This document describes the roles and functions of different layers in the OSI model. It explains that the access layer is the entry point for end stations to the network. The distribution layer aggregates traffic from the access layer. The core layer provides fast transport across the enterprise with no packet manipulation. It also summarizes that hubs operate at the physical layer, switches and bridges at the data link layer, and routers at the network layer.

1. Objectives On completion of this chapter , you will be able to perform the following tasks: Describe how data traffic is exchanged between source and destination devices Identify the roles and functions of a hub, switch, and router, and where they best fit in the network Select the appropriate Cisco equipment for a given set of network requirements

2. Network Structured Defined By Hierarchy Core Layer Distribution Layer Access Layer

3. Access Layer Characteristics Access Layer End station entry pint to the network

4. Access Layer Aggregation Point Router traffic Broadcast / Multicast Domains Media Transiation Security Possible point for remote access Distribution Layer Characteristics

5. Core Layer Characteristics Core Layer Fast transport to enterprise services No packet manipulation

6. OSI Model Overview Application (upper) Layers Reduces complexity Standardizes interfaces Facilitates modular engineering Ensures interoperable technology Accelerates evolution Simplifies teaching and learning Data Flow Layers Application Presentation Session Transport Network Data Link Physical 7 6 5 4 3 2 1

7. Application Presentation Session Transport Data Link Network Physical EXAMPLES Telnet HTTP ASCII, EBCDIC JPEG Operating System / Application Access Scheduling Role of Application Layers

8. Reliable Or Unreliable delivery Error correction before retransmit Provide logical addressing which routers use for path determination Combine bits into bytes and bytes into frames Access to media using MAC address Error detection not correction Move bits between devices Specifies voltage , wire speed and pin-out cables Role of Data Flow Layers EXAMPLES TCP UDP SPX IP IPX 802.3/802.2 HDLC WIA/TIA-232 V.35 Application Presentation Session Transport Network Data Link Physical 7 6 5 4 3 2 1

9. Application Presentation Session Transport Network Data Link Physical Data Data Upper layer data Upper layer data TCP Header Data IP Header LLC Header MAC Header 0101110101001000010 Encapsulation Data PDU Bits Frame Packet Segment

10. Application Session Presentation Transport Data Link Network P hysical Upper Layer Data Upper Layer Data TCP+Upper Layer Data IP+TCP+Upper Layer Data MAC Header TCP Header LLC Hdr+IP+TCP+Upper Layer Data LLC Header IP Header 0101110101001000010 De-Encapsulation Data

11. Physical Layer Functions Signaling Type Connector Media Type Defines Physica l Ethernet 802.3 V.35 EIA/TIA-232

12. Host 10 Base 2__ Thick Ehernet 10 Base 2__ Thick Ehernet Hub Hosts 10 Base T _ Twisted Pair Physical Layer :Ethernet/802.3

13. Physical All devices in the same collisions domain All devices in the same broadcast domain Devices share the same bandwidth Hubs operate at Physical Layer A B C D

14. Data Link Layer Functions 802.2 HDLC Data Link Defines Physical source and destination address Higher layer protocol (Service Access Point) associated with frame Network topology Frame sequencing Flow control Connection-oriented or connectionless Physical Ethernet 802.3 EIA/TIA_232 V.35 Frame Relay

15. Data Link Layer Functions 0000.0c xx.xxxx 8 Preamble Dest Add Source Add Length Data FCS 6 6 2 Variable 4 Ethernet II uses “Type “ here and does not use 802.2 IEEE assigned Vender assigned MAC Layer -802.3 # Bytes

16. OR Data Link Switches and Bridges Operate at Data Link Layer Each segment has its own collision domain All segments are in the same broadcast domain

17. Physical Ethernet 802.3 EIA/TIA_232 V.35 Frame Relay Network Data Link IP, IPX 802.2 HDLC Network Layer Functions Interconnects multiple data links Defines paths through network Defines logical source and destination addresses associated with a specific protocol

18. Network Layer Functions Network Layer End Station Packet Source address IP Header Destination address Data 172.15.1.1 Logical Address Network Node

19. 172.16.122.204 255.255.0.0 10101100 00010000 01111010 11001100 11111111 11111111 00000000 00000000 172 16 122 204 255 255 0 0 Host Network Layer Functions(cont.) Network Address Mask Binary Address Binary Mask

20. Routing Table NET INT Metric Routing Table INT NET Metric 1 2 4 E0 S0 S0 0 0 1 1 2 4 S0 S0 E0 1 0 0 1.1 1.2 1.0 E 0 S 0 1.3 2.1 2.2 4.3 S 0 E 0 4.0 4.1 4.2 Network Layer Function (cont.) Logical addressing allows for hierarchical network Configuration required Uses configured information to identify paths to networks

21. Routers: Operate at the Network Layer Broadcast control Connects to WAN services Traffic management Multicast control Multicast Path determination

22. Telecommuter Mobile User Branch Office Main Office Internet Using Routers to Provide Remote Access Modem or ISDN TA

23. 16 Copyright © 1998, Cisco Systems, Inc. ICRC_revision_11.3 Connection-Oriented Sessions Synchronize Synchronize Acknowledge Negotiate Connection Data Transfer (Send Segments) Sender Receiver Connection Established Connection Established Connection Established