Serial Point-to-Point Overview
Point-to-Point connections are used to connect LANs to a service providers WAN in a serial connection, or leased line.Serial VS Parallel connections
Serial connections compared to parallel connections.Serial connections
- Transmit bits sequentially over a single channel
- Bidirectional
- Also called bidirectional port or a communications port
- Used for external communications and signal conversion
- Less expensive than parallel ports, requiring fewer wires, cheaper cables, and use fewer pins.
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Parallel Connections
- Allow bits to be simultaneously transmitted over multiple channels
- Uni-directional (outbound from hard drive)
- Theoretically eight times faster than Serial connections
- Problems with crosstalk that worsens with the length increases
- Used on PCs for communicating to printers, other peripheral devices, and for internal connections.
- Problem with Clock Skew
Parallel ports and RS-232 serial ports have been replaced by faster Universal Serial bus (USB) interfaces.
Serial Communications
Frames are encapsulated by the communications protocol then are sent and decapsulated by the same protocol on the receiving router. There are three main serial connection standards of LAN to WAN connections, RS-232, V.35, High Speed Serial Interface (HSSI).RS-232/RS-232s
- RS-422/RS-423 are the newer standards
- Used for peripheral devices but are mostly replaced by USB
- RJ-45 connecters conform to the original RS-232 standard
- Uses both 9 and 28 pin connectors
V.35
- Used for modem-to-multiplexer connections
- ITU standard for high-speed synchronous data exchange
- Used in the US to connect routers and DSUs to T1 carriers
- Connects DTEs and DCEs over digital lines
HSSI
- Transmission rates up to 52Mbps
- Connects routers to WANs over T3 lines
- High-speed connectivity between LANs using Token Ring or Ethernet
- DTE/DCE interface
- Developed by Cisco Systems and T3 Plus Networking
- Addressed need for high-speed communication over WAN lines
Point-to-Point communication links
Point-to-Point serial links provide a single, pre-established, WAN communications path that connects two distance locations through a service provider. Service provider dedicates specific resources for the leased line. The links are not limited to land and are more expensive than shared connections. The Dedicated capacity removes latency/jitter. The constant availability is useful for VoIP and Video over IP. |
| ©2014 Cisco Press, Connecting Networks Companion Guide |
Even tough resources are dedicated for a leased line, the links may be shared using multiplexing. There are two types used: Time-Division Multiplexing (TDM) and Statistical Time-Division Multiplexing (STDM).
TDM
Physical layer concept, invented by Bell Industries, to maximize traffic carried over a single link. Before TDM, phones required individual links.How TDM Works
- The Multiplexer (MUX) accepts multiple separate channel/links
- Assigns each channel a time slot
- Divides the signals into segments
- Puts each segment on a single channel in its assigned time slot
- The receiving MUX reassembles the stream into the separate streams based on the arrival time of each bit.
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| ©2014 Cisco Press, Connecting Networks Companion Guide |
STDM
Developed to use all time slots when fewer streams are active. Channels compete for free slots on a first-come first-serve basis. STDM temporarily stores data in buffer memory during high-traffic. Each transmission requires an identification number or channel identifier. |
| ©2014 Cisco Press, Connecting Networks Companion Guide |
TDM/STDM examples are SONET/SDM.
This chapter will be continued in Part 2.
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