Modems

Modem standards are set by the International Telecommunications Union (ITU).
Modem speed is measured in bits per second (different than the baud rate)

Asynchronous Modems

Use plain old telephone lines (POTs).
Most popular modem type.
25% of data stream is flow control and data coordination.
Data compression is standard.

Synchronous Modems

Works on a dedicated line with coordinated (synchronized) timing between devices.
Significantly faster than asynchronous modems.
Primary communication protocols include:
SDLC Synchronous Data Link Control used in IBM's SNA
HDLC High Level Data Link Control An ISO Data Link Protocol
BiSync Binary Synchronous Communications Protocol Another IBM protocol

Digital Modem

Used for ISDN lines.
Composed of:
Network Termination (NT)
Terminal Adapter (TA)

Carriers
Three factors when considering modems and connections

1. Throughput
2. Distance
3. Cost

Three carrier options:

1. Public switched telephone network.
2. Dial up ISDN.
3. Dedicated leased lines

Remote Access Service (RAS)

RAS with NT server can allow up to 256 remote dial inclients.
The client side name depend on the O/S.
For NT 3.51 and WfWG, it is called RAS client.
For NT 4.0 and 95, it is called Dial-Up Networking (DUN).

SLIP Dial up Protocols

Serial Line Internet Protocol (SLIP)
Older protocol
Essentially a physical layer protocol that provides no error correction.
Only supports TCP/IP

PPP Dial up Protocol

Point to point (PPP)
Provides both Physical and Data Link services.
Effectively turns a modem to a NIC
Supports Dynamic IP addressing, compression and error checking.

Connecting Networks

Networks, like the organizations that use them, are dynamic.
Dynamic network tasks include:
Physically expanding to support additional computers.
Segmenting to filter network traffic.
Extending to connect separate LANs.

Connecting two separate computing environments.

Connection Devices


Layer	Function
Seven	Gateways
three	Brouters,Routers
two	Brouters, Switches, Bridges
one	Repeaters

Repeaters regenerate the network signal effectively extending the network length.

Can only connect like networks.
Can connect different media.
For coaxial cable, 5-4-3 Rule applies.

Bridges

Works at the Data Link layer.
Segment a network.
Filter (on MAC addresses) traffic between the segments.
Can connect different media and architectures.
Forwards appropriate packets between network segments.
Requires unique data paths.

Bridge Features

Bridges build a bridging table that contains a comprehensive list of MAC addresses and the network segment of each address.
Bridges check each packet's source and destination addresses. If it is on an external network segment, the packet is forwarded. If not, it is discarded.
Bridges do not reduce broadcast traffic.

Routers

Routers connect separate networks to form an internetwork.
Operate at the Network level.
An internetwork is created when two or more independent networks are connected yet continue to function separately.
Can select among multiple network paths.

Router Features

Routers analyze a packets destination node (MAC) address and its destination network address.
When a router receives a packet with a destination network address it does not know, it discards that packet.
A router's table consists of network addresses and the addresses of the routers that handle those networks.

Router Path Choice Methods
Two TCP/IP protocols that can help a router to choose a packet's best path.

RIP (Routing Information Protocol)
A distance vector algorithm. It chooses the path with the fewest hops.
OSPF Open Shortest Path First
A link state algorithm that also considers factors such as network traffic and speed.

Populating a Router Table
With static routing, a table must be updated manually.

The administrator must individually enters each route.

Dynamic routers communicate with each other.

When multiple LANs have more than one path, the spanning tree algorithm (STA) is used to eliminate these redundant routes,

Routable vs Nonroutable Protocols

Routers are only able to work with protocols that include Network layer information.
Routable protocols include: TCP.IP, and IPX.
Nonroutable protocols include NetBEUI, and DLC..

Brouters
Brouters combine both bridge and router functionality.
Brouters maintain both a bridging table, which contains hardware addresses and a routing table, which contains network addresses.

Gateways
A gateway translates information between two different network architectures or data formats.
Operates at the Applications Layer.
NT services for Macintosh functions as a gateway that translates between NetBEUI and AppleTalk.

Switches
In essence, a high-speed multiport bridge that can create a VPN.
An intelligent hub that maintains a bridging table.
Can dedicate bandwidth to each of its ports which ensures that the maximum bandwidth is available to all computers on the network.
Can serve as 100/10Mbs interface.