PC and Industrial Networks

How Applications Access the Media

The physical layer provides the mechanical, procedural, and functional aspects of

activating the physical link between end systems:

Voltage levels

Data rates

Maximum transmission distances

Physical connections

The data link layer provides data transport across a physical link. The data link handles:

Physical addressing (address resolution)

Network topology (ethernet, token ring, fddi)

Line discipline (hdlc, ppp, frame relay)

Error notification

Orderly access to the physical medium

Flow control (optional)

The TCP/IP Model calls this the Network Interface Layer

Each machine that connects to the network usually contains a device known as a Network

Interface Card (NIC) that connects the machine directly to the network. In order for the

device to communicate with other devices, there must be an addressing mechanism that is

used to specify the destination of the packet. The addressing schemes differ depending

on the network technology utilized.

Ethernet Characteristics:

Shared bus

Best-effort delivery: hardware provides no mechanism for informing sender whether packet was delivered.

Carrier Sense Multiple Access/ Collision Detect – distributed, no central authority for transmission. (use switching to reduce contention)

48 bit addressing scheme: expressed as hex: 00:00:c0:c8:b2:27

24 bits: manufacturer address

00:00:0c cisco

08:00:20 sun

08:00:07 apple

00:50:da 3Com

24 bits: host id

hardware address, physical address, MAC address, layer 2 address

2

Utilizes broadcast technology – all stations receive every transmission host interface receives all packets.

host interface examines the packet and decides if it should be sent up the

stack.

host interface discards other packets.

Broadcast address of all 1’s causes all hosts to respond.

Ethernet Frame Format:

Preamble Destination Address Source Address Frame Type Frame Data CRC

8 octets 6 octets 6 octets 2 octets 46-1500 octets 4 octets

Preamble: 1010 1010  AAh

Start Delimiter: 1010 1011  ABh

Destination Addy: MAC Address

Source Addy: MAC Address

Frame Type: 0800h IP

(RFC 1700) 0806h Address Resolution Protocol (ARP)

8037h Novell IPX

809Bh AppleTalk

Payload: 46 octets minimum (may require “padding”)

1500 octets maximum

Notes:

1. The Frame Type identifier makes the Ethernet frame “self-identifying” and accounts for the concept of multi-protocol networks.

2. NICs respond by extracting the frame when the destination address in the header contains the corresponding MAC address (unicast).

3. NICs respond by extracting the frame when the destination address in the header contains a broadcast address: FF:FF:FF:FF:FF:FF (broadcast).

4. NICs also respond by extracting the frame when the destination address in the header contains a multi-cast address. The NIC must be programmed by the

software application to respond to the multi-cast address.

3

The Data Link Layer in Ethernet

Contains 2 sub layers:

Logical link control

Media access control

LLC

Manages communications between devices over a single link of a network

Allows multiple higher layer protocols to share the data link

Provides interface flexibility (i.e. upper layers operate without regard to the LAN media)

Can provide flow control

MAC

Provides access to the LAN medium in an orderly manner

For multiple stations to share medium and identify each other, they must have unique addresses

MAC defines a unique hardware (physical) or data link address for each LAN interface

Address structure is flat (compare to logical or virtual addresses in layer 3 – IP – which is hierarchical: numbers relate to the network(s) they attach

to)

When 2 devices on a network communicate with each other, they don’t use IP. They use

protocols specific to the wire itself. Devices on an Ethernet segment use a predefined

series of electrical impulses to communicate with each other (raise and lower voltage on

the shared medium). The changes in voltage are dictated by the protocols and are

specific to the physical network. Thus, ethernet networks data link protocols will not

work on a Token Ring network.

4

Encapsulation of the Packet into Datagrams and then Ethernet Frames

Packet Header Packet Data

Datagram Data Datagram Header

Ethernet Frame Data Ethernet Frame Header

Application Data