Network Adapter Cards

Network Adapter Cards

The role of the network Adapter cards it to:
· Prepare data from the computer for the network cable
· Send the data to another computer
· Control the flow of data between the computer and the cabling system

NIC's contain hardware and firmware (software routines in ROM) programming that implements the
· Logical Link Control and
· Media Access Control
Functions of the Data Link layer of the OSI

Preparing Data
· Data moves along paths in the computer called a BUS - can be 8, 16, 32 bits wide.

· On network cable, data must travel in a single bit stream in what's called a serial transmission (b/c on bit follows the next).

· The transceiver is the component responsible for translating parallel (8, 16, 32-bit wide) into a 1 bit wide serial path.

· A unique network address or MAC address is coded into chips in the card

· card uses DMA (Direct Memory Access) where the computer assigns memory space to the NIC
if the card can't move data fast enough, the card's buffer RAM holds it temporarily during transmission or reception of data

Sending and Controlling Data
The NICs of the two computers exchanging data agree on the following:
1. Maximum size of the groups of data being sent
2. The amount of data to be sent before confirmation
3. The time intervals between send data chunks
4. The amount of time to wait before confirmation is sent
5. How much data each card can hold before it overflows
6. The speed of the data transmission

Network Card Configuration
· IRQ: a unique setting that requests service from the processor.

IRQ #---- Common Use---- I/O Address
IRQ 1 - Keyboard
IRQ 2(9)- Video Card
IRQ 3 - Com2, Com4 2F0 to 2FF
IRQ 4 - Com1, Com3 3F0 to 3FF
IRQ 5 - Available (Normally LPT2 or sound card )
IRQ 6 - Floppy Disk Controller
IRQ 7 - Parallel Port (LPT1)
IRQ 8 - Real-time clock
IRQ 9 - Redirected IRQ2 370 - 37F
IRQ 10 - Available (maybe primary SCSI controller)
IRQ 11 - Available (maybe secondary SCSI controller)
IRQ 12 - PS/2 Mouse
IRQ 13 - Math Coprocessor
IRQ 14 -Primary Hard Disk Controller
IRQ 15 - Available (maybe secondary hard disk controller)

Base I/O port: Channel between CPU and hardware
Specifies a channel through which information flows between the computer's adapter card and the CPU. Ex. 300 to 30F.

Each hardware device must have a different base I/O port

Base Memory address: Memory in RAM used for buffer area
Identifies a location in the computer's RAM to act as a buffer area to store incoming and outgoing data frames. Ex. D8000 is the base memory address for the NIC.

Each device needs its own unique address.

some cards allow you to specify the size of the buffer ( 16 or 32 k, for example)

Transceiver:
Sometimes selected as on-board or external. External usually will use the AUI/DIX connector: Thicknet, for example

Use jumpers on the card to select which to use

Data Bus Architecture
The NIC must
· match the computer's internal bus architecture and

· have the right cable connector for the cable being used
· ISA (Industry Standard Architecture): original 8-bit and later 16-bit bus of the IBM-PC.
· EISA (Extended Industry Standard Architecture): Introduced by consortium of manufacturers and offers a 32-bit data path.
· Micro-Channel Architecture (MCA): Introduced by IBM in its PS/2 line. Functions as either 16 or 32 bit.
· PCI (Peripheral Component Interconnect): 32-bit bus used by Pentium and Apple Power-PC's. Employs plug and play.

Improving Network Card Performance
Direct Memory Access (DMA):
Data is moved directly from the network adapter card's buffer to computer memory.

Shared Adapter Memory:
Network adapter card contains memory which is shared with the computer.
The computer identifies RAM on the card as if it were actually installed on the computer

Shared System Memory:
The network adapter selects a portion of the computer's memory for its use.
MOST common

Bus Mastering:
The adapter card takes temporary control of the computer's bus, freeing the CPU for other tasks.
moves data directly to the computer's system memory
Available on EISA and MCA
can improve network performance by 20% to 70%

RAM buffering:
Ram on the adapter card acts as a buffer that holds data until the CPU can process it.
this keeps the card from being a bottleneck

·On-board microprocessor:
enables the adapter card to process its own data without the need of the CPU

Wireless Adapter Cards
· Used to create an all-wireless LAN
· Add wireless stations to a cabled LAN
· uses a wireless concentrator, which acts as a transceiver to send and receive signals

Remote-Boot PROMS (Programmable Read Only Memory)
· Enables diskless workstations to boot and connect to a network.
· Used where security is important.

