12-21-2000
The concept of service layers is primarily a software concept. The purpose of the concept is to allow network interoperability between computing systems.
The physical and link layers are the lowest level of networking and discusses physical connectivity protocols. One example is ethernet. Ethernet works between all servers and NIC cards because manufactures follow a very rigid open standard. Physical is the wire. Link layer is mostly the software protocol.
As you move up the stack, interoperability tends to be more difficult. At the network and transport layer, TCP is the common way computers move datagrams between end-to-end systems. Above the link layer this is done with software data structures that interface the link layer device drivers. In UNIX this is typically with sockets.
Sockets can be of many flavors and varieties. A TCP/IP socket takes care of end-to-end reliability using well developed protocols to react to congestion, sequencing, lost packets in a noisy environment, etc.
As you move further up the stack, you being to deal with information as it 'goes in and comes out' of the network tunnel, so to speak. The includes the session, application, and presentation layer. Session is a layer that manages state information between two client/server processes (for example) during a given session.
Application layer activity includes specific tasks performed on the end-to-end platforms to accomplish a task. This application is independent of the network. Often the task could just have easily been done from a console without networking or by moving the information manually (and very inefficiently) from one platform to another.
The presentation layer is the layer the user sees. This is the area that gets the most attention by Windows users and the least attention in true client/server environments. In addition, a true client/server environment completely decouples the presentation layer from the application, so the 'window' into the application is platform neutral.
This construct is the most important construct in modern day networking. Understanding how it intimately works takes years of software experience. It is really pretty Zen in that the concepts are so simple but the results so wonderful.
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INTRO(4) Kernel Interfaces Manual INTRO(4)
NAME
intro - introduction to special files and hardware support
DESCRIPTION
This section describes the special files, related driver functions, and networking support available in the system. In this part of the
manual, the SYNOPSIS section of each configurable device gives a sample specification for use in constructing a system description for the
/sys/conf/configscript, the autoconfig(8), program and descibes the major and minor device numbers and their encoding. The DIAGNOSTICS
section lists messages which may appear on the console and/or in the system error log /usr/adm/messages due to errors in device operation.
This section contains both devices which may be configured into the system, ``4'' entries, and network related information, ``4N'', ``4P'',
and ``4F'' entries; The networking support is introduced in intro(4N).
PDP DEVICE SUPPORT
This section describes the hardware supported on the DEC PDP-11. Software support for these devices comes in two forms. A hardware device
may be supported with a character or block device driver, or it may be used within the networking subsystem and have a network interface
driver. Block and character devices are accessed through files in the file system of a special type; c.f. mknod(8). Network interfaces
are indirectly accessed through the interprocess communication facilities provided by the system; see socket(2).
A hardware device is identified to the system at configuration time and the appropriate device or network interface driver is then compiled
into the system. When the resultant system is booted, the autoconfiguration facilities in the system probe for the device on either the
UNIBUS (or Q-bus) or MASSBUS and, if found, enable the software support for it. If a device does not respond at autoconfiguration time it
is not accessible at any time afterwards. To enable a device which did not autoconfigure, the system will have to be rebooted.
The autoconfiguration system is described in autoconfig(8). A list of the supported devices is given below.
SEE ALSO
intro(4), networking(4), config(8).
LIST OF DEVICES
The devices listed below are supported in this incarnation of the system. Pseudo-devices are not listed. Listed also are devices which
are in various stages of porting to 2.11BSD from 4.3BSD.
Devices are indicated by their functional interface. If second vendor products provide functionally identical interfaces they should be
usable with the supplied software. (Beware, however, that we promise the software works ONLY with the hardware indicated on the appropri-
ate manual page.) Occasionally, new devices of a similar type may be added simply by creating appropriate table entries in the driver.
The following are known to work:
acc ACC LH/DH IMP communications interface
de DEC DEUNA 10Mb/s Ethernet controller
dh DH-11 emulators, terminal multiplexor
dhu DHU-11 terminal multiplexor
dz DZ-11 terminal multiplexor
ec 3Com 10Mb/s Ethernet controller
hk RK6-11/RK06 and RK07 moving head disk
ht TM03 MASSBUS tape drive interface (with TE-16, TU-45, TU-77)
il Interlan 1010, 1010A, 2010A 10Mb/s Ethernet controller
lp LP-11 parallel line printer interface
qe DEC DEQNA Q-bus 10 Mb/s Ethernet interface
ra DEC UDA-50, RQDX, KLESI disk controllers
rk DEC RK05 disk controller
rl DEC RL-11 disk controller
rx DEC RX02 floppy interface
si SI 9500 disk controller
tm TM-11/TE-10 tape drive interface
tmscp TMSCP-compatible tape controllers (e.g., TU81, TK50)
ts TS-11 tape drive interface
vv Proteon proNET 10Mb/s and 80Mb/s ring network interface
xp General purpose SMD disk controller
The following should work:
dr DR-11W general purpose DMA UNIBUS interface
The following worked in the past but will probably require work:
css DEC IMP-11A communications interface
dmc DEC DMC-11/DMR-11 point-to-point communications device
en Xerox 3Mb/s Ethernet controller (obsolete)
sri DR-11C IMP interface
It should be possible to port these from 4.3BSD:
ex Excelan 10Mb/s Ethernet controller
ix Interlan NP-100 10Mb/s Ethernet controller
np Interlan NP-100 10Mb/s Ethernet controller (intelligent mode)
pcl DEC PCL-11 communications interface
No idea whether any of the following could be ported:
ad Data translation A/D interface
ct C/A/T or APS phototypesetter
ddn ACC ACP625 DDN Standard Mode X.25 IMP interface
dmf DEC DMF-32 terminal multiplexor and parallel printer interface
dmz DEC DMZ-32 terminal multiplexor
dn DEC DN-11 autodialer interface
hdh ACC IF-11/HDH IMP interface
hp MASSBUS disk interface (with RP06, RM03, RM05, etc.)
hy DR-11B or GI-13 interface to an NSC Hyperchannel
kg KL-11/DL-11W line clock
mt TM78 MASSBUS tape drive interface
tu VAX-11/730 TU58 console cassette interface
un DR-11W interface to Ungermann-Bass
up Emulex SC-21V, SC-31 UNIBUS disk controller
ut UNIBUS TU-45 tape drive interface
uu TU58 dual cassette drive interface (DL11)
va Benson-Varian printer/plotter interface
vp Versatec printer/plotter interface
4th Berkeley Distribution January 27, 1996 INTRO(4)