CAPÍTULO 1. EL 11-S COMO ACONTECIMIENTO: UNA VISIÓN
1.2. Realidad Mediática
1.2.6. De noticia a infotainment
and SQ: Sequence Indicator, for correct reassembly LCAS
CTRL: Control:
0000 fixed:indicates no LCAS 0001 add: add a member to VCG 0010 norm: normal LCAS transmission
0011 eos: end of sequence, highest member of VCG 0101 idle: member is not part of VCG or to be removed 1111 dnu: do not use, receive side reported MST FAIL GID: Group Id. Constant value for all VCG members
(part of 215 – 1 PRBS)
CRC-3: Generated on H4/K4 over previous frames
MST: Member status multiframe, 1 bit per each VCG member (0 = ok, 1 = fail)
RS-ack: Re-Sequence acknowledgement after a new eos member Source to
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VCAT Overhead
If the CTRL (Control) field is any value other then 0000, then LCAS is in operation and the LCAS fields are active. The VCAT/LCAS header includes the following fields:
MFI (Multiframe Indicator)
SQ (Sequence Indicator)
LCAS CTRL
VCG GID (Group Identification)
CRC (Cyclic Redundancy Check)
MST (Member Status)
RS-ack (Re-Sequence Acknowledgement)
The MFI is two separate multiframes operating in bits 1 to 4 or 5 to 8 of H4 and bit 1 or bit 2 of K4. MFI1 operates in bit 1 and is used as an MFI, whereas a superframe operates across bit 2.
The SQ is 8 bits long and is used to identify the VCG member, i.e. VC-n.
If set to 0000, LCAS CTRL indicates fixed bandwidth, hence no LCAS operation. It can otherwise be set to identify the LCAS operation, i.e. 0010 norm, for normal transmission.
GID is a single bit within the multiframe and is set in accordance with a 215– 1 PRBS (Pseudo Random Binary Signal) at the source. All members of a group will have the same GID in the same multiframe.
The CRC is an 8-bit HVC or 3-bit LVC calculated over VCAT header multiframe for validation of the VCAT overhead.
MST is used over the multiframe and superframe to indicate the status of VCG members, i.e. 0 = OK, 1 = fail. This is normally carried in association with messages such as ADD, NORM, EOS and DNU.
RS-ack is a single bit which is toggled to indicate that a change has been detected at the sink node. This could be the result of a member addition or removal or the change of a SQ number.
J1
1A 10 Gigabit Ethernet 1B GFP
MFI1: Multiframe indicator part 1 [0...255]
MFI2: Multiframe indicator part 2 [0...255]
SQ: Sequence indicator, one per member of the VCG [0...255]
MFI: Combination of MFI2-MFI1 [0...4095]
TY2702/v3.1 © Wray Castle Limited 5.5
VCAT Support for HVCs
The multiframe and superframe for the HVC VCAT is shown in the diagram. As this is in support of HVCs such as VC-4, H4 is used.
MFI1 can be seen in operation on bits 5 to 8 of H4 and these will record the frame number as part of a 16-frame multiframe from 0000 to 1111 (binary), a period of 2 ms.
Within this multiframe a single VCAT overhead will have been transported in the associated bits 1 to 4 of H4.
In bits 1 to 4, the MFI2 is in operation and this is always found covering frame 0 and 1 of the multiframe, creating an 8-bit value i.e. 0 to 255. Thus the VCAT superframe is 256 x 16 or 4096 frames (512 ms).
Note how the GID is found in bit 4 of the fourth frame and the CRC being used is a CRC8, as this is a HVC. The SQ is carried at the end of the multiframe and again is an 8-bit value allowing for 256 VCG members.
VCAT techniques similar to those described for HVCs may be used for LVCs.
Next Generation SDH
VC-N VCAT will reorder
VC-Ns as source VC-N
3 2 1
Legacy SDH
TY2702/v3.1
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VCAT Example
This VCAT example illustrates three members of a VCG together with their payloads over successive frames. Only the source and sink nodes need to know anything about VCAT and LCAS; the intermediary nodes that make up the legacy SDH network are cross connecting VC-4s along individual trails.
For every additional node in a path, the signal at the receiver will be delayed by a frame. Thus it is possible for information to be received at the far end over different path lengths. To compensate for this the receiving node is equipped with buffers that can deal with delays around 256 ms or 2048 SDH frames.
The receiver will be using the MFI 1 and 2 to time-align the information received as well as the GID and SQ numbers to keep track of separate information flows within a VCG.
RS Ack toggles
TY2702/v3.1 © Wray Castle Limited 5.7
LCAS Example
The LCAS example shows an established VCG with two members, 1 and 2. The intention is to add another VC-n as member 3, which is the current highest sequence number plus one.
To start, the EOS (End Of Sequence) is transmitted in the multiframe associated with member 2.
Member 3 is showing IDLE with an MST set to fail (1).
First, the source node sends an LCAS CTRL ADD for member 3. The correct response is for the sink node to respond with the MST set to OK (0).
Next, the source node informs the previous EOS member that it is now normal, i.e. SQ=2.
The source node informs the new member that it is EOS.
The sink nodes indicate this change by flipping the RS-ACK bit in the multiframe.
If any resources are removed permanently from a VCG then the sequence numbers will be reordered to fill any gaps in the sequence.