Putting Data on the Cable

Access Methods
The 4 major methods
Carrier Sense Multiple Access Methods
1. with collision detection (CSMA/CD)
2. with collision avoidance (CSMA/CA)

Token passing that allows only a singe opportunity to send data

A Demand Priority method

Carrier Sense Multiple Access with Collision Detection. (CSMA/CD)
1. Computer senses that the cable is free.
2. Data is sent.
3. If data is on the cable, no other computer can transmit until the cable is free again.
4. If a collision occurs, the computers wait a random period of time and retransmit.
o Known as a contention method because computers compete for the opportunity to send data. (Database apps cause more traffic than other apps)
o This can be a slow method
o More computers cause the network traffic to increase and performance to degrade.
o The ability to "listen" extends to a 2,500 meter cable length => segments can't sense signals beyond that distance.

Carrier Sense Multiple Access with Collision Avoidance　 (CSMA/CA)
in CSMA/CA, the computer actually broadcasts a warning packet before it begins transmitting on the wire. This packet eliminates almost all collisions on the network because each computer on the network does not attempt to broadcast when another computer sends the warning packet.
All other computers wait until the data is sent.
The major drawback of trying to avoid network collisions is that the network traffic is high due to the broadcasting of the intent to send a message.

Token Passing
Special packet is passed from computer to computer.
A computer that wants to transmit must wait for a free token.
Computer takes control of the token and transmits data. Only this computer is allowed to transmit; others must wait for control of the token.
Receiving computer strips the data from the token and sends an acknowledgment.
Original sending computer receives the acknowledgment and sends the token on.
the token comes from the Nearest Active Upstream Neighbor　 and when the computer is finished, it goes to the Nearest Active Downstream Neighbor
uses "beaconing" to detect faults => this method is fault tolerant
NO contention => equal access to all computers on the network
NO collisions

Demand Priority
0. 100 Mbps standard called 100VG-AnyLAN. "Hub- based".
1. Repeaters manage network access by performing cyclical searches for requests to send from all nodes on the network. The repeater or HUB is responsible for noting all addresses, links and end nodes and verifying if they are all functioning. An "end node" can be a computer, bridge, router or switch.
2. Certain types of data are given priority if data reaches the repeater simultaneously. If two have the same priority, BOTH are serviced by alternating between the two.

Advantages over CSMA/CD　
1. Computers Uses four pairs of wires which can send and receive simultaneously.
2. Transmissions are through the HUB and are not broadcast to all other computers on the network.
3. There is only communication between the sending computer, the hub and the destination computer.
　
Other methods 　
Appletalk
The cabling system for an AppleTalk network is called LocalTalk.
LocalTalk uses　 CSMA/CA
AppleTalk has a dynamic network addressing scheme.
During bootup, the AppleTalk card broadcasts a random number on the network as its card address. If no other computer has claimed that address, the broadcasting computer configures the address as its own. If there is a conflict with another computer, the computer will try to use different IP combinations until it finds a working configuration.
　
　
ARC Net
ARC Net uses a token passing method in a logical ring similar to Token Ring networks.
However, the computers in an ARC Net network do not have to be connected in any particular fashion.
ARC Net can utilize a star, bus, or star bus topology.
Data transmissions are broadcast throughout the entire network, which is similar to Ethernet.
However, a token is used to allow computers to speak in turn.
The token is not passed in a logical ring order because ARCNet does not use the ring topology; instead the token is passed to the next highest numerical station
Use DIP switches to set the number (the Station Identifier) of the workstations, which you want to be beside each other so the token is passed to the next computer efficiently.
ARC Net isn't popular anymore => ARC Net speeds are a mere 2.5 Mbps.

Most important ARC Net facts for you to know:
ARC Net uses RG-62 (93 ohms) cabling;
it can be wired as a star, bus, or star bus; and
it uses a logical-ring media access method.

Summary Chart 　

Type of Communication
CSMA/CD - Broadcast-based
CSMA/CA - Broadcast-based
Token Passing-Token-based
Demand Priority-Hub-based


Type of Access Method
CSMA/CD - Contention
CSMA/CA -Contention
Token Passing-Non-Contention
Demand Priority-Contention


Type of Network
CSMA/CD -Ethernet
CSMA/CA -Local Talk
Token Passing-Token Ring ARCnet
Demand Priority-100VG-AnyLAN