Network Protocols Course:
	Instructor: Dr. Debby Koren
	Group Members:
	Ido Ish-Hurwitz, Mor Miller, Oded Cohen & Assaf Frenkel
	Tel-Aviv University / Computer Engineering      Spring 2003

	The tutorial is best seen as a Slide Show
	(See button in the bottom right corner)

	E1 and T1 are standardized TDM technologies
	This tutorial will explain:
		TDM
		Basics of E1 and T1
		Higher hierarchies of E1 and T1
		Details of E1 and T1 frames
	In the end there is a short quiz to verify your understanding

	The Multiplexed channels can carry data or audio
	Audio is sampled and coded to data

	Suppose we have a channel with a rate of 64Kbs
	That means that we need to transmit a bit every 1/64000 seconds = 15.6 micro seconds. Why not transmit bits in the middle?
	Let’s see a sample for 3 channels, each transmitting a bit every 3 seconds

	E1 and T1 are both TDM interfaces.
	They belong to the first two layers of the ISO (International Standards Organization) OSI (Open System Interconnect) model: The physical and data-link layers. When using signaling mechanism the E1 and T1 can be considered as a data-link layer and when they are used as a raw bit stream it can be considered as a physical layer.
	T1 technology was developed by AT&T in 1957 and is used in America and Asia
	E1 Technology is used in Europe

	The E1 and T1 are common in the telephony world for connection between switches.
	Today they are used also for connections between ISPs inside states and between states.

	PDH - Plesiochronous Digital Hierarchy
	Uses E1 (30 channels) up to E4 (1920 channels).
	The PDH is restricted to the E4 because in each level more bits are added for framing until it’s getting hard to get the channels.
	In  order to get to one E1 in a switch, all the hierarchy need to be de-multiplexed to E1s

	SDH - Synchronous Digital Hierarchy
	Base hierarchy of SDH uses E1-E4
	SDH is based on high synchronization using atomics clocks rather then adding framing bits
	Thanks to the synchronization it gets to higher rates that goes from 1980 channels (STM-1) to 120000 channels (STM-64), still allowing adding and dropping separate E1s without de-multiplexing all the channels.

	Point-to-Point Protocol (PPP) -  a common layer two protocol that provides a standard method for transporting multi-protocol data-grams over point-to-point links.
	PPP was designed for simple links that transport packets between two peers. These links provide full-duplex simultaneous bi-directional operation, and are assumed to deliver packets in order.
	E1 / T1 are used as links for PPP.

	There are several E1 modes, all use 2048 Kb/s:
	Unframed  (UNF) - stream of 2048 Kb/s with no channel association
	Framed (FR) – all 32 slots are used for data, detection of boundaries is gained with TS0
	Multi-Framed (MF) – TS0 is used for synchronization, all  other channels are unaffected
	MF + Channel Associated Signaling (CAS)1
	MF + Cyclic Redundancy Check (CRC)
	MF + Common Channel Signaling (CCS)/CAS + CRC1

	A Frame is composed from 256 bits that are divided to 32 Time Slots (TS) x 8 bits per TS
	Each channel rate is 64 Kb/s
	The channels are in consecutive time slots numbered 0-31
	Frame rate is 8 KHz

	TS 1-15, 17-31 are used for user data and are referred as channels 1-30
	TS0 is used for synchronization, alarms and messages (future use)
	TS16 is used for signaling (but can be also used for data)

	A multi-frame is formed from consecutive 16 frames numbered 0-15
	It is used for adding information regarding the data:
		In TS0 – synchronization between frames & Error Correcting
		In TS16 – Signaling

	Synchronization is gained in every 2nd frame. The synchronization pattern is 0011011. Specifically in the even numbered frames (0, 2…) bits 1-7 (from 0-7) holds the mentioned pattern.
	Note that this synchronization does not involves a clock, but bits are transmitted constantly even when the line is idle.

