
Chapter 1  Networking Components

22. Demo Network Repeater is a physical layer device
Amplifies the signal
Can extend the length of the LAN
Hub is a repeater with multiple I/O ports
A physical layer device
Demo network has a hub in subnet 2
Bridge is a link layer device
Only forwards frame onto appropriate link(s)
“Transparent” since self-learning
Chapter 1  Networking Components

23. Demo Network A switch is a fancy bridge
So a switch is a layer 2 device
In “switched Ethernet”, can have simultaneous comm. between hosts on LAN without collisions
Layer 3 and layer 4 switches exist
These switches can use info in higher layers
Eg, layer 4 switch could give retransmission priority to packets that require high QoS
Chapter 1  Networking Components

24. Demo Network Routers Layer 3 devices
Like bridges/switches, routers isolate collision domains
Routers also isolate broadcast domains
Routing tables use IP address
For small network, static table is OK
For larger network, use RIP, OSPF, etc.
Routers route between networks, not hosts
Chapter 1  Networking Components

25. Demo Network
Probes
Gather information and pass it to “management station”
Management station analyzes the network traffic
Probe also called a “remote monitor”
In demo network, Probe 1 can monitor 2 segments simultaneously
Chapter 1  Networking Components

26. Chapter 1 Summary Overview of networking OSI reference model
TCP/IP protocol stack
Networking devices
Demo network
Book includes lots of pictures of hardware
Chapter 1  Networking Components