Examples of 2nd layers

IBM’s SDLC

CCITT X.25 LAPB (Link Access Procedure Balanced)

ISO HDLC

LLC(802.2)+ MAC(CSMA/CD/Token Ring, …)

Ethernet II

ITU-T LAPD, LAPM(for Modem connections)

HDLC is a superset of LAPB

LLC(Logical Link Control=802.2) + MAC(802.3)

-Derivative of HDLC used for LANs(LLC+MAC = LAN’s 2nd layer)

-Services: 3 optional services

Unacknowledged connectionless(used when a higher layer provides error & flow controls

--ex. In TCP/IP networks, TCP provides those. So the 2nd layer avoids overheads)

Acknowledged connectionless(used when Request-Reply type of interaction is required(ex. Automated process control--to send an alarm and get response) & to avoid overhead of connection-oriented services(initialize, maintain, Terminate) for time-critical messages.

Connection-Oriented(used for simple device connections--ex. a terminal controller which may not run a highler layer protocol)

Ethernet II(version 2--original Ethernet Specification from DIX is version 1)

IEEE802.2+802.3 vs Ethernet II frame formats

IEEE 802.2/802.3 Encapsulation(RFC 1042):

Synch bytes 802.3 MAC

Preamble S Destination Source Length LLC + Data CRC

(101010……) F address address (bytes) SNAP

7Bytes 1 6 6 2 8 38 - 1492 4

802.2(LLC) 802.2 SNAP

DSAP SSAP Control org code type Data

AA AA 03 00

1 1 1 3 2

type IP datagram

0800

2 38 -- 1492

type ARP

PAD

0806 Request/ Reply

2 28 10

Ethernet II Encapsulation(RFC 894)

Synch bytes Ethernet Header

Preamble S Destination Source type Data CRC

(101010……) F address address

7Bytes 1 6 6 2 46 -- 1500 4

type IP Datagram

0800

2 46 -- 1500

type ARP

PAD

0806 Request/ Reply

2 28 18

-Both formats starts with 7 Byte Preambles(56 bits of 10101010……….)

-Starting Delimiter= 10101011

-Addresses--Each Ethenet hardware(called as Network Adapter or Network card or NIC(Network Interface Card) has a unique address: Example: In hex

00:00:1D:12:98:AC, 00-00-1D-12-98-AC

-IEEE assigns each manufacturer a pool of addresses:

00000C  Cisco

00000E  Fujitsu

00000F  NeXT

000010  Sytek

00001D  Cabletron

…………

…………

00AA00  Intel

02608C  3Com

080009  Hewlett-Packard

-Main difference is between  (802.3 Length field) --  (Ethernet II type field)

-To accommodate the “type” field in IEEE format, the new fields called

SNAP(SubNetwork Access Protocol) is included

LAPD(Link Access Procedure D-Channel)

- Subset of HDLC for use in ISDN signaling channel(D-Channel)

-Services--connectionless(used to transfer network management related messages and protocols which do not require connections) & connection-oriented(used to transfer call setup messages)

2nd layer

-Manage the link--for Connection-oriented service only

-Synchronize & delimit frames

-Flow control--for Connection-oriented service only

-Error control

Network Layer(Packet Layer)

-Delivery of Packets through the "network"----“packet switching”

3rd Layer Design Issues

-Layer services

-Routing Algo.

-Congestion

-Internetworking Controversy--which service should we provide in Network layer(packet

switching layer):

-Connectionless service(Datagram)?

-Connection-Oriented service(Virtual Circuit)?

Connection-oriented(VC) vs Conncectionless(DG)

ISSUES Connection-oriented Connectionless

-------------------------------------------------------------------------------------

Initial setup Required No

and termination

of the VC

Addresses Full address needed Always full addresses

During initial setup are needed

Afterwards only VC#

Is needed

Packet Guaranteed Not guaranteed

Sequencing

Error Control Network layer will Left to a higher layer

Check error and

retransmit

Flow Control Done by Network layer Not done

Overhead High Low

Examples: CCITT X.25 IP, IPX, ISO-IP

Packet layer(X.75)

Examples of 3rd layer Protocols

X.25 Packet Layer

-Connection-Oriented(VC)

-PVC / SVC

-Flow / Error Control similar to 2nd layer

IP(Internet Protocol)

ISO-IP(Both IP & ISO-IP are connectionless)

Novell Netware IPX