	Error correcting (optional) is done using a Cyclic Redundancy Check (CRC-4) that uses 4 bits for every half multi-frame (8 frames). specifically the 4 bits are placed in bit 0 of every 2nd (even) frame, before the synchronization pattern.
	Error indication bits using CRC-4 are held in frames 13 and 15
	Remote alarm indication bit is held in bit 3 of odd numbered frames (1, 3…)
	Other bits in the odd frames are spare bits.

	Common Channel Signaling (CCS) – at least one channel (usually TS16) is used for signaling and serves asynchronously all the channels
	Channel Associated Signaling (CAS) – in each multi-frame, for each channel, there is a frame that half of it’s TS16, is dedicated for that channel signaling:
		Frame 0 is used for alarm indication and spare bits
		Frame 1 is used for channels 1 and 16 (4 bits each)
		Frame 2 is used for channels 2 and 17 (4 bits each)
		…
		Frame 15 is used for channels 15 and 30 (4 bits each)

	When the bits aren’t used for signaling 2nd and 4th bits should be ‘1’ and the 3rd should be ‘0’.
	The bits can be used for signaling of 2 states (1 bit), 4 states (2 bits) or 15 states (4 bits minus ‘0000’).
	This was used mainly to the  on-off-keying slow dial method (OOK). Today DTMF is used in each data channel as part of the data

	All modes uses 1544 Kb/s:
	Unframed  (UNF) - stream of 1544 Kb/s with no channel association
	Framed
	Super-Frame (SF)
	SF + CAS1
	Extended Super Frame (ESF)
	ESF + FDL1
	ESF + CA/CRC/FDL1
	CCS1

	The T1 frame is composed from 24 channels that come in consecutive time slots numbered 0-23
	The frame is 193 bits that are composed from  1 framing bit + 8 bits * 24 Time Slots (TS
	Framing bit creates an additional channel of 8 kb/s
	Frame rate is 8 KHz

	The T1 Super Frame (SF) is composed from 12 frames that are numbered 1-12
	The SF structure includes a synchronization mechanism (pattern of ‘001001’) and can include a signaling mechanism.
	These mechanisms are using the framing bit that is added to each frame. Some of them are used for frame boundaries and some for the SF boundaries

	Channel Association Signaling (CAS) is also optional. It uses 2 bits for every channel, 1 bit of the 6th frame and the 12th frames. I.e. every channel looses 2 bits in each SF
	The CAS therefore forms a 10.666 kb/s channel.
	Accordingly the channels rate decreases from 64 kb/s to 56 kb/s which will not interfere to audio channels, but may interfere to data transmissions

	The Extended Super Frame (ESF) is composed from 24 frames. It is also known as D5 and Fe
	Signaling – using 1 bit from each 6th frame (6, 12, 18, 24) for each channel. Also known as A/B/C/D signaling. Can for up-to 16 features (4 bits) similar to the SF’s CAS.

	There are 3 types of framing information:
		Synchronization – using the pattern ‘001011’ in every 4 frames – 4, 8, 12, 16, 20 and  24
		CRC-6 in frames – 2, 6, 10, 14, 18, 22 The CRC-6 detects all errors of less then 6 bits, and 98.4 % of errors in more bits. It also prevents synchronization loss that can occur from identical pattern to the synchronization pattern.
		Data link in the odd frames – 1, 3 … 23 creating a 4 kb/s channel for maintenance and supervisory control.

	E1 and T1 are:
		Microwave technology
		Fiber technology
		TDM Interface

	While multiplexing:
		We send same data on several channels (for backup)
		We send several transmissions on a same link
		We send several transmission on several links

	E1 Channel Rate is:
		8 Kb/s therefore it isn’t good for voice
		64 Kb/s therefore it is a waste to use it for data
		2.048 Mb/s therefore it is good for voice, music and data
		None of the above

	T1 Frame:
		12 channels à86 bits
		12 channels + framing bit à 87 bits
		24 channels à192 bits
		24 channels + framing bit à 193 bits
		None of the above

	www.epanorama.net/links/tele_general.html
	www.dcbnet.com/notes/9611t1.html
	www.hardware-guru.com/SystemDesign/E1_T1_Tutorial.htm
	www.lucent.ca/certification/pdf/study001.pdf
	Personal knowledge