User guide

A-10 Cisco 2500 Series Access Server User Guide
LANs and WANs
Each layer of the OSI reference model specifies particular network functions such as
addressing, flow control, error control, encapsulation, reliable message transfer, and data
representation. The upper layer (the application layer) is closest to the user; the lowest layer
(the physical layer) is the closest to the cables and wires.
Each layer of the OSI reference model relies on the layers below, and offers its services to
the layers above.Table A-4 summarizes the layers and their functionality.
Table A-4 OSI Reference Model of Networks
LANs and WANs are generally defined by the lower two layers of the OSI reference model.
Higher layer protocols (such as TCP/IP, IPX, or AppleTalk) can use the foundation laid at
the lower layers to transport data between network devices.
1. EBCDIC = Extended Binary Coded Decimal Interchange Code.
2. TCP = Transmission Control Protocol.
3. SPX = Sequenced Packet Exchange.
4. ATP = AppleTalk Transaction Protocol.
5. ADSP = AppleTalk Data Stream Protocol.
6. IPX = Internetwork Packet Exchange.
7. DDP = Datagram Delivery Protocol.
8. HDLC = High-Level Data Link Control.
9. PPP = Point-to-Point Protocol.
Layer Title Purpose Example
7 Application Services to users File transfer, e-mail, virtual
terminals
6 Presentation Data representations ASCII text, EBCDIC
1
, ASN.1
5 Session Control of sessions Printing, file sharing
4 Transport Reliable delivery of packets over
networks
TCP
2
, SPX
3
, ATP
4
, and ADSP
5
3 Network Logical addressing, routing IP, IPX
6
, AppleTalk DDP
7
2 Data link Physical addressing, topology
(bus or ring), line access method
Ethernet, Token Ring, HDLC
8
,
PPP
9
1 Physical Electrical, mechanical 10BaseT, 10Base2