Next Generation SDH
Core Header
Payload Information
Field
Payload FCS (Opt) Payload
Header
Payload Length Indicator (PLI) Core HEC (cHEC)
User Payload Identifier (UPI) Type HEC (tHEC) Channel Identifier (CID)
Spare
GFP Extension HEC (eHEC)
PTI PFI EXI
GFP Payload
pFCS (Optional)
TY2702/v3.1
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GFP (Generic Framing Procedure)
GFP is designed for any number of packet-based technologies over SDH. This is achieved by providing a framing structure at the SDH layer, which in turn allows for the encapsulation of the higher-layer protocols.
To cater for different service requirements there are two modes defined for GFP-F (GFP: Frame Mapped GFP) and GFP-T (Transparent Mapped GFP).
GFP-F
GFP-F is designed to support variable frame services such as Ethernet and PPP (Point to Point Protocol). GFP-F maps entire frames into GFP payloads before transmission.
GFP-T
GFP-T is as layer 1 for encapsulating layer 1 signals into constant-sized frames. Typical uses include encapsulation of Ethernet 100Base-T, fibre channel and ESCON (Enterprise Systems Connection)¹ data streams. These go straight into GFP without waiting for a complete frame, so T is faster than GFP-F. The CID (Channel Identifier) of the GFP frame is primarily used with this mode because it provides a mechanism for indicating the contributing source, since there may not be frame header information.
¹ ESCON is a serial interface used between IBM computers and peripheral devices.
Payload Length Indicator (PLI) Core HEC (cHEC)
User Payload Identifier (UPI) Type HEC (tHEC) Channel Identifier (CID)
Spare
GFP Extension HEC (eHEC)
PTI PFI EXI 0010 ring (not currently
used)
000 0000 0001 Frame Mapped Ethernet 000 0000 0010 Frame mapped PPP 000 0000 0011 Transparent Fibre Channel 000 0000 0100 Transparent FICON 000 0000 0101 Transparent ESCON
000 0000 1000 Frame Mapped Multi Access Protocol over SDH
000 0000 1001 Transparent DVB MPEG transport streams
000 0000 0110 Transparent Gigabit Ethernet 000 0000 1010 Frame Mapped Resilient Packet
Ring (RPR)
000 0000 1011 Frame Mapped Fibre Channel 000 0000 1100 Asynchronous Transparent Fibre Channel 000 0000 1101 Frame Mapped MPLS
100 0000 0001 Signal Fail 100 0000 0010 loss of character sync
TY2702/v3.1 © Wray Castle Limited 5.9
Generic Framing Procedure (GFP) (continued)
The GFP frame is split into four main parts: core header, payload header, GFP payload and pFCS (payload Frame Check Sequence).
The core header is scrambled to assist frame delineation. It also contains a cHEC (core Header Error Code), a CRC-16 checksum, and single bit error correction of the core header.
The payload header indicatex the payload being carried and proides additional checksum facilities.
The GFP payload is the encapsulated frame, i.e. the Ethernet frame.
The pFCS is an optional field and is only present if indicated by the PFI (Payload FCS Indication) bit in the payload header.
The PTI (Payload Type Identifier) indicates if the frame is carrying user data or an alarm (client management).
The EXI (Extension Identifier) indicates that the payload is a linear frame (0001) or nothing, null (0000).
The UPI (User Payload Identifier) details the payload or alarm condition associated with the GFP frame.
Next Generation SDH
VCG Member 1 VCG Member 2 VCG Member 3 VCG Member 4
Virtual Concatenation Group VC-4
VC-4 VC-4 VC-4
GFP
TY2702/v3.1
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Next Generation SDH Summary
The diagram shows how VCAT and GFP can be used to support a packet-based service such as Ethernet.
A VCG (Virtual Concatenation Group) comprising four VC-4s is to be used. The effect on GFP is that the GFP frame information can be carried using the larger pipe provided by VCAT. Although the VCAT members are seen as concurrent, the VC-4s they use within STM-n payloads do not have to be. The containers and subsequent cross connects are taken from those that are free within the SDH node.
This example could be used to carry a Gigabit Ethernet frame, even though only four VC-4s have been allocated instead of the expected seven. This is correct, assuming that the traffic loading on the Gigabit Ethernet is operating at a lower rate.
A next generation SDH network could be used to support traffic between Gigabit Ethernet ports on Ethernet switches located in distant offices interconnected by an optical transmission system utilizing SDH technology.
1 2 1 2
3 2
1
3 3 3
4
4 4 4
3
LCAS
GF P
VC AT
Legacy SDH NG SDH
LCAS
GF P VC AT
NG SDH
Sink Source
TY2702/v3.1 © Wray Castle Limited 5.11
Next Generation SDH Summary (continued)
The Ethernet frames could be bursty and as such would be supported by GFP-F.
These frames are encapsulated into GFP-F frames and carried using the available VCG. The VCAT group is managed by LCAS at the source and sink nodes.
The VCAT is using four VC-4s to carry the GFP frames. The cross connects supporting the VCs do not have to follow the same path as long as they are switched through to the same final destination. The sink node will have sufficiently large buffers to cope with any excessive path lengths when reassembling the GFP frames from the incoming VCs.
Next Generation SDH
TY2702/v3.1